HYBRIDIZING
Working with nature to develop new varieties of trees is fascinating
although it requires infinite patience and study combined with skill and
concentration. A person without experience may taste of this pleasure,
however, by trying his hand at cross-pollination, and there is no end to
the number of hybrids possible.
In attempting to make crosses, one must necessarily understand the
botanical relationship between the trees to be crossed. Trees of the
same species cross readily in almost all cases; trees of the same genus
are not as easily crossed; trees belonging only to the same family are
usually difficult to cross. It is generally assumed that trees not in
the same family are impossible to hybridize. The plum serves as a
practical example of this. The American wild plum crosses readily with
almost any other plum and particularly well with the Japanese plum.
These crosses have resulted in such phenomenal fruit as the Underwood
plum, a cross made between species. If a cross were made between a
chestnut and a walnut, it would be between members of different
families. I recommend to anyone who is attempting to cross-pollinate for
the first time, that he limit his work to crosses made within species.
His chances of success will be greater and such success added to the
experience he is acquiring, will give him the background needed for more
difficult hybridizing.
Crosses made between filberts and hazels usually produce great changes
in the resulting fruit. J. F. Jones won considerable horticultural fame
from crosses he made between the wild American hazel known as the Rush
hazel, and such varieties of the European filbert as the Italian Red and
Daviana. Hazel and filbert cross readily and the resulting seedlings
will usually bear after only three or four years. For both these
reasons, they are good material for a beginner to work with. If the wild
hazel is to be used as the female, or mother, of the cross, it is
necessary to pick off all the male blossoms, or staminate blooms. This
should be done long before they begin to expand. The pistillate, or
female blossoms, should be enclosed in bags, about six of the
three-pound, common kraft bags should be enough. These are slipped over
those branches which bear female blossoms and are tied around a heavy
packing of absorbent cotton, which has been wound around the branch at
approximately the place where the opening of the bag will be. In
fastening the mouth of the bag around the cotton, I find that No. 18
copper wire, wrapped several times around and the ends twisted together,
is more satisfactory than string. This makes a pollen-tight house for
the pistillate blossoms but not one so air-tight as to cause any damage
to either the plant or blossoms.
In order to have pollen available at the proper time, it is necessary to
cut a few filbert branches which bear staminate blooms and store them in
a dark, cold place to prevent the pollen from ripening too soon. I
recommend keeping such branches in dampened sphagnum moss until it is
time for the pollen to ripen, or if a cold cellar is available, burying
the cut ends of large branches carrying male catkins one foot deep in
clean, moist sand. When the pollen is wanted, the branches should be
placed in a container of water and set near a window where sunlight will
reach them. Usually, after one day of exposure to bright sunlight, the
staminate blooms will expand and begin to shed their pollen. The pollen
may easily be collected by allowing an extended catkin to droop inside a
vial or test tube and then, as the catkin rests against its inner wall,
tapping the outside of the tube sharply with a pencil to jar the pollen
grains loose. A separate test tube must be used for each variety of
pollen to be experimented with. By following this procedure for several
days with all the staminate blooms that have been gathered, the
experimenter should have enough pollen for work on a small scale. The
test tubes containing this pollen should never be stoppered with corks,
but with plugs of absorbent cotton, which will allow the passage of air.
Pollen may be stored in this manner for several days, possibly as long
as two weeks, if it is kept dry. By a close observation of the blooming
period of the wild hazels, one is able to determine the best time for
placing the filbert pollen on the pistillate blossoms. No attempt should
be made to do so until the male catkins of the wild hazel species are so
entirely exhausted that no amount of shaking will release any grains of
pollen. When this condition exists, it is time to move the stored
filbert branches to strong sunlight. A quiet day should be chosen to
pollinize the hazels for two reasons. If there is a wind, it will blow
away the pollen and so make the work more difficult. A wind will also
increase the danger of the hazels being fertilized by native hazel
pollen which may still be circulating in the air and which the flowers
may prefer to filbert pollen.
