Transcriber Note
Whole and fractional parts are shown as 2-1/2.
Emphasis shown as _Italics_ and =Bold=.
UNITED STATES DEPARTMENT OF AGRICULTURE
=BULLETIN No. 816=
Contribution from the Bureau of Plant Industry
Wm. A. Taylor, Chief
-------------------------------------------------------
Washington, D. C. [Illustration] January 19, 1920
-------------------------------------------------------
=STREET TREES=
By
=F. L. MULFORD, Horticulturist=
Office of Horticultural and
Pomological Investigations
-------------------------------------------------------
CONTENTS
Page
Importance of Shade Trees 1
Public Control of Street Trees 6
Planning for Trees on City Streets 8
Spacing Trees 9
Conditions for Tree Growth 10
Kinds of Trees Suitable for City Streets 14
Qualities Necessary 14
Trees for Different Regions 16
Trees for Special Purposes 20
Descriptions of Street Trees 20
Culture of Street Trees 43
Selection of Individual Trees 43
Preparation of Holes 44
Planting 45
Pruning 50
Stakes and Guards 51
Later Care 52
Care of Mature Trees 53
Pruning 53
Feeding 55
Spraying 55
[Illustration]
WASHINGTON
GOVERNMENT PRINTING OFFICE
1920
=BUREAU OF PLANT INDUSTRY.=
William A. Taylor, _Chief_.
K. F. Kellerman, _Associate Chief_.
James E. Jones, _Assistant to Chief_.
J. E. Rockwell, _Officer in Charge of Publications_.
Office of Horticultural and Pomological Investigations.
SCIENTIFIC STAFF.
L. C. Corbett, _Horticulturist in Charge_.
Truck Crop Production Investigations:
J. H. Beattie.
F. E. Miller.
C. J. Hunn.
B. J. McGervey.
Irish Potato Production Investigations:
William Stuart.
C. F. Clark.
W. C. Edmundson.
P. M. Lombard.
J. W. Wellington.
L. L. Corbett.
Truck Crop Improvement Investigations:
W. W. Tracy.
D. N. Shoemaker.
Landscape Gardening and Floriculture Investigations:
F. L. Mulford.
W. Van Fleet.
Bulb Culture Investigations:
David Griffiths.
Fruit and Vegetable Utilization Investigations:
J. S. Caldwell.
C. A. Magoon.
C. W. Culpepper.
Fruit Production Investigations:
H. P. Gould.
L. B. Scott.
C. F. Kinman.
George M. Darrow.
E. D. Vosbury.
Grape Production Investigations:
George C. Husmann.
Charles Dearing.
F. L. Husmann.
Elmer Snyder.
G. L. Yerkes.
Fruit Breeding and Systematic Investigations in Pomology:
W. F. Wight.
Magdalene R. Newman.
Fruit Improvement through Bud Selection:
A. D. Shamel.
Nut Investigations:
C. A. Reed.
E. R. Lake.
Fruit and Vegetable Storage Physiology:
L. A. Hawkins.
R. C. Wright.
J. R. Magness.
J. F. Fernald.
Extension Work (in cooperation with States Relations Service):
W. R. Beattie.
C. P. Close.
UNITED STATES DEPARTMENT OF AGRICULTURE
[Illustration] BULLETIN No. 816 [Illustration]
Contribution from the Bureau of Plant Industry
WM. A. TAYLOR, Chief
Washington, D. C. [Illustration] January 19, 1920
=STREET TREES.=
By F. L. Mulford, _Horticulturist, Office of Horticultural
and Pomological Investigations_.
-----
CONTENTS
Page
Importance of Shade Trees 1
Public Control of Street Trees 6
Planning for Trees on City Streets 8
Spacing Trees 9
Conditions for Tree Growth 10
Kinds of Trees Suitable for City Streets 14
Qualities Necessary 14
Trees for Different Regions 16
Trees for Special Purposes 20
Descriptions of Street Trees 20
Culture of Street Trees 43
Selection of Individual Trees 43
Preparation of Holes 44
Planting 45
Pruning 50
Stakes and Guards 51
Later Care 52
Care of Mature Trees 53
Pruning 53
Feeding 55
Spraying 55
-----
IMPORTANCE OF SHADE TREES.
The comfort to be derived from shade trees has long been recognized. The
early settlers of this country saved fine trees about their homes, on the
village greens, along the country roads, and in the fields. Later, as
villages grew, the householders planted trees adjoining their properties,
and the result has been the beautiful elm-shaded villages of New England,
the maple-shaded towns of New York and the Ohio Valley, and the oak-shaded
streets of the Southeastern States. (Fig. 1.)
With time, the villages and towns became cities, and the woodlands were
largely destroyed. Conditions for tree growth were less favorable in the
cities, and nurseries had to be depended upon for planting material. With
these changed conditions the native trees of a region became less dominant
in the city planting and were largely replaced by those trees listed in
nursery catalogues which took the fancy of each property owner along the
street. (Fig. 2.) The quickest growing trees were considered first, and
as some of these made a big showing the first few years and were easily
transplanted, they have become the dominating trees in street planting
from the Atlantic to the Pacific and from the Great Lakes to the Gulf of
Mexico. (Figs. 2, 10, and 13.) A few have planted better and more lasting
trees (figs. 1, 3, and 4); but the tree growth on the streets of the
average town or city is ragged and unkempt in appearance, while that of
the suburb or small village is not much better unless the planting has
been done under municipal control and the plantings on a street have been
confined to a single kind of tree.
