Among the many factors that indirectly affect the cold hardiness of a particular plant by 

 affecting its general condition are drought, soil fertility, the extent of any injury due to insect 

 pests or diseases, the weakening effect of heavy crop production, time of fertilizer application, 

 mechanical or chemical injury, or rodent injury. 



The task of horticultural scientists seeking fruit and nut varieties which will tolerate cold 

 is obviously complicated by the many variables involved such as drought. Characteristics that 

 promote cold hardiness under one combination of circumstances may make a plant more sus- 

 ceptible to cold under another set of circumstances. Consequently plant explorers and breed- 

 ers need to obtain plants that are adapted to growing under widely varying conditions or are 

 fitted to the particular area in which they will be grown. 



Horticulturists generally agree that early maturity combined with a long rest period is a 

 reliable index to cold hardiness but other more objective indices for accurately predicting cold 

 hardiness are being sought. 



The terms rest period and state of dormancy are sometimes confused by non-technical 

 readers. Rest period refers to an inactive state of the plant that is brought about by internal 

 plant controls. Dormancy refers to the state in which the plant is either in its required 

 rest period or is inactive due to environmental factors such as drought. 



Duration of the rest period varies inherently with the kind of plant. Until its rest 

 period is complete, the plant remains inactive regardless of the weather. And even after the 

 rest period has been completed, the plant continues in an inactive state until stimulated into 

 growth by moisture and warm weather. The amount of warm weather required to stimulate 

 growth following completion of the winter rest period differs with the kind and variety of 

 plant. By affecting readiness to bloom, these differences in rest period requirements and 

 dormancy affect the vulnerability of some plants to frost damage. 



Cold hardiness is only one of the traits needed to make a plant commercially desirable. 

 High yield, good quality, and resistance to insect pests and diseases are also sought by plant 

 researchers in selecting and developing cold-hardy fruits and nuts. Such combinations of 

 desirable traits are rarely found in native plants or imports brought home by plant explorers 

 because of superior quality or other specific characteristics. Therefore recourse to plant 

 breeding is frequently necessary. And plant breeding takes time — especially in the case of 

 certain crops, such as apples, which require many years to get into commercial production. 

 Furthermore, the various areas and the different crops have their particular problems which 

 must be solved. Some crops, for example, have a short minimum required rest period. Con- 

 sequently a few days of mild weather in the winter or spring can break their dormancy and 

 render these plants extremely vulnerable to cold damage. Extreme winter cold, such as char- 

 acterizes the Northern Great Plains, is an example of a regional problem which has limited 

 the economic production in those areas of fruits that are damaged by prolonged exposure to 

 low temperatures. 



DECIDUOUS TREE FRUITS 



Deciduous fruit trees require a certain amount of winter cold in order to bear fruit and 

 most kinds can stand temperatures far below freezing while dormant. However, the differ- 

 ent kinds and varieties vary in their ability to withstand cold damage and all kinds are more 

 susceptible to cold during periods of growth. Consequently, varieties that have a short rest 

 period and that require only a few days of warm weather following completion of rest to 

 induce bloom are especially liable to freeze damage in the early spring or after unseason- 

 ably warm weather in winter. 



