Sunlight is the main energy source for growing plants, and most vegetables grow best with at least 7 hours of direct sun each day. But in midsummer, when days are long and warm, a string of sunny days can put plants under stress that can be partially alleviated with shade. Shade covers also slow evaporation of soil moisture, reduce heat build-up in ripening fruits, and cool the plant canopy by 4 to 5 degrees.
Shade covers for vegetables are easy to make using cloth or fabric row cover, or you can use knitted mesh fabrics made for use as shade cloth. Their advantage over many other materials is hail resistance, which is no small matter in late summer. Shopping for shade cloth can get confusing, because you will need to choose between products with varying levels of light transmission. There is a clear answer: the sweet spot for shade covers for vegetables is about 30 percent reduction in light.
Shade Covers for Peppers
Big, juicy sweet peppers are at high risk for sunscald, which happens when hot sun burns a patch in mature green fruits. In research plots in Mexico, Spain, Israel, and the US, bell pepper yields under 30 percent shade increased by 50 percent, in large part because of less sunscald. When it comes to growing peppers in hot summer areas or in a greenhouse or tunnel, leafiness, fruit set and fruit size all improve with light shade, and sunscald is reduced to almost nothing.
Timing is important, because you don’t want to slow peppers by shading them too soon. Let the plants grow uncovered until the soil is warm and the plants start blooming and setting fruit, and then install a shade cover. I use a piece of 30 percent shade cloth secured over the peppers, but you also could use a panel of row cover or an old sheer curtain as a shade cover for peppers. Either will reduce light transmission by about 20 percent.
Try Red Overhead
Most plants have no preference when it comes to the color of cloth shade cover, which is typically black or green. However, basil, cilantro and lettuce have been found to grow bigger and bushier under red shade covers. Red shade covers increase the red and far-red light spectrum, and reduce blue, green and yellow light. Plants respond by growing more foliage and roots, so red shade covers are a good choice for transplanted seedlings.
Red shade cover is hard to find in small quantities, but you can make your own with a double or triple thickness of red tulle (wedding net), which is sold at fabric and craft stores. Red cotton gauze or thin muslin can work well, too. When you can see light through any cloth held to bright light, it will probably do as a shade cover for vegetables.
Light Shade for Tomatoes and Beans
In my climate tomatoes grow so bushy that the fruit is well shaded, but in situations where ripening fruits are baking in hot sun, shade covers can save your crop from widespread sunscald. Install tall stakes to which you can attach pieces of cloth or row cover, which can usually be secured in place with wood clothespins. If the plants are too tall to cover, hanging a row cover “fence” on the south or west side of the planting will keep the fruits from overheating the afternoon sun.
Snap beans are naturally shade-tolerant, but when high temperatures and blooming coincide, the beans shed their blossoms or set scrawny pods. A cooling shade cover can make a huge difference for bean plants that wilt badly in midday heat despite adequate watering. I use a double layer of tulle, which also excludes Japanese beetles.
Transplanting Under Cover
From midsummer to early fall, any seedling I set out will get some kind of shade cover. I can start fall salad greens three weeks sooner by setting out seedlings under shade covers, and in August I fill a shade cloth tunnel with cabbage, Chinese cabbage, and broccoli seedlings. The sun-hungry plants don’t need such shade for long, only until they are rooted, after which I switch to regular row cover, which admits more light while keeping out pests.