[Illustration: P15311HP
Fig. 1.--An oak-shaded street in the South. Willow oaks in Birmingham,
Ala., in late summer.]
[Illustration: P18826HP
Fig. 2.--A street with mixed plantings. The trees are of different
kinds, some unsuited for the purpose, planted at varying distances
apart, according to the inclination of the property holders. A street in
Stockton, Calif., photographed in early summer.]
[Illustration: P12515HP
Fig. 3.--American elms on a city street in midsummer. All these trees were
planted at one time at uniform distances apart by the Commissioners of
Washington, D. C.]
The advent of such civilizing agencies as the telegraph, the telephone,
the electric light, and the trolley car have added each its share toward
the mutilation or destruction of the good trees that were in existence
at the time of their coming. Faulty methods of pruning also have caused
much disfigurement and ruin. (Figs. 5 and 21.) To this mutilation has been
added the unnecessary destruction of many trees in centers of business
(fig. 6), because they excluded a little daylight, or made a store
less prominent, or were somewhat in the way of using the sidewalk for
merchandise.
In spite of all these troubles tree planting has continued because people
love trees, enjoy well-shaded streets, and are willing to make efforts to
get them. The trees on well-shaded streets are not only pleasing, but also
contribute toward the health of the community by transpiring moisture into
the atmosphere and by producing a restful effect on eyes and nerves. Red,
especially, is known to have an exciting effect on human beings, and where
city streets are well-shaded it makes less prominent those colors that
might otherwise prevail and offend.
[Illustration: Fig. 4.--Trees 18 years old on adjacent streets: _A_, Pin
oaks; _B_, ginkgos; _C_, Norway maples. Note the differences in size.]
Good shade is so appreciated that its presence adds a value to adjoining
properties. Real-estate men recognize this factor and plant shade trees
as early as practicable on land which they develop. That the beauty of a
city is improved by good street trees is becoming recognized more and more
and is finding expression in the desire of garden clubs, civic improvement
associations, and boards of trade for information on this subject.
Success in planting street trees can be attained only by planning and
controlling the planting as a whole, by selecting the most suitable
varieties, by securing trees in the best condition and planting them
properly, and by giving the necessary later care.
While towns were small, conditions for tree growth favorable, and
woodlands plenty, so that native trees were easily obtained and started,
the practice of each householder planting his own trees as he saw fit gave
good results. As towns became larger and impervious pavements took the
place of earth roads, the conditions for tree growth became more severe
and the results from the individual planting of trees less uniform. In
large cities the conditions to be met are so extreme that it has become
practically impossible for the average householder to grow street trees
successfully, or to do so only at excessive cost. Then, too, a lineman
in a few minutes often undoes what the individual has achieved with care
and years of patient waiting (see fig. 5). The trees and the lines are
both needed by the public, but when provided by individual initiative
at private expense, but trimmed for the benefit of electric lines by
employees of corporations intent on maintaining service at the least cost,
the trees suffer unduly.
[Illustration: P16692HP
Fig. 5.--A tree mutilated by linemen. An otherwise beautiful red oak in
Louisville, Ky., as it appeared in midsummer.]
In order to have good shade trees at a reasonable cost which receive
timely and efficient attention, with the effective control of wire lines,
the care of the trees needs to be vested in some adequate authority.
PUBLIC CONTROL OF STREET TREES.
Providing shade on city streets is as much a municipal function as
providing lights or sidewalks and should, therefore, be cared for by
public officials. All street trees should be directly under the care of
duly appointed officers, who should be responsible for their planting
and care, as well as for their pruning or removal. Negative control by
requiring permits for planting, pruning, and removal is little better than
no control.
[Illustration: P16986HP
Fig. 6.--A desert of asphalt in the business center of a city having less
than 100,000 population.]
The officials in charge should have the necessary authority and should be
required to initiate and carry forward planting and all other needed work
connected with the establishment and maintenance of street trees. Probably
the most satisfactory way of securing supervision is through an unpaid
commission of three or five members, which in turn employs an executive
officer. In a small place a commission of three persons may be best, one
being appointed every 2 years for a 6-year term. In large places five
members may be better, and the ideal term would be 10 years. A compromise
would be a 5-year term, a new member being appointed each year. The great
need of long-term appointees is that it takes two or three years for a
member of such a board or commission to see and realize the things needed
to be done and the policies that should be carried out. Because it takes a
long time to get results in growing street trees, the policies should be
as nearly continuous as possible and the terms of the members long enough
to insure a majority of experienced persons on the board at all times.
The method of appointing the commissioners is not so important as that
each shall be selected from the territory as a whole rather than from a
part of it. In some places where the term of service is 10 years, each
one's successor is appointed by the remaining commissioners, subject to
confirmation by the court. Where this is done a member is not permitted to
succeed himself. In other places the commission is appointed by the court;
in others, it is elected by the city legislative body or is appointed by
the mayor subject to the approval of the legislative body. The important
point is to keep the administration as nearly as possible on a purely
business basis.
