Environmental problems for landscape plants are related to the weather or the site in which a plant is growing. A landscape site is made up of a number of small parts that have varying environmental factors. The broad-leaved evergreen that burns each winter and scorches each summer in a sunny southern exposure would be at its best in shaded and wind-protected northern exposure. When a growing site does not meet the needs of a plant, environmental problems can be expected.

In northern climates, cold temperatures are an annual winter occurrence. Cold temperatures will cause the injury to plants that are growing slightly farther north of the climatic zones to which they are adapted. They are not able to develop the cold temperature hardiness necessary to withstand the more severe winters. Various plant parts can be injured at different temperatures. Twigs and leaf buds, for instance, often survive low temperatures that kill flower buds. This explains the failure of some plants to bloom, even though they survive.

How do cold temperatures injure plants? Plants are killed when the temperature drops below the temperature the plant can withstand. There are several situations in which this can happen. Depending on the situation, the same plant species may be killed at different temperatures.

It is possible for winter temperatures to drop below the levels to which plants have adapted. This will be a rare occurrence for plants native or adapted to northern states. Plants native or adapted to warmer climates are likely to be injured more frequently during the winter because they may not have the ability to develop the same hardiness levels as hardier plants. Plants native to a state may have broad geographic ranges. Plants of a species from the southern parts of the range will not have the same ability to withstand cold temperatures as their northern relatives.

A second situation in which plants are injured by cold temperatures is most likely to occur in spring or autumn. In spring, plants lose the ability to withstand very cold temperatures; they regain the ability in the fall. Unseasonably cold temperatures during these times can injure plants after they lose large amounts of hardiness or before they develop adequate hardiness. Unseasonably warm weather during the winter, followed by a rapid change to unseasonably cold weather, can also be a problem.

A third type of injury occurs when plant tissue is rapidly warmed and then the temperature drops quickly. When the sun shines on dark green foliage or dark colored bark, the foliage or bark may become many degrees warmer than the air around it. When the sun stops shining on the foliage or bark, the plant temperature drops very quickly to the air temperature. These episodes of rapid freezing and thawing can severely injure plants.

Curt Peterson, Ph.D., is a professor of landscape horticulture and nursery management on faculty at Florida Southern College. He resides in Union Springs during the summer months. He can be reached at curt.peters@hotmail.com