ee. Hardiness is only a relative
term and can be determined only when the different kinds of plants are
in the same physiological condition as regards growth or activity. Just
what it is that makes a difference in the hardiness or ability to
withstand low temperatures without injury is not known. However, over
the years, experience and research have taught us that there are a
number of factors that affect the hardiness of woody plants.

There is a very great difference between the temperature that will cause
injury to a tree tissue when it is in active growth and most tender in
the spring and that required when it is most resistant in midwinter.
With some trees this difference in temperature is as much as 50 deg. to
60 deg.F. or even more. With woody plants, the tissues are least hardy in
spring when they are growing rapidly, and as the season progresses
hardiness normally increases provided that second or late growth does
not occur. There are many changes that take place in the tissues of a
tree as hardiness is developed: the moisture content is reduced; cell
walls are thickened; the concentration of sugars, starches, and other
carbohydrates becomes greater; there is the formation of pentosans,
gums, and waxes; and the respiration and other life processes become
slower. However, none of these offer a full and satisfactory explanation
of why the plant becomes as resistant to cold as it does. All of these
changes and probably many others play a part in developing hardiness in
woody plants.

Maximum hardiness is developed only by trees that support a large area
of normal leaves continuously from the time of foliation in the spring
until late fall when they are killed by frost. Attacks by insects or
diseases that injure the leave or cause partial or complete defoliation
at any time during the spring, summer, or before the occurrence of frost
in the fall, not only prevent the development of maximum hardiness of
the trees, but such defoliation results in reduced growth of the trees
and in poor filling of the nuts. The importance of maintaining a large
area of healthy leaves on the trees during the entire growing season can
hardly be too strongly stressed. This is because trees that hold their
leaves are strong, vigorous trees and are the ones best able to
withstand cold, as well as other adversities, without injury. This,
however, does not mean that fertilizer applications should be made in
late summer or that cultivation s