A good board can accomplish nothing without liberal funds. There are two
methods of providing these: (1) By an appropriation from the general tax
levy and (2) by direct assessment against the properties, collectible
with the other taxes. If the funds are provided by appropriation, a
fixed minimum, expressed in millage of the tax rate, should be provided
in the organization of the commission. This minimum should be such that
a fair amount of maintenance work can be done when no other funds are
available. Councils that appropriate money sometimes hamper boards by
withholding appropriations. Work of the nature of tree planting should
not be permitted to suffer or be lost by a year's neglect. The fund
provided by this minimum amount should not be so large that regular
additional appropriations will not be needed to carry on the work
properly, as this will give a desirable point of contact of the commission
or board with the ordinary channels of expressing public sentiment in
the district interested. The minimum appropriation mandatory should be
sufficient to prevent injury from lack of care of work already begun.
A period of minimum care and attention while a board and the people or
their representatives are coming to a new understanding of one another's
position is not necessarily a detriment, provided a reasonable maintenance
has been possible in the interim, but without such care the results are
ruinous and work would better not be started than be undertaken with the
possibility of such a period of neglect occurring.
It is probably desirable to assess the cost of tree planting against
the adjacent property owners at a proportional cost per front foot and
to provide for maintenance out of a general fund. Boulevards and other
unusual developments are sometimes maintained with satisfactory results by
regular assessments against the abutting properties.
After a proper governing board is provided, the securing of a competent
executive is a matter of ordinary business procedure. It is usually
desirable that he shall be not only a good executive but also a man with
a knowledge of trees and trained in their care, so that he may be a
competent adviser of the board as well as its executive.
[Illustration: P18857HP
Fig. 7.--Increased attractiveness due to trees on a city street, as shown
by contrasting the two sides of the thoroughfare. If trees like red oaks,
American elms, or the Eucalyptus in the distance had been used, the effect
on this wide street would have been comparable to figure 3. The trees in
the left foreground are umbrella trees. Merced, Calif.; midsummer.]
PLANNING FOR TREES ON CITY STREETS.
With the help of one who knows trees and the local conditions to be
met, the town should be studied and a suitable kind of tree selected
for each street or for a large portion of a street (figs. 1 and 3),
and as conditions warrant the plan should be carried out as outlined.
Mixed plantings of different sorts of trees (figs. 2 and 7) are not as
pleasing and effective as the use of a single species for considerable
distances. The use of only one or two kinds for a whole town is likely to
be monotonous, and it is also undesirable because the variety most used
may become subject to serious disease or insect attacks. The species and
varieties of trees suitable for city planting are few enough, if all are
used, so an endeavor should be made to include as many different kinds as
practicable, assigning one variety for a long stretch of street unless
there is a marked change in its character, in which case a change of trees
would be warranted.
Where trees are already on a street, the problem of planning for the
future is frequently much complicated, especially if there are several
kinds in good condition. Where there is but one good kind, gaps can be
replanted with young trees of that sort. If there are poor trees of a good
variety or trees of a poor or short-lived variety it would be advisable
to remove these and do all the replanting at one time, so as to have the
trees on the street as nearly uniform as possible. Where there are several
good varieties in good condition the sensible thing is to care for the
trees that are there and then, after careful study, decide on one variety
for all future plantings on that street.
[Illustration: P14631HP
Fig. 8.--Trees 20 feet apart that should be at least 60 feet apart.
Sycamores in Washington, D. C., as they appear in late winter.]
SPACING TREES.
A common fault in all street planting is to put the trees too near
together. (Fig. 8.) This is more evident where the work has been done by
the abutting property owners than by municipalities.
After trees are started and have attained some size it is extremely
difficult to get them removed, even where the good of the remaining trees
demands it. The removal of a fairly good tree merely because it is short
lived in order to make room for a good one that will be permanent does not
appeal to the average citizen. Where trees which have been planted by the
property holder come under city control a strong feeling of proprietorship
still remains, which is outraged by the suggestion of the removal of even
poor trees. Where all the work is under city control good work is often
hampered by a strong public sentiment against the removal of trees, even
though they are poor or crowding.
Because of this difficulty it is extremely important that young trees be
planted farther apart than at that time seems reasonable. If they are
planted as far apart as is proper for mature trees the distance will be
so great as to make planting seem a joke. If they are planted half the
distance apart they should be when mature, good results would follow if
the intermediate trees were removed when they nearly touch those to be
left. As the intermediate trees would probably not be removed, or not
until too late for the good of the remaining ones, planting had better
be sufficiently far apart in the beginning to avoid the necessity of
later removals. In the beginning the trees will be too far apart and when
mature too close together, but it seems to be the alternative imposed by a
misguided public opinion.
There is scarcely a community that would permit the removal of
interplanted trees from a street of fine elms, oaks, or other worthy
varieties without a protest that would be the almost sure political
death of the administrative authorities responsible, no matter how great
the need or how much expert support they might have. If short-lived
intermediate trees were used they would not be likely to be taken out
before they died, and they probably would not die before they had
irreparably injured the permanent trees. The removal of surplus or
interplanted trees can be made with least shock to the community by
gradually narrowing the tree tops by severe pruning from year to year on
the sides next the permanent trees until finally they are so narrow they
may be removed and leave only small openings between the permanent trees.
