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Larry Sagers: Some plants have tough time beating Utah's heat

Published: Sunday, Aug. 5 2012 3:00 p.m. MDT

Sun scald on a tomato

Larry Sagers

Across the country, the news is grim. Record heat and no rain have much of the country baking in the heat and withering from the drought. While vegetable gardens in Utah are irrigated, they are not immune to the prolonged hot temperatures, the scorching wind and the low humidity.

The leaf and other tissues of most plants die at about 115°F. While plant temperatures are normally just above air temperature, they can reach critical levels under certain conditions.

Plants have three major ways to get rid of excess heat. They can reflect the sun's rays and the associated heat, transfer the heat to the air by convection and transpiration, or allow water from the plant to cool the plant through evaporative cooling.

Our high elevation and cloudless days in Utah mean there is little you can do about the solar radiation. We normally consider it a positive growth factor, but when coupled with other factors, it reduces crop production dramatically.

Transpiration cools the plant by moving water from the soil through the plant. Plants that suffer from lack of water, root or stem injury or are exposed to extremely high temperatures stop transpiring and cannot get rid of damaging heat.

Very hot, dry winds also cause plants to build up heat. The leaves lose water more quickly than roots can absorb it. Photosynthesis or plant food manufacturing decreases rapidly above 94°F, so high temperatures limit production.

High daytime temperatures cause major heat problems, but warmer nighttime temperatures burn up the food that is produced during the day. That food is diverted from the produce so your vegetables produce poorly.

Adding to the problem is the vegetable growing environments. Reflected heat from soil and walls raises the daytime temperatures; the surface temperatures on black plastic mulch can exceed 150°F.

So getting past the scientific of why, what is happening to your garden?

The most frequent complaint is about tomatoes and why the vines are not setting fruits.

Ideal tomato fruit set has a narrow range of night temperatures (60°-70° F). Night temperatures above 75°F prevent pollen tube growth and cause blossom drop. Daytime temperatures over 95°F also cause similar problems. In addition to the blossoms aborting, fruits that have set are misshapen or are much smaller in size than normal.

High temperatures also affect fruit set and fruit quality in beans, melons, cucumbers, squash and pumpkins. While some cultivars are more heat- tolerant than others, none is immune to this problem.

Poor pollination of these crops makes misshaped fruits.

Blossom-end rot is another problem that becomes worse with the hot temperatures. The symptom is a brown, water-soaked spot on the blossom end of partially grown fruits. It is most common on tomatoes but it also affects peppers, squash and melons. The water-soaked areas become sunken and leathery.

Keep plants uniformly moist and protect the roots from injury.

Mulch helps maintain a more uniform moisture supply. Blossom-end rot is caused by calcium deficiency inside the plant, but in Utah we have too much calcium in the soil so adding more does not help.

Fruit cracking is another problem that is intensified with hot weather. While you might think rain would help the plants, it can have an unintended consequence.

The excessive heat and lack of humidity make the skin on the tomato fruits shrink slightly. An afternoon thunderstorm, or sprinkling tomatoes that have not been previously watered that way or too much water causes the fruit to swell. This cracks the skin and makes the fruit less marketable and more likely to spoil.

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