"Plant Hardiness"
Dr. Griffith J. Buck,
Iowa State University

Editor's Note: This article was written in the late 1970's, but we thought it was well-worth repeating! Hardiness is a vital concern to gardeners everywhere, for, to a large degree, it influences one's choice of plant materials and success in the garden. While the concept of the word "hardy" is universal, it is in its application that misunderstanding arises. A hardy plant is one capable of surviving without injury the climatic extremes of a given environment. This should imply not only winter hardiness but also summer hardiness. However, it is only with respect to the winter climate that the term is most frequently used. Yet, winter climates vary from place to place, and it does not always follow that a plant hardy in one area is hardy in another.

Hardy plants have the ability to bind their water against freezing to a greater extent than non-hardy plants. Maximum plant maturity, which results in maximum hardiness, is realized when there is a high level of carbohydrates accumulated in the plant. Hydrophilic colloids are produced by the metabolism of carbohydrates within the cells. These colloids have a large surface in proportion to their size and adsorb large quantities of water on these surfaces. This adsorbed water is called bound water because it is held, not in, but to the colloids and has more of the characteristics of a solid than a liquid, and cannot be frozen at the usual temperatures.

A high carbohydrate and high bound water content are characteristic of mature tissues. Immature tissues lack these. Thus, the rate of carbohydrate manufacture, the relative rates of storage and use, and the ability of the plant to produce hydrophilic colloids are all concerned with plant hardiness. Plants native to temperate regions undergo a growth phase beginning in midsummer and lasting well into winter. During this period the plant is most resistant to growth stimulation by environmental factors. (A combination of cooler temperatures, adequate moisture, and large quantities of growth material produce almost ideal conditions for growth). Yet, this is the rest period, a time of dormancy.

The rest period is usually terminated by cold exposure. After termination of the rest period, the plant is kept non-active by environment factors, principally temperature. The rest period's length varies from one kind of plant to another and even from one clone (horticultural variety) to another. Usually the greater the chilling requirement, the number of hours of exposure to low temperature required to terminate the rest period, the hardier the plant and the more resistant it is to freeze injury, especially in later winter. The rest period, in itself, does not cause hardiness, but it does contribute to hardiness by preventing growth at a time when such would be a liability.

The degree of cold and the rapidity of temperature change is, of course, closely related to bound-water conversion by the plant. Injury to plants from freezing comes from ice formation within the tissues. When temperatures drop rapidly, plant tissues freeze rapidly, with a resulting formation of ice crystals within the vacuoles (watery sacs within the cytoplasm of the cells) and cytoplasm, the living gel-like substance surrounding the vacuoles. Regardless of their state of maturity of hardiness, all cells, and therefore the tissue involved, frozen in this way are killed because of the disruption caused by the freezing. When plant tissues freeze slowly, ice crystals form in the spaces between the cells. As freezing continues, water extracted from the cells increases the size of these intercellular ice crystals. Dehydration either by this water withdrawal, or a deficiency of water supplied to the cells, concentrates the fluids within the cell, reducing its freezing point, and preventing ice crystal formation inside. If the freezing process continues long enough, the cells shrink and finally collapse. When the ice crystals thaw slowly, the water returns to the cells and restores their pre-freezing condition. Dehydrated protoplasm becomes highly viscous, then coagulates and clots. Rapid rehydration or renewal of the water in such cells, such as occurs during rapid thawing, sets up mechanical stresses which may rupture and destroy the cell.

Casual freezing of mature cells does little harm, but repeated freezings and thawings tend to increase this type of injury. Many of the methods used to protect plants from freezing in winter slow the freezing rate and thus reduce the danger of injury from freezing temperatures. Preparation of a plant for winter begins not in the late summer or fall, but in the early spring. The degree of maturity produced in the plant by the end of the growing season is directly related to the kind and quality of culture it received during the growing season.

The diligence of the gardener in satisfying a plant's cultural requirements can increase its hardiness to its maximum level but no farther. However, poor culture can definitely lower the level of a plant's hardiness. In order to realize a plant's maximum winter hardiness, gardeners should assess their cultural methods critically as well as their effects on the development of high carbohydrate reserves within the plants. The goal they should work toward is a steady growth rate through the growing season with a careful cessation from mid-August until killing frost.

To do this means giving careful attention to fertilizing, irrigation and photosynthesis. It also means that violent fluctuations in growth rate such as mid-summer dormancy followed by active growth late in the season as a result of pruning, fertilizing or irrigation is to be avoided.

Not the least of the cultural operations is pest control. Plants which have been defoliated by disease or insect attack are especially vulnerable to premature fall freezes. Premature defoliation interferes with photosynthesis and, therefore, carbohydrate production, by removing the organs concerned. Also, it can lead to the stimulation of late growth, which is not only too immature to survive winter temperatures, but requires carbohydrates for its production, thus depleting the plant's carbohydrate reserve and reducing its hardiness.