Even this method will not materially lessen the public protest at the
final removal.
A common practice is to set street trees 35 feet apart. If it were
practicable to remove one-half the trees at the proper time this would be
a good distance, but in the eastern half of the United States and on the
Pacific slope 50 feet apart is close enough for most varieties, and for
the larger growing trees 60 to 70 feet would be better.
CONDITIONS FOR TREE GROWTH.
In order to grow, trees must have a soil of suitable texture, in proper
mechanical condition, that contains sufficient available mineral elements
and plenty of organic matter, and, last but not least, a constant supply
of moisture and air. In addition to these there must be in active growth
in the soil many forms of organic life that are in various ways preparing
the material in the soil for the use of the larger plants. Not only must
these things be present, but others that are deleterious must be absent,
whether the substance is hurtful in itself or whether it is an excess of
one that is otherwise beneficial.
Above the soil three things must be present--air, sunlight, and
moisture--and, as in the soil, harmful things must be absent in order to
have success. Among the deleterious substances are sulphur and other fumes
and soot and other products from incomplete combustion.
[Illustration: P14633HP
Fig. 9.--The irregularity in the size of the trees shown is due to a
part of the first planting having been killed by illuminating gas from
defective pipes. Norway maples as seen in Washington, D. C., in late
winter.]
Some of the more obvious things with which a city tree has to contend are:
Water-tight pavements, both on the sidewalk and street, that prevent the
admission of air and water; the removal of the topsoil in street grading,
thus forcing the tree to exist on the good soil provided in the hole;
careless digging near the tree for gas, water, and electric service, and
especially for the placing of curb-stones; the saturation of the soil with
illuminating or sewer gas from defective pipes (fig. 9); the pouring of
salt water from ice-cream freezers into gutters, where it may find its way
into the soil near tree roots; the gnawing of the trunks by horses; and
the cutting of the tops by linemen and tree trimmers.
Because of the uncongenial conditions for the growth of trees on city
streets comparatively few kinds are satisfactory for such use. Among
those available are some that will grow under extremely trying conditions.
Kinds can be found that will thrive wherever it is suitable for human
beings to live. If it is impossible to grow trees on a street, as a health
measure that street should be closed for human use until conditions are so
improved that it will support trees.
[Illustration: P15298HP
Fig. 10.--A business center relieved by a parking with Carolina poplars.
Macon, Ga.; late summer.]
More kinds will thrive under suburban conditions where only a small
portion of the roadway is covered by an impervious coating, where the
parking spaces are liberal, and where the street is lined with open lawns
than under the conditions in a city, where the street is covered with a
water and air proof coating and the sidewalks with an impervious material,
where parking spaces are limited, and where adjoining lawn areas are small
or lacking. By a careful selection of kinds, all conditions in a city can
be met. In some places bad conditions could have been improved greatly by
a little forethought; in others, such conditions can be bettered. These
details, like many other matters connected with city planning, have been
ignored, but should be considered immediately, especially by villages and
small cities. Figure 6 shows how an opportunity for creating a beauty spot
has been lost sight of, while figure 10 shows how a city has utilized less
ground to increase the comfort and attractiveness of its business center.
[Illustration: P15278HP
Fig. 11.--A street well proportioned in width of roadway, sidewalk, and
parkings, with willow oaks on the left, American elms on the right, and
young Carolina poplars near the roadway that should be removed. Columbus,
Ga.; midsummer.]
[Illustration: P14359HP
Fig. 12.--A street with too much pavement and too little parking space.
Carolina poplars in Baltimore, Md., in midautumn.]
A common mistake in ambitious young cities and many old ones is to pave
more of the width of the street for traffic purposes than, is likely ever
to be needed. By reducing the roadway and throwing the remainder into
liberal parking spaces much is added to the attractiveness and comfort
of a city. A contrast in the two methods of treatment is illustrated
in figures 11 and 12. The recommendation that the roadway prepared for
travel be made narrow is not to be interpreted as a reason for lessening
the area dedicated to the public use; in fact, in most cities, especially
in the northeastern quarter of the United States, too little space has
been reserved from houseline to houseline (fig. 13). By reserving more
room between the houses and the street for use as lawns and gardens the
conditions would be made more livable, opportunity would be offered for
widening the public way without prohibitive expense if traffic or business
demanded it, and the growing of street trees would cease to be a serious
problem.
[Illustration: P16842HP
Fig. 13.--A street with too little room from houseline to houseline. Note
the more attractive appearance of the side with trees. Norway and silver
maples in Frederick, Md., in midsummer.]
KINDS OF TREES SUITABLE FOR CITY STREETS.
QUALITIES NECESSARY.
Compared with the whole number of trees used for ornamental planting, the
number of kinds suitable for street planting is very small. For use under
city conditions a tree must be adapted to the climate and to the soil upon
which it is to be grown. It must have healthy foliage that withstands
dust and smoke and a root system not easily affected by unusual soil
conditions, by restricted feeding areas, or by root pruning when street
improvements are made. The top should be in proportion to the width of
the street upon which it is used, and it should be rather high headed or
easily trained to that form and of open growth without being too spreading
or sprawling.
