Hot, dry summer has different effects on different plants

Published 10:59 pm Saturday, August 4, 2012

Plants sweat a lot in hot weather.

I think my tomato plants have been sweating more than the okra and the pole butterbeans. Transpiration is the correct term for the evaporation of water through plant leaves into the atmos-phere. Transpiration serves plants like sweating does us, it helps cool plants in hot weather.

Here lately, vegetable garden plants have needed some cooling. Then again, nearly 100 degrees the last of July is nothing new. Neither was the dry spell before the daily rain spell. But high temperatures can start taking a toll when vegetable plants can’t get cooled down enough. Blooms won’t become tomatoes or squash or beans when the plant is too hot. I don’t know that it ever gets too hot for okra.

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Nighttime temperatures sometimes cause problems with pollination and fruit set as much as daytime highs, if not more. Most of the plants we depend on in summer gardens are happiest with daytime highs in the low 90s and overnight lows in the 60s to low 70s.

Plants depend on cooler night temperature and use it as sort of a daily rest period since photosynthesis and most transpiration and other processes shut down or slow down for the night. The nighttime cutoff point for our hot-weather vegetables is somewhere around the 80-degree mark.

When the temperature at night doesn’t drop below 80, the plant uses too much energy trying to stay cool. And anytime a plant is stressed, be it from heat, too much or too little water, or not enough sunlight, the first thing to go is reproduction. The system gets shed of blooms, pollen and young fruit and goes into a sort of survival mode.

By the way, daytime temperatures in the mid-90s basically don’t help, but don’t hurt. It’s the high 90s up to 100 or more when plants need some relief.

Again, this is all based on vegetable species that grow and reproduce in hot weather. We have some “warm” weather species that do fine planted on time to be eaten and long gone by now.

The best example is probably sweet corn. It comes up and grows well through the spring into early summer but wants no part of our mid- to late-summer weather. Sweet corn planted in mid-April will silk, tassel and pollinate in mid-June and be ready to pick for a Fourth of July cooking.

Plant sweet corn the first day of June and it will come up just fine. But regardless of fertilizer, weeding, irrigation and TLC, the outcome is not pretty. That’s because pollination time would be right about now and sweet corn pollen is pretty much sterile in this heat. There would be a puny corn cob with a few scattered kernels due to poor pollination.

Throw in a dry spell with the heat at pollination and the resulting corn would be what old-timers called a nubbin’ ear.

Terry Rector writes for the Warren County Soil and Water Conservation District, 601-636-7679 ext. 3.