Emmalea Ernest, Scientist – Vegetable & Fruit Crops; emmalea@udel.edu
Over the past four years I have been testing the practice of using shade cloth in bell pepper production. Shade cloth is a knitted, weather resistant fabric that can be used to block a portion of the sunlight that would otherwise reach plants. It is commonly used in greenhouse and nursery plant production and could be useful in preventing heat stress in some vegetables. Different colors and levels of shade are available. For vegetables, shade cloth that blocks 30% of the sunlight is typically recommended.
In 2018 and 2019, I tested different colors of shade cloth for green bell pepper production on drip irrigated black plastic mulch. In those trials, the 30% black shade cloth treatment produced the highest yields and increased marketable yield to three times the marketable yield of no shade cloth (Fig. 1). Shade cloth did not increase the number of peppers produced, rather it increased pepper fruit size and marketability. Shade cloth can prevent heat stress induced quality defects like sunscald and reduces plant heat stress, resulting in larger fruit. In these trials black shade cloth produced significantly higher marketable weight than the other shade cloth colors (white, red and aluminized).
Figure 1. In the 2019 trial an unshaded plot (foreground) with poor plant growth and fruit sizing compared to a shaded plot (background).
In the 2018 and 2019 trials the peppers were transplanted in early June and the shade cloth was applied in early July, after the plants had been staked and tied once. Trials conducted in 2020 and 2021 showed a benefit to applying shade immediately after transplanting in early June. Earlier shading protects young plants from girdling that can result from heat damage to the stem soon after transplanting and help transplants to establish successfully (Fig 2).
In the 2021 trial three colored bell varieties (Early Sunsation, Mandarin Perfection, Aristotle) and a sweet Italian variety (Carmen) were used. The bell pepper varieties all had significantly higher yields with shade, but Carmen did not have a significant yield increase from shade, indicating that this variety may be more heat tolerant than the bell pepper varieties. One goal of the 2021 trial was to determine the best timing for shade cloth use in peppers. The treatments with the highest marketable yield were those with shade cloth in both June and July, as opposed to only June or only July. Keeping the shade cloth on for the first two weeks of August increased marketable yield slightly over the June & July shade treatment.
Figure 2. Transplant with stem girdling from heat damage soon after transplanting
In the 2021 trial I used data loggers to measure air temperatures in the leaf canopy of shaded and unshaded peppers throughout July (Figure 3). Average daily temperatures were 2 °F cooler in the shaded plants. Differences in maximum daily temperatures were even larger, with the shaded plants having, on average, 8 °F lower maximum temperatures. The reduction of maximum temperatures may be especially important in avoiding plant stress and fruit damage from sunscald.
You may be wondering how best to implement shade cloth on your farm. Shade cloth is durable and can be reused for many years. In the experiments described the shade cloth was draped over the pepper stakes and secured to the ground with landscape staples or aluminum tent stakes. Shade cloth can also be applied over low tunnels or larger structures to create “shade houses”. Shade cloth might also benefit high tunnel grown peppers during the hottest months. In my trials we did not remove the shade cloth for sprays and did not notice differences in disease incidence between shaded and unshaded plots. Unless used with a large structure, shade cloth will have to be moved to access plants for harvest.
Figure 3. Daily average and maximum temperatures in the plant canopy of shaded and unshaded pepper plants in July 2021
I am repeating the 2021 shade cloth timing study this summer. It was planted on June 1 and already there are noticeable differences with less stem girdling from heat stress in shaded plots. With what is likely to be a hot summer ahead I will be interested to see whether shade cloth continues to be a useful tool and think about how it can be efficiently implemented on farms.