Of minor consideration is the character of the foliage masses, whether
dark or light, heavy and somber or open and airy, and also whether they
have vivid autumn colorings. Only in the most southern parts of the
country and in western California should evergreen trees be considered
for street planting, and then only the broad-leaved evergreens, such as
magnolias and live oaks. In the North the lack of sunshine during the
short cloudy days of winter makes it desirable to admit all the light
possible. Even in the South the question of sunshine should be considered
when selecting varieties.
[Illustration: P12536HP
Fig. 14.--Narrow upright trees (Lombardy poplars) on a Barrow Conditions,
it is better street. Washington, D. C.; midsummer.]
Narrow streets should be planted with columnar trees (fig. 14) or
sometimes with small trees. Broad streets may be planted with spreading
trees (figs. 3 and 16), or, if provided with a central parking space, with
moderate-sized trees in the center and on the sides, or with trees on the
sides suited to the space and formal trees in the center. (Fig. 15.)
[Illustration: P18856HP
Fig. 15.--Formal trees in a central parking, but appropriate trees wanting
on the sides of the street. Canary Island date palms in Merced, Calif.;
midsummer.]
As a rule, trees native to the locality that have been successfully grown
in other cities should be given the preference. When a choice must be made
between untried native trees and those tested in a city or town under
different soil or climatic to give the native trees the first trial. There
are many native trees that are promising which have not been planted on a
sufficient scale or under sufficiently varied conditions to demonstrate
their real value for street planting over any considerable area. Many of
the trees mentioned in this bulletin may prove valuable far beyond the
areas for which they are suggested. The burr oak, the swamp white oak, the
scarlet oak, the chestnut oak, the white oak, the sour gum, and others may
be found on further trial to be as valuable as those already demonstrated
to be valuable over large areas. Those mentioned have all been tested in a
small way.
Caution should be used in selecting trees with conspicuous flowers and
those with edible fruits or nuts, as in many parts of the country such
trees are badly mutilated by the public. Even horse-chestnuts, although
the nuts are not edible, are often broken by boys clubbing the trees.
That public opinion can prevent such vandalism is in evidence all along
the Pacific coast and at a few places in the East. Every effort should be
made to create a sentiment that will protect these attractive additions to
street adornment, but where the sentiment does not exist it is better to
avoid the planting of such trees except in a limited way.
[Illustration: P15394HP
Fig. 16.--Live oaks, the handsomest southern street tree for broad street:
Biloxi, Miss.; late summer.]
Besides the native trees there are many introduced trees that have proved
valuable and many more that are worthy of trial.[1] A fair trial of
promising introduced trees should be made, and the native kinds should be
thoroughly tested.
[1] As examples of this are a number of new elms such as _Ulmus
pumila_ and _Ulmus densa_, besides lindens, poplars, and _Koelreuteria
paniculata_, while _Pistacia chinensis_ is suitable for warm regions.
The Office of Foreign Seed and Plant Introduction of the United States
Department of Agriculture will be glad at any time to suggest new trees
that are promising for any region.
TREES FOR DIFFERENT REGIONS.
To simplify the discussion of kinds of street trees likely to prove
satisfactory, the United States has been arbitrarily divided into the
regions shown in figure 17. An endeavor has been made to make each
division cover an area having similar growing conditions, so that the
trees suggested will be likely to thrive in all its parts. A discussion
of the strong and weak points of the different kinds will be found with
the description of the kinds farther on in this bulletin.
_Region 1._--Region 1 comprises the mild humid portion of the northern
Pacific coast east to the Cascade Mountains, including the western
third of Washington and Oregon and a portion of northern California.
The trees native to western Europe are adapted to this region, as the
climatic conditions are quite comparable. Most of our American trees
also succeed here.
Some of the desirable varieties for street planting in region 1 are
the Oregon, Norway, sycamore, and sugar maples; California walnut;
tulip; European linden; basswood; sycamore; London plane; white and
European ashes; English and American elms; English, red, and pin oaks;
ginkgo; and the black locust.
[Illustration: Fig. 17.--Outline map of the United States, showing the
regions within which essentially similar conditions for tree growth exist.]
_Region 2._--Region 2 is that portion of California lying between
the Sacramento and San Joaquin Valleys and the Pacific Ocean. Many
varieties of trees will succeed here if given water. Because of the
lack of water, unless specially irrigated the more drought-resistant
species should be used.
Among the deciduous trees useful for this region are the London plane;
the California and common sycamore; English, Huntingdon, and American
elms; Oregon, Norway, sycamore, and English maples; white, green, and
European ashes; red, English, and pin oaks; European linden; basswood;
California walnut; honey and black locusts; horse-chestnut; Albizzia;
and the Japanese varnish tree, or Sterculia.
Evergreen trees which will probably be successful in region 2 are the
Eucalyptus[2] in variety, acacias, rubber, magnolia, California live
oak, Victorian and poplar-leaved bottle trees, and in the southern
portions the California pepper, silk oak, and jacaranda. Palms are
much planted, but they do not make good street trees except where a
formal effect instead of shade is desired.
