Horticulture: July 1998

The past several months have been extremely dry in southern Oklahoma and the thermometer has been pushing 100°F quite regularly. Considering summer doesn't officially begin for another three weeks (I'm writing this article June 1), memories of 1980 are beginning to haunt me. If you lived in these parts during 1980, no doubt the summer left quite an impression on your mind. Every living thing suffered through one of the hottest summers on record.

Could such an extreme summer happen in 1998? No one knows for sure but we do know something about how plants react to high temperature and what can be done to lessen heat related stress.

Blossom drop and resulting lack of fruit set on tomato, pepper and bean plants during the summer is most likely due to high temperatures. The fruit that does set will usually be small, contain few seeds and generally be of poorer quality.

Fruit can scorch if subjected to direct sunlight. Tomato fruit fail to develop a deep red color when exposed to high temperature during ripening. Sweetcorn ears maturing during scorching weather are often poorly filled. Potatoes can rot if left in soils having temperatures above 90°F. High temperatures increase the rate of ripening fruit, consequently decreasing the storage life. Melons, cucumbers, sweet potato, okra and eggplant are the most tolerant vegetables to high temperature. However, persistent temperatures above 100°F can reduce yield and quality in these vegetables.

To lessen the adverse effect of high temperatures on the food garden, begin by selecting heat tolerant varieties. The smaller "cherry" tomatoes are more reliable in hot weather compared to the larger slicing types. Use a shade cloth to reduce air temperature several degrees and reduce incidence of sunburn.

Water vapor exiting leaf surfaces (transpiration) cools the leaf and the air in the immediate vicinity of the plant. Therefore, avoid excessive pruning of foliage. Maintain a uniform water supply to avoid moisture stress. Mulch beds to reduce evaporation from the soil surface.

Ornamental plantings aren't immune to the effects of high temperature stress. High temperature in combination with drought can cause yellowing (chlorosis) and scorching (necrosis) of foliage. Cottonwood trees react to hot dry weather by dropping their leaves.

Catalpa wilt very easily despite irrigation. Maple, Bradford pear, Dogwood and Flowering quince tend to retain their leaves but often show heat scorch. During hot weather continue to irrigate and fertilize trees as usual. As long as 50-75% of the tree leaves remain green there shouldn't be a problem.

In closing, let me leave you with this thought. During periods of high temperature, any plant that has been subjected to other types of stress can be expected to perform poorly or even die. The key to plant survival during hot weather is to prevent or suppress other types of stress.