_Heat._—Where the conditions of soil and moisture are favorable the
almond will endure the intense heat of the interior valleys and even of
the Imperial Valley, provided it is pruned properly to shade the main
branches so as to prevent sunburn. Where trees, by severe pruning, are
opened up suddenly to the intense heat of the summer sun, almonds will
sunburn, but if the necessary opening up is done gradually, the bark will
become inured to the new conditions without danger. The nuts grow and
ripen more satisfactorily in the greater heat of the interior than along
the coast.
_Frost._—The almond tree is hardy and will endure fully as much cold
as the hardiest peach without injury. Trees are found growing well in
Illinois, Ohio, New York and other Eastern states. In very favorable
seasons they may even bear fruit, though this happens very seldom, due to
the extremely early habit of blooming before the spring frosts are over.
The first warm weather seems to start the trees into bloom, especially
where the enforced dormant season of winter is very long.
The blossoms, on the other hand, are very tender. There is a great range
in the degree of frost which will cause injury, depending largely on the
condition of the tree during the time that the fruit buds are forming and
developing, as well as on the duration and severity of the frost. Buds
and blossoms on trees which have been forced into premature dormancy,
either by lack of moisture or by severe attacks of red spider, are much
more susceptible to frost than those on trees which have continued
growth late enough in the fall to provide for the proper development and
maturity of the buds. After differentiation of fruit buds commences in
the summer, the almond leaves should remain on the tree until late into
the fall in order to strengthen and develop the fruit buds and store up
the elaborated food material for the use of the buds in their normal
development through the winter. Studies of almond buds gathered from
healthy trees which held their leaves until late fall frosts at Davis,
showed the first evidence of differentiation between fruit and flower
buds commencing about August 18, while the flower was not completely
developed until February 18 following. During the intervening time
development proceeded unchecked through the winter even though the tree
was apparently dormant. During the time the crop is ripening on the
trees, little is done toward storing food material for the buds. If the
leaves turn yellow or drop soon after harvest, the trees do not have the
opportunity of storing a sufficient supply of plant food for their normal
requirements and the buds are insufficiently nourished during the winter
period. The resulting buds are weakened and the indications are that they
are unable to endure unfavorable climatic conditions in the spring, such
as light frosts, continued cold weather or sudden changes from warm to
cold weather.
The most tender stage in the blossoming and development of the young
fruit seems to be immediately following the dropping of the calyx lobes
from the young fruit as it first commences to swell rapidly. The blossom
becomes more and more tender as it opens out and reaches the above stage.
After the young fruit has attained the size of a pea it rapidly becomes
more resistant to low temperatures. Blossoms with the petals exposed but
not yet opened have been known to stand temperatures of 24 degrees F.
and blossoms with petals beginning to fall have stood 28 degrees F. No
records are available as to the duration of these temperatures. In other
cases, blossoms with the petals falling have been killed by temperatures
of 30 and 31 degrees F. It must be remembered in this connection that
the almond blooms earlier than other orchard fruits and, therefore, is
often subjected to much more severe frosts than occur during the blooming
period of the later fruits. The greatest injury is likely to occur when a
frost follows one or more days of warm weather. When the mean temperature
both day and night remains low, frosts that might otherwise kill the
flowers or setting fruit do no harm. This is what occurred in February,
1917, at the University Farm, when repeated frosts at blooming time did
no harm whatever.
In determining the desirability of a location in regard to its freedom
from frost, the possibility of adequate air drainage is an important
item. For this reason the lands along the lower foothills immediately
above the floor of the valleys are ordinarily much less subject to
frost—because the cold air is free to drain away to the lower levels.
Generally the lands along the banks of streams which have been built
up higher than the other lands of the Great Valleys through which they
flow, are less subject to frost by reason of the natural flow of the
cold air from them to the lower lands adjacent. For the same reason the
planting of almonds in the lower lands of the valleys, no matter how
large the valleys may be, should be avoided, unless the locality has been
thoroughly tested for a long period of years and has proved to be an
exception to the rule because of some peculiar situation with favoring
air currents or air drainage, such as might exist near a natural draw
in the hills where the settling of the cold air in some portions of the
adjacent valley might be prevented. Such locations are generally confined
to very small areas. Oftentimes an opening or draw in the hills may
serve as an outlet for the drainage of much colder air from considerably
higher elevations beyond, and then the danger from frost is very greatly
increased. This is very common where cañons act as drains to conduct the
cold air from the high Sierras to the valleys below.
Variable weather conditions, and especially as regards temperature in
the spring after growth commences, are highly undesirable. Warm weather
immediately followed by cold tends to produce sour-sap, fruit drop and
kindred physiological ills. Oftentimes crops have been lost where no
frosts occurred after blooming commenced, simply due to sudden changes
in the weather. However, it is highly probable that the greater portion
of the trouble with fruit dropping, when of the size of peas or larger,
is due to improper pollination. When the soil is not sufficiently well
drained at such a time, the sour-sap effect is greatly augmented.
_Humidity._—Foggy or moist weather during ripening or harvesting is
highly objectionable. The nuts do not dry out rapidly enough on the trees
to prevent the growth of molds and consequent darkening of the shells.
The nuts then require much heavier bleaching to brighten them properly
for the demands of the market. The damp weather prevents the rapid and
thorough drying-out of the kernel; the sulphur fumes are absorbed by the
moist kernel and it sometimes becomes rancid before it is six months old.
Much damp weather in the spring encourages the growth of “shot-hole”
fungus in the blossoms and fruit, often causing the loss of a
considerable portion of the crop; the loss of leaf surface from the
fungus infection is sometimes so great as to materially affect the vigor
and vitality of the tree.
_Rainfall._—It is impossible to state any definite amount of rainfall
which will or will not maintain the trees and enable them to bear regular
crops of nuts, for so much depends not only on the variation in rainfall
in different sections and in different years, as regards distribution
throughout the year, but also on the time and intensity of the fall,
the character of the weather following the rains and the ability of the
soil to receive and retain the rain that falls. Ordinarily, however,
with the above factors favorable, it is conceded that where the winter
rainfall averages sixteen inches, almonds can generally be grown without
supplementing the water supply by irrigation, if the orchardist exercises
reasonable care to conserve the moisture for the use of the trees. Where
the rainfall is inadequate some means of irrigation must be found to make
up the deficit.
In some sections the annual rainfall varies greatly from year to year.
Often it falls in such a way that a large proportion of it is lost in
the surface run-off. In many places the soil is so leachy that it is
incapable of holding sufficient water for the use of the trees throughout
the summer, much of the winter rainfall being lost in the underground
drainage. Under either of these conditions, 40 inches of rainfall might
not be sufficient. Very often winter rains are followed by desiccating
winds so that a considerable portion of the rain which falls is lost by
evaporation before anything can be done to hold it.
Continued rainy, damp and cold weather at the time of blooming is
apt to sour the pollen or actually wash it away and thus prevent the
fertilization of the blossoms, without which a crop is impossible.
Bees and other insects are the principal means of accomplishing the
pollination of almonds and such weather prevents them from working.