[2] Some cities have ordinances against the planting of certain trees
because their roots sometimes obstruct sewers. Among these trees are the
Eucalyptus in California and some of the poplars in several of the States.
_Region 3._--Region 3 comprises the Sacramento and San Joaquin Valleys.
The deciduous trees for this region are the California walnut; London
plane; California and common sycamores; Oregon, Norway, and sycamore
maples; white, European, and green ashes; red, English, valley, and
pin oaks; European linden; basswood; English and Huntingdon elms;
honey locust; and horse-chestnut. Chinaberries and Texas umbrellas are
much planted in these valleys, but are not good street trees. Olives
and palms are suitable only for formal effects, while eucalypti are
satisfactory but are liable to make trouble with defective sewers.
Acacias grow especially well in this region except in the extreme
north.
_Region 4._--Region 4 includes the country from the Sacramento and San
Joaquin Valleys to the crest of the Sierra Nevada Mountains. It varies
in elevation and correspondingly in temperature and the amount of
available moisture.
Where there is sufficient moisture, the deciduous trees recommended
for region 3, except the valley oak and possibly the California
sycamore, may be used. Where there is less moisture the thornless
honey locust, black locust, green ash, hackberry, poplars, ash-leaved
maple, and the American elm if it can be watered the first few years
may be planted. In the warmer sections the chinaberry and Texas
umbrella may be used.
_Region 5._--Region 5 comprises the hot semiarid country of southern
California and southwestern Arizona which is dependent on irrigation.
The best deciduous trees for this region are those suggested for the
drier portions of region 4. With ample irrigation the deciduous trees
recommended for region 3 might grow.
Among the evergreens the Texas palmetto, Parkinsonia, and the
Washingtonia and some other palms can be used where other trees do
not succeed. The red and desert gums may be used also in the drier
regions. With ample irrigation the evergreens suggested for region 2
should succeed.
_Region 6._--Region 6 comprises the intermountain section and extends
from the crest of the Cascade and Sierra Nevada Mountains eastward to
the eastern base of the Rocky Mountains. The region includes great
variations in growing conditions, often in very short distances. As a
whole it is semiarid, and in most places trees can hardly be expected
to thrive without more or less irrigation, although in some of the
mountain valleys and on some of the mountain slopes almost ideal
conditions for tree growth exist.
In the drier parts of the region only those deciduous trees that are
weeds under more congenial conditions can be grown. Those that can
be planted with the greatest hope of success are the thornless honey
locust, black locust, green ash, hackberry, and where the others do
not succeed, the poplars and ash-leaved maple. If it can be watered
for a few years the American elm usually can be grown, and in the
southern half of the region the Mississippi hackberry will probably
succeed. Near the southern border, on lower elevations, the chinaberry
and Texas umbrella can also be planted. In the locations most favored
naturally or where irrigation is possible, the trees suggested for
region 9 can be used.
Evergreens that may be used for the drier portions of the southern
part of region 6 are the Parkinsonia and the Texas palmetto.
Native trees may be found that will prove of greater value for limited
areas than any suggested. Cities and towns contemplating street tree
planting would do well to consult the nearest State agricultural
experiment station or the United States Department of Agriculture if
it is thought possible that something better has been found than the
trees suggested.
_Region 7._--Region 7 is the northern part of the Great Plains area
from the foot of the Rocky Mountains at about the 5,000-foot contour
line east to the ninety-eighth meridian. It is rather uniform in
general conditions, the character of soil having no wide divergence
and the elevation increasing gradually from south to north and east
to west. The rainfall gradually increases from west to east until at
about the ninety-eighth meridian the conditions are more favorable for
tree growth.
The trees to be relied on are the thornless honey locust, common
hackberry, black locust, green ash, ash-leaved maple, the poplars,
the Chinese elm, and the American elm if it can be watered the first
few years after transplanting. The mossy-cup oak is another tree
worth testing in a small way, as it is native a little east of the
ninety-eighth meridian. The basswood and Norway maple would probably
succeed if supplied with plenty of water.
_Region 8._--Region 8 is the southern part of the Great Plains.
In addition to the deciduous trees recommended for the northern Great
Plains (region 7) the Mississippi hackberry, Texas umbrella, and
chinaberry may be successfully grown.
Evergreen trees that may be used in region 8 are the Texas palmetto
and Parkinsonia.
_Region 9._--Region 9 is the upper Mississippi Valley, including
the area from that already considered to Lake Michigan and south to
southern Kansas. It is more favorable to tree growth than regions 6
and 7.
Trees which will succeed here are the American elm; red, pin,
mossy-cup, and other native oaks; white ash; sycamore; basswood; and
Norway and sugar maples.
_Region 10._--Region 10 includes the northeastern part of the country
from eastern Illinois to the Atlantic Ocean, and extends southward
through the Appalachian Mountains. It is most favorable for tree
growth.
The best trees for street planting in region 10 are the red and pin
oaks, London plane, sycamore, the staminate form of the ginkgo,
basswood, tulip, Norway maple, white ash, thornless honey locust,
American elm, and in the southern portion of the region on light land
the sweet gum. The red and sugar maples are among the best trees for
suburban conditions. The hackberry will grow, but should be discarded
in favor of better varieties. The mossy-cup and chestnut oaks are
worthy of trial on gravelly soils in the suburbs.
