e United States in determining the amount of cold injury that may
be sustained by woody plants. Furthermore, it explains why certain
plants may be successfully grown in much colder parts of the world and
yet fail here. Our winter weather conditions are not uniform, in that it
is quite common for us to have quite long periods of alternating warm
and cold weather. Too often during mid-or late winter the weather may be
quite warm for several days, with above-freezing temperatures even at
night, only to be quickly followed by a sudden and extreme drop in
temperature. Such conditions are almost certain to result in cold injury
to at least certain kinds of woody plants in which the rest period had
been broken prior to the occurrence of warm weather, especially so if
conditions are favorable for initiation of growth. The plants that were
still in the rest period at the time of the warm weather or those with
high heat requirement to start growth (as for example, the pecan) would
be the only ones that would escape injury. To illustrate with an
example: The Chinese chestnut tree has a shorter rest period or less
chilling requirement than does the average Persian walnut tree. Now
suppose that during the months of November and December a sufficient
number of hours of chilling temperatures were experienced to break the
rest period or to satisfy the chilling requirement of the Chinese
chestnut but not that of the Persian walnut. Then suppose there was a
period of two weeks or more of warm weather in January and it was ended
by a very sudden drop to below freezing temperatures. Later we would
expect to find that some parts or tissues of the Chinese chestnut trees
had been injured while the Persian walnut trees had survived without
injury. Similar differences would be expected with other crops, such as
peaches and apples, that have a difference in rest period or chilling
requirement. Under the conditions just described the parts or tissues of
the tree that are most likely to be injured are those that first become
active with the coming of warm weather, such as the pith in the wood,
the lower buds, and later the cambium or the leaf buds. This explains
why peach fruit buds and the catkins of the European filbert are often
killed in the East during the winter.

Some kinds of woody plants are very much hardier than are other kinds.
For example, the butternut is hardier than the eastern black walnut and
the almond is hardier than the tung tr