Tomatoes growing in high tunnels are in the middle of or close to harvest. Developing and maturing fruit are under leaf canopies. On the top of the plants, many flowers are still blooming. These flowers will contribute to the second big harvest. Although tomatoes in June are most valuable, we certainly appreciate big, red and delicious tomatoes in July and August. To ensure a sustained yield, it is important for these flowers to set fruit. The process of fruit set is very sensitive to excessively high temperatures. When temperatures rise above 100°F, even just for a few hours for a handful of days, tomato flowers may be aborted and fruit set fail. Night temperatures above 75°F may also cause tomato fruit set failure. In addition to fruit set, high temperatures affect fruit ripening process. Ethylene associated ripening decreases markedly at a temperature above 93°F. As a result, there might be an increasing number of yellow-shoulder tomatoes.
Because of the negative effects of high temperature on excessive-heat-sensitive crops, for example, tomatoes, it is reasonable for Indiana growers to consider adding shade to high tunnels. In general, growers have two options, shade cloth (Figure 1) and shade paint. Black shade cloth is the most commonly used shade materials in our region. It comes with different percentages, which indicates the percentage of light blocked by the shade. For example, the definition of 40% shade is that 60% of light can pass through it. In general, 30% shade is the most common recommendations for growing vegetables. It should be noted that the percentages do not directly translate to heat reduction. In a comparison of high tunnels with and without a 30% black shade cloth, we found the 30% shade cloth reduced the temperature from 119°F to 109°F in a sunny day but with little effects on night temperatures. An alternative option is reflective shade clothes. Reflective shade cloth might be white or aluminet shade clothes. They may provide addition cooling effects during the days because they tend to reflect the sun energy instead of blocking and absorbing it. Shade cloth with other colors are also available; they are used to grow specific crops by filtering different wavelengths of light.
Figure 1. A 30% black shade cloth was added to one of the high tunnels at SWPAC
Shade paint is another option some growers prefer. It is a liquid designed to spray on the outside of high tunnels or greenhouses. A commonly used product is called Kool Ray Liquid Shade®. It is normally applied in the early summer, and slowly weathers off. Manufacturers recommend different dilution rates to vary the light blocking effect. Shade paint is easy to apply and there is no need to take it off. Using shade paint also avoids the problem of having shade cloth moving around in the wind. Regarding cost, using shade paint to cover a high tunnel in one year is cheaper than purchasing shade clothes. However, it should be pointed out that shade cloth can be used for more than 10 years with good protection while shade paint needs to be purchased every year.