_Region 11._--Region 11 includes the lower Mississippi Valley and the
country east of the southern Appalachian Mountains, extending from the
light lands near the South Atlantic and Gulf coasts to the northern
limits of the distinctively southern flora.
The typical street trees of this region are the willow oaks (fig. 1)
and water oaks, the former a valuable street tree, the latter good
when young but comparatively short lived, with no advantages over the
willow oak. Other good trees are the red, Spanish, laurel, Darlington,
and pin oaks, tulip, sweet gum, American elm, red and Norway maples,
and the ginkgo.
_Region 12._--Region 12 is the land near the coast from Wilmington, N.
C., to the Mexican border, exclusive of the southern part of Florida.
Good deciduous trees for this region are the willow, laurel,
Darlington, and Spanish oaks, tulip, sweet gum, sycamore, London
plane, American elm, and the staminate form of the ginkgo. The honey
locust, red or scarlet maple, Norway maple, and the hackberries are
not so good.
The live oak is the characteristic tree of region 12 (fig. 16) and is
the pride of the cities that have used it. Even though an evergreen,
it is an excellent street tree, as it is large, spreading, and open.
The palmetto and palms thrive and may be used for producing formal
effects. The evergreen magnolia is a good broad-leaved evergreen.
_Region 13._--Region 13 consists of the southern part of Florida. The
deciduous trees suitable for this section are the willow, Spanish, and
southern red oaks; American elm; Mississippi hackberry; and in the
southern half of the region the Poinciana.
Evergreen trees are better suited to region 13 than to, any other
portion of the United States except possibly southern California.
Among the best are the live and laurel oaks, evergreen magnolia,
camphor, rubber, silk oak, or grevillea, and casuarina. Eucalypti are
planted to some extent in Florida, but the climate is such that only
on the drier grounds of the interior are they likely to succeed, and
even there they are not to be compared with other excellent species of
trees that may be cultivated successfully.
TREES FOR SPECIAL PURPOSES.
In the heart of a city, where the greatest difficulty is experienced in
getting trees to grow, the ailanthus will probably thrive when nearly all
other kinds fail. The sycamore and the London plane are also good for such
places. The Carolina poplar will frequently grow under these conditions,
and its use may sometimes be warranted.
For very narrow streets the Lombardy poplar is the best tree. (Fig. 14.)
Trees suitable for use within the reach of ocean spray or on sandy lands
near the coast are the red oak and the red or scarlet maple south to
Charleston, S. C., while the sweet gum and the live oak are equally good
from Norfolk southward and along the Gulf of Mexico. The red oak, sweet
gum, red maple, and eastern live oak are all grown successfully along
the Pacific Ocean, while the California live oak can be used from San
Francisco southward. The trees that endure the most alkali appear to be
the bladder-nut tree,[3] London plane, peppermint gum,[4] blue gum,[5]
the Washingtonia and other hardy fan palms, Canary Island date palm, the
camphor tree, and _Acacia cyclops_ and _Acacia retinodes_. Only the first
two withstand severe freezing weather. The red oak and the red maple are
worth testing for these conditions.
[3] _Koelreuteria paniculata_.
[4] _Eucalyptus amygdalina_ Labill.
[5] _Eucalyptus amygdalina angustifolia_.
DESCRIPTIONS OF STREET TREES.
ACACIA.
The acacias, or wattles, are a large group mostly of small trees with
showy yellow flowers. Although much used in California, many of them are
too small to make satisfactory shade trees, and because of shallow rooting
they are injurious to sidewalks. They also stump-sprout badly. They thrive
in regions 2 and 3 and in restricted portions of regions 1 and 5.
The Australian blackwood,[6] blackwood acacia, or wattle, is a strong,
upright tree, growing to a height of 75 feet and forming a well-shaped
head. It is badly affected by citrus scale, and on this account its
planting is sometimes prohibited.
[6] _Acacia melanoxylon_ R. Br.
The black wattle[7] is a strong-growing round-headed tree that reaches a
height of 40 feet and has dark-green leaves.
[7] _Acacia decurrens mollis_ Lindl.
The green wattle[8] is a rapid-growing tree, reaching a height of 60 feet
and forming a round head with finely cut leaves.
[8] _Acacia decurrens_ Willd.
The silver wattle[9] is much like the black wattle except that its leaves
and young branches are covered with a whitish down.
[9] _Acacia decurrens dealbata_ F. Muell.
AILANTHUS.
The ailanthus,[10] or tree of heaven, is a tall, broad, handsome tree that
is especially valuable in the heart of closely built or smoky cities.
The staminate and pistillate flowers are borne on separate trees. Only
the pistillate trees should be used, as the odor of the blossoms of the
staminate ones is very objectionable for about 10 days in late spring.
These may be produced by grafting from pistillate trees or by propagating
from suckers or root cuttings from such trees if they have not been
grafted. The ailanthus may not succeed in regions 5 and 13.
[10] _Ailanthus altissima_ (Mill.) Swingle (_A. glandulosa_ Desf.).
ASH.
There are three kinds of ash trees that are useful for street planting.
