EFFECTS OF GRAFTING ON UNLIKE STOCKS
It is unquestionably a great shock to a tree when 90% of its top is cut
off. If it is healthy and vigorous, the root system will try to recover,
using every means possible to do so. If a new top is grafted to it, the
stock must either accept and nourish that foreign and sometimes
incompatible new part, or give up its struggle for life. Nature and the
tree stock usually accept the challenge and the graft begins to grow. In
an attempt to continue with its own identity, the stock will bring into
activity adventitious buds. These are tiny microscopic buds imbedded in
the bark of a tree that are not apparent to the eye but are nature's
protection against destruction of the individual plant. But these must
be removed by the horticulturist to insure proper nourishment of the
grafts.
Because the root system is striving hard to live, and because it is
usually the stronger, it may force the top to accept certain of its
characteristics. Occasionally, it may assume some qualities of the
original top. Such cooperation is necessary if either is to survive.
First of all, the grafted scions must accept the vital quality of
climatic hardiness, a powerful factor developed through ages spent in a
certain climate. To hasten the acclimatization of a tender variety, I
cut scionwood from such unions early in the winter, storing it until
spring. When these scions are grafted on new root systems, I find that
they are much more readily accepted than the first grafts were. The
following season, I allow the grafts of this later union to go through
their first winter of exposure. Early each spring I continue to cut
scions from the most recent unions and graft them to new root systems,
so hastening and setting the factor of hardiness through frequent
asexual propagation.
Because my observations of the effects of scion on root and vice versa,
have not extended over a sufficient period of time, I think it is
possible that the changes I have seen may be only transient. In any
case, I do know that the phenomenon occurs, for I have seen many
examples of it.
One instance in which the stock was apparently affecting the scions,
occurred in the case of several varieties of black walnuts which had
been grafted on wild butternut stock over a period of sixteen years. The
walnut top flourished but tended to outgrow the butternut, so that the
caliber of the walnut was greater than that of the stock a few inches
below the graft union. I also noticed that, although the graft began to
bear about as early as black walnuts do when they are grafted on their
own species, the nuts did not mature at all during the first few years
of bearing. In 1938, after a favorable season, I found mature nuts on
one variety, the Thomas. These nuts varied in size more than they do
when grafted on black walnut. The most surprising thing about them,
though, was that they did not have the characteristic black walnut
flavor. When properly dried and cured, they could have passed as an
entirely different nut since they tasted like neither the black walnut,
the butternut nor the Persian walnut.
The overgrowth of the Ohio black walnut, grafted on butternut, was even
more apparent than that of the Thomas. These nuts were, as I have said,
immature the first few years they appeared and they, too, lacked the
usual black walnut flavor. In their case, however, the most striking
change was in the shape and structure of their shells which were
elongated like butternuts, with corrugations typical of those found on
butternuts and nearly as deep and sharp. (See Illustration in Chapter 1,
Page 5.)
In 1937, I made experimental graftings on native black walnut stocks of
the Weschcke No. 4 butternut, a variety I found to be superior to
hundreds of other native trees tested. The grafts grew luxuriantly and
in 1940, produced about two pounds of nuts. These nuts were
approximately 30% larger than those on the parent tree. They cracked
well and the kernels were similar to those from the parent tree. They
definitely distinguished themselves, however, by being a free-hulling
nut, which is not true of the mother tree nor of most butternuts. Soon
after the nuts had dropped to the ground and were still green, they were
hulled and their hulls peeled off like those of the Persian walnut,
leaving the nuts clean and free from remnants. Apparently this
phenomenon was a transient one since later crops did not display this
free-hulling feature.
I have mentioned, elsewhere, the seedling apricot which came into
bearing in St. Paul, and how I obtained grafts before it died during a
very cold winter. I have grafted scions of this apricot on both hybrid
and wild plum stocks repeatedly and this apricot now exhibits a material
gain in hardiness. It overgrows the plum stock, but this does not seem
to inhibit its bearing, the fruit growing to greater size than that of
the mother tree.
These are some of the instances in which I have seen stock exert a
definite, and, mainly a beneficial influence on its grafted top. It may
easily be that these are only of a temporary nature and until I have
seen them maintained for many more years, I must consider them to be
transient effects.