Last week, the highest temperature reached 110°F for a few successive days inside of our high tunnels. As a result, we observed some blossom drop on tomatoes. More information on high temperature effects on tomato fruit set can be found here. In addition to blossom drop, high temperature and high light intensity contribute to sunscald injury, uneven ripening, and cracking of tomato fruit. To protect tomatoes from damage caused by excessive heat, we placed 30% black shade cloth on top of the high tunnel. By installing the shadecloth, we expect there will be less cracking and more uniformly ripe tomatoes. Tomato marketability will increase. However, using shade cloth also has some negative effects. In this article, we review the effects of high temperatures on tomatoes, and discuss positive and negative aspects of using a shade cloth.
Excessive high temperature (above 100°F) lasting for a few hours for successive days could cause tomato flower abortion and affect fruit set. Night temperatures above 75°F might also cause tomato fruit set failure. In addition to fruit set, high temperature affect fruit ripening process as well. With temperatures above 93°F, ripening-associated ethylene production was markedly decreased. As a result, we see more yellow-shoulder tomatoes in middle summer. More information regarding yellow shoulder disorder can be found here. Studies showed that shade cloth with 15% to 50% shade reduced temperature for 3-4°F at the time when light intensity was high. However, at night and when light intensity is low shade cloth had little effects on reducing temperatures.
In general, using shade cloth reduces the number of unmarketable fruit. The most significant effect is to reduce skin crack. Tomato skin crack is partially caused by irregular watering, especially when soil-moisture goes from very dry to very wet. Shade cloth reduces water evapotranspiration, thus preventing dramatic variation of soil moistures, especially when plants are not watered frequently. Lower temperature under the shade also help reduce tomato cracking. This is because when temperature is high, the pressure exerted by the pulp on the skin is increased, and at the same time skin strength is decreased.
Using shade cloth does not always generate positive results. When shade level is more than 50%, plants developed larger but thinner leaves, longer internodes and less vegetative biomass. Water, nitrogen and potassium uptake was declined as shade density increase. A study conducted in northeastern United States showed that when 50% shade was applied at the time when greenhouse tomato began to ripe, total yield was reduced after 3 weeks of shade application even though the early yield was not affected. Studies also showed that shade might reduce tomato sugar content because of the reduced photosynthesis rate.
Now the question is whether we should apply shade cloth to the high tunnel tomatoes. Most of us who grow tomatoes in high tunnel target the early market. Having perfect tomatoes in the early season is quite important. To avoid those early season tomatoes that become unmarketable if temperature is too high, my suggestions is to apply shade cloth, but only use the ones that have moderate shade level such as 30% shade or less. It always good to apply shadecloth only in the days when temperatures are extremely high, and take them off in cloudy days. However, this is not always possible. If shadecloth is used till the end of the season, this practice might increase tomato yield in a hot summer, considering there would be less blossom drop because of the lower temperature. But in a mild summer, this practice might turn out reduce overall tomato yield in late seasons. In another case that shade cloth might be more helpful is when indeterminate tomatoes are grown and pruned and trellised to a single stem. Because of less foliage and higher temperature in upper level of high tunnels, those tomatoes are more vulnerable to damage by excessive heat and more likely to produce better fruit under the shade.