The white ash[11] is a large oval-headed tree, reasonably satisfactory
on rich lands in regions 1, 2, 3, 4, 9, 10, 11, and 12, but it is better
adapted to suburban than urban conditions.
[11] _Fraxinus americana_ L.
The green ash[12] is one of the few successful trees in regions 6, 7, and
8 and may succeed in region 5. It grows well throughout the remainder of
the United States, but is of less value than other trees there. It is much
smaller than the white ash, with a broad round top.
[12] _Fraxinus lanceolata_ Borck.
The European ash[13] is a large, handsome, round-headed tree suited to
regions 1, 2, 3, and 4.
[13] _Fraxinus excelsior_ L.
CAMPHOR.
The camphor tree[14] is a large, handsome, oval-headed evergreen that will
succeed in the southern half of region 2, in regions 3, 5, and 13, and in
the warmer parts of region 12. It will endure more frost than the orange,
and where it is successfully grown it is deservedly popular.
[14] _Cinnamomum camphora_ (L.) Nees and Eberm.
CHINABERRY.
The chinaberry,[15] sometimes known as the China tree, is a small,
round-headed, short-lived tree that will grow in regions 2, 3, 5, 8, 11,
12, and 13 and near the southern edge of region 6. It is too short lived
to be considered for planting where other trees will grow.
[15] _Melia azedarach_ L .
The umbrella tree,[16] or Texas umbrella, is a small, compact form of the
chinaberry with an umbrella-shaped top. It is useful for formal effects,
as in the parking on a wide street where taller trees are used on the
side. It will grow in regions 2, 3, 5, 8, 11, 12, and 13 and in the
southern parts of region 6.
[16] _Melia azedarach umbraculiformis_ Berckmans and Bailey.
ELM.
The elms are large, handsome shade trees suitable for use over a wide
range of territory.
[Illustration: P12460HP
Fig. 18.--An American elm with crotches liable to be split by heavy winds.
Note the supporting chains.]
The American elm[17] sometimes called the white elm and water elm, is
one of the handsomest American shade trees. (Fig. 3.) It has been the
standard street tree of New England, giving to the roadsides and village
streets the characteristic appearance which is so attractive to summer
visitors.
[17] _Ulmus americana_
The American elm is tall and spreading, and where planted as near together
as is customary on streets and country roads the effect of the mature
trees is that of an arch formed by the growing together of their spreading
tops. It is of rapid growth and long lived.
This elm drops its leaves very early in the fall, but it comes into leaf
early in the spring. Because of its manner of branching it is especially
liable to be split by heavy winds. This trouble may be lessened by
selecting and planting specimens with a close, compact habit of growth or
possibly also by great care in training young trees. Two limbs separating
from one another by a very small angle, that is, when they start to grow
in nearly the same direction, make a crotch that is liable to split. (Fig.
18.) Where two limbs separate at nearly a right angle or where three or
more limbs of about equal size grow from a common point or very nearly so,
the crotch is likely to be much stronger. Careful pruning and training to
provide a proper system of branches may be especially helpful with this
elm.
Because of the attacks of the elm leaf-beetle[18] and the European elm
bark louse,[19] many handsome trees have been severely damaged or killed
before communities were properly equipped for fighting them, for with
careful spraying these insects may be kept in check. However, on account
of the existence of these pests and because they are gradually spreading
to new territory, tree planters should consider carefully whether it
is advisable to plant the elm in their localities. Where there is no
danger from these insects, this elm is one of the best of street trees.
Consultation with the nearest State agricultural experiment station or
with the Entomologist of the United States Department of Agriculture would
be advisable in order to determine this point.
[18] _Galcrucella luteola_ Mull.
[19] _Gossyparia spuria_ Mod. (Data regarding both insects furnished by
the Bureau of Entomology.)
The best specimens are to be found in the northern part of region 10,
although the elm is being grown all over the United States and is proving
a valuable street tree even in towns and villages of regions where the
rainfall is as low as 15 inches. It is not recommended for planting in
regions 3 and 5.
The English elm[20] is a tall, oval-headed, compact, handsome tree with
leaves smaller than the American elm and which stay on much later in the
fall. In regions 1 and 2 it is at its best, in the former equaling the
American elm and in the latter excelling it. It also thrives in regions 3
and 10 and in the eastern part of region 11.
[20] _Ulmus campestris_ L.
The Huntingdon elm[21] is a comparatively round-headed European variety.
[21] _Ulmus hollandica vegeta_ (Lindl.) Rend.
It is a large, handsome tree with good foliage and is more compact in
growth than the American elm. It succeeds well in regions 1, 2, 3, and 4.
The wahoo, or winged elm,[22] is native to the South Atlantic and Gulf
States near the ocean. It has larger leaves than the American elm and is
not as spreading in its growth, but it succeeds well on city streets in
regions 11, 12, and 13.
[22] _Ulmus alata_ Michx.
EUCALYPTUS.
There are a large number of species of Eucalyptus, many of which can be
used for street planting in regions 2, 3, and 5. Some cities prohibit
their planting because their roots are liable to penetrate defective
sewers, and in other cities they must be kept at least 70 feet from a
sewer, though even this distance may not prove permanently effective.