t is quite
possible that the clones and seedlings of the Persian walnut brought to
this country a few years ago by the Rev. Paul Crath from the Carpathian
Mountains of Poland may require the longest rest period of all.

The question may be asked what causes or brings on this rest period in
plants and what breaks it? The scientific answers to these questions are
not known at this time, but we do know some of the factors which cause
the initiation of rest and how it is broken.

Tree growth is initiated in the spring with coming of warm weather and
other suitable conditions. At first the rate of growth is slow; but the
rate increases and goes through a maximum and then slows up again and
finally ceases. On the cessation of growth in length, a terminal bud is
formed and the tree begins to go into rest. This period of growth is
determined by the age of the tree, the suitability of moisture and
nutrient supply. Young trees grow longer during the spring and summer
than do old ones. Deficiencies of soil moisture or nutrients or both
cause the cessation of growth and the beginning of rest. In some trees,
such as tung, cessation of growth and the initiation of rest is caused
by the change from long to short day-lengths.

After rest has begun, the longer it continues the more profound or
deeper it becomes until a maximum is reached, i.e., it becomes
increasingly difficult, up to a certain time, to make the trees start
growth again even though optimum conditions are provided. Some trees
such as Persian walnuts and pecans, for example, are slow to go into
deep or profound rest in late summer or fall. For this reason, there may
be several cycles or periods of growth during the summer and early fall,
depending on weather conditions and whether the leaves on the trees have
remained in a healthy condition. Under conditions of dry weather growth
stops on the Persian walnut and pecan and when this is followed by a
rainy period and warm weather growth begins again. In fact in early
summer a walnut or pecan tree may form terminal buds on all the shoots
and remain without growth long enough for an apple or pear tree to go
into complete or profound rest; then later, new shoot growth may be made
from all or nearly all of the walnut or pecan shoots. Not only is this
an important factor in promoting susceptibility to cold injury but in
the case of bearing trees more often than not this late growth prevents
the proper development of the kernels