At this time of year gardeners are nervous about the appearance of late frosts. By definition, a frost is a sudden and untimely onset or return of temporary freezing temperatures.

These freezing temperatures injure plants that have started growing, in the case of perennials, or that have been planted too early, in the case of annuals.

Chill injury is a related problem that occurs when an actively growing plant is injured but not frozen. Even air temperatures that remain just above freezing for a prolonged time will kill many tender flowering annual plants. This is because air temperature affects many of the plant's basic physiologic processes, such as the absorption of nutrients.

To Each Variety Of Flower, Their Own Optimal Temperature

Every species in the plant world has a critical maximum, minimum temperature and also an optimal temperature at which it will attain its greatest growth. Each plant also has lethal temperature extremes or cut offs, i.e., both high and low limits of temperature beyond which it will die.

Alternation between freezing and thawing temperatures is bad for our perennials that are just beginning to end hibernation at this time of the year. Tender annuals that have been newly planted are especially vulnerable, however, and must be covered at night if the temperature will go near freezing.

We think of 32 degrees Fahrenheit as the magic number for freezing, but low lying areas in the garden may be damaged even if the thermometer is above this number.

Also, frost damage may occur even if there is no visible white coating on the plants.