Hot Weather Effects on Tomato Production

Tomato production sensitive to heat, humidity

Bill Sevier: Ask a Master Gardener

Saturday, July 16, 2016

Q: My tomatoes have stopped producing; they look OK and have a few blossoms but no tomatoes. What could cause this? Brad, Tulsa

A: Tomatoes often have diseases, pests or problems related to the environment that affects their production. The main causes of interruption of tomato production this time of year is hot temperatures.

Problems with disease and insect pests are usually evident on inspection of the leaves. Therefore, if the leaves are green and appear healthy, it is likely related to environmental factors. The lack of blossoms or the sudden loss of developing blossoms in summer is usually due to heat, water and humidity issues.

Tomatoes are self-pollinating and are dependent on wind to move the pollen from male to female blossom structures, which results in fertilization and development of fruit. If the pollen is not fertile, the flower withers and falls from the plant, called “blossom drop.” One of the main causes of blossom drop in summer is faulty pollination and is the reason tomato production decreases this time of year.

Pollen is finicky, and its fertility is sensitive to temperature and humidity. Evening temperatures out of the range of 55 to 70 degrees or persistent temperatures topping 90 degrees (especially with hot winds) during the day will prevent successful pollination in most tomato plants. This is the major cause of decreased summertime blossom and fruit production.

Tomato plants may also be stressed to extremes of heat, hot wind and water issues. This may cause the whole plant to go into survival mode. It will then grow leaves, which it needs to survive, at the expense of producing blossoms and fruit.

Two other situations reducing tomato productivity at any time of the growing season are excessive nitrogen fertilization and too much shade. Excess nitrogen causes the plant to grow tall with dark green leaves and few tomatoes. When there is not enough sunlight, the plants produce leaves to gather sunlight and not fruit. Tomatoes need full sunlight for best production

The good news is that in the early fall, as it cools, the pollen becomes more fertile, plants healthier and tomato production begins again. Tomatoes are often planted by some gardeners in mid-summer for a fall crop. Some of the tastiest tomatoes may be found in the fall production.

Another tomato heat-related fact is that when temperatures are over 85 degrees most tomatoes are unable to develop red coloring. Lycopene and carotene needed for red color development are not produced at these temperatures. However, mature-sized green-pink tomatoes will ripen and become red indoors, at room temperature and with no sunlight.

To reduce the likelihood of blossom drop, select tomato cultivars that demonstrate more heat tolerance. Examples are Traveler 76, Brandywine and Porter Improved (cherry tomato) suggested by OSU Extension. There are others.

Go to the tomato section of the Master Gardener website and review the four OSU fact sheets on growing and dealing with problems of tomatoes.

Garden tips

§  When watering your lawn, ornamentals or vegetables, always do so in the morning if possible. If watered in the evening, plants will go into the night still being moist. Most disease-causing organisms need moisture, and because they grow best at night, leaving leaves wet in the evening will promote many plant diseases.

§  Bulb onions are ready to harvest when the tops fall over. They should be removed and allowed to dry in a well-ventilated, shaded area. After the tops are completely dry, they may be stored in a cool, dry area.

§  For detailed information from OSU on mosquitoes and Zika virus in Oklahoma, do an internet search for Oklahoma State and “Pest e-alert.” This should find the document.