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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) 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.

Blossom end rot on a tomato           (Larry Sagers) Blossom end rot on a tomato (Larry Sagers)

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.

Blossom end rot on a watermelon           (Larry Sagers) Blossom end rot on a watermelon (Larry Sagers)

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.

Cracked tomato (Larry Sagers) Cracked tomato (Larry Sagers)

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.

Cracking on a tomato           (Larry Sagers) Cracking on a tomato (Larry Sagers)

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.

Some tomato cultivars are more resistant to cracking than others. Unfortunately, many heirlooms are very prone to the problem, so select tomatoes that resist cracking.

Try to keep the water off the fruit and pick and use any cracked fruit as promptly as possible.

Other high temperature tomato problems include sunscald and poor color development. Sunscald is a problem when the plant has not developed enough leaves. The exposed part of the tomato turns tan and does not look attractive. Very high temperatures also prevent the fruits from developing good color.

Heat stress on tomatoes           (Larry Sagers) Heat stress on tomatoes (Larry Sagers)

While the heat is causing problems, hopefully it will cool enough so you can still get a good crop. Unfortunately, there is little else you can do to solve the problems.

Larry A. Sagers is a horticulture specialist for the Utah State University Extension Service at Thanksgiving Point.

Garden Tips

(USU classes at Thanksgiving Point)

Basic Landscape Design, August 14, 21, and 28 from 10 a.m.-noon or 6-8 p.m.: Learn the proven step-by-step process for creating a successful landscape design, designing the public and service areas and learn water-wise landscape principles. The class includes a consultation on your plan by a USU Extension Service master gardener.

Poorly pollinated cucumber           (Larry Sagers) Poorly pollinated cucumber (Larry Sagers)

Spectacular Spring Flower Bed Designs, August 14, 21, and 28, 2-4:30 p.m.: Wonderful spring flower gardens don't just happen. They are created by gardeners who know what to plant in the fall. Learn to choose the bulbs, winter annuals, biennials and spring blooming perennials that will create the wonderful, showy gardens at Thanksgiving Point and Temple Square. Cost: $43 (for each 3-week course).

Garden Talks in the Park are complimentary garden talks at Brigham Young Historic Park on the Southeast corner of State Street and North Temple. The Care of Our Urban Forest is August 8 at 7:30 p.m. No tickets are required and all ages are welcome.Garden tips

USU classes at Thanksgiving Point

Basic Landscape Design, Aug. 14, 21, and 28 from 10 a.m.-noon or 6-8 p.m.: Learn the proven step-by-step process for creating a successful landscape design, designing the public and service areas and learning water-wise landscape principles. The class includes a consultation on your plan by a USU Extension Service master gardener.

Sunburn on watermelon           (Larry Sagers) Sunburn on watermelon (Larry Sagers)

Spectacular Spring Flower Bed Designs, Aug. 14, 21, and 28, 2-4:30 p.m.: Wonderful spring flower gardens don't just happen. They are created by gardeners who know what to plant in the fall. Learn to choose the bulbs, winter annuals, biennials and spring blooming perennials that will create the wonderful, showy gardens at Thanksgiving Point and Temple Square. Cost: $43 (for each three-week course).

Garden Talks in the Park are complimentary garden talks at Brigham Young Historic Park on the southeast corner of State Street and North Temple. "The Care of Our Urban Forest" is Aug. 8 at 7:30 p.m. No tickets are required and all ages are welcome.

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