When good conditions are present, then, the hybridizer proceeds to his
work. A brush with which to transfer pollen from the vial to the
pistillate blossoms is made by wrapping a little absorbent cotton around
the end of a match. The paper bag is removed from around a group of
hazel blossoms, a small amount of pollen is dabbed on each blossom and
the bag is immediately replaced, to remain on for two more weeks. When
the bags are finally taken off, the branches should be marked to
indicate that the nuts will be hybrids. Before receiving pollen, each
pistillate blossom has, emerging from its bud tip, a few delicate red or
pink spikes which are sticky enough to make pollen adhere to them.
Within a few days after receiving pollen, these spikes may dry up and
turn black, a fair indication that the pollen has been effective. If the
pollen does not take hold, the spikes of the staminate blooms are sure
to continue pink for a long time. I have seen them in the middle of the
summer, still blooming and waiting for pollen which would let them
continue on their cycle. This ability of hazel flowers to remain
receptive for a long period allows the nut-culturist ample time to
accomplish his work. It is not so true with all members of the nut tree
group, some, such as the English walnuts, being receptive for such a
short period that only by very frequent examination and many
applications of pollen can one be sure of making a cross.
Early in the fall, the hybrid nuts should be enclosed in a wire screen
to prevent mice and squirrels from taking them before they are ripe.
Such wire screens may be used in the form of a bag and fastened around
each branch. When the husks turn brown and dry, the nuts are ripe, and
ready to be gathered and planted. Careful handling of the nuts is
advisable to preserve their viability. They should be planted in an
outdoor bed which has been fully protected against the invasion of
rodents. A screen such as I described for other nut seed is satisfactory
for these hybrid nuts but it need not be as large as that. After the
nuts have sprouted and the plants have grown for one season, they may be
transplanted into a permanent location where they should again be well
protected against mice by a trunk screen, and against rabbits by driving
a stout stake deep into the ground on the south side of the tree and
tying it to the tree. This use of a stake discourages rabbits from
cutting off the tree.
There are innumerable other crosses that can be made as well as those
between hazels and filberts. It is possible, for example, to cross the
English walnut with the black walnut. Many such crosses have been made
although none of them is known to have produced superior nuts. Thousands
of crosses exist between butternuts and Japanese heartnuts. Many of
these are of some worth and are being propagated. Crosses between
heartnut and butternut are easily made, following the same procedure
used in crossing hazels and filberts, except that larger bags are
necessary for covering the female blossoms. Also, these bags should have
a small, celluloid window glued into a convenient place, so that the
progress of the female blossoms toward maturity can be observed.
When hybridizing walnuts, it is necessary to use a pollen gun instead of
removing the bag from around the female blossoms and applying the pollen
with a cotton-covered applicator. Such a pollen gun can be made by using
a glass vial which does not hold more than an ounce of liquid. An
atomizer bulb, attached to a short copper or brass tube soldered into a
metal screw-cap, is fitted to the vial. Another small copper or brass
tube should also be inserted in the screw-cap close to the first one.
The second tube should be bent to a right angle above the stopper and
its projecting end filed to a sharp point. Without removing the bag from
around the pistillate blossoms, the hybridizer forces the point of the
atomizer through the cotton wadding between bag and branch. The pollen
in the vial is blown through the tube into the bag in a cloud, covering
all the enclosed blossoms. It is advisable to repeat this on several
successive days to make certain of reaching the female blossoms during
their most receptive period.
[Illustration: _8 x 8 x 8 foot tightly woven sheet of unbleached muslin
stretched over mother hazel plant during pollination period in the
process of making controlled crosses between it and filbert parents.
Photo by C. Weschcke._]
[Illustration: THE WESCHCKE POLLEN GUN
Taper end of copper tube ... not absolutely necessary, but it saves
pollen.
Long fibre cotton wad wired to intake side of bulb to strain out foreign
pollens that may be in atmosphere.
De Vilbiss atomizer bulb.
Pollen grains
Any small glass bottle with a wide mouth and screw cap.
Tubes A and B--3/16" outside diameter copper tubing can be purchased at
any garage. Solder both tubes to screw cover C.
Drwg by Wm. Kuehn
_How to make pollen gun._]