The same cool-season vegetables that delight gardeners in the spring can bring dismay at the height of summer. That's because many early bearing "cool weather crops" tend to bolt during the height of summer. You can prevent these veggies from turning bitter or woody by using shade cloth. Most commercial shade cloth makers sell their product by the percentage of sun blockage it provides. Use either hoop houses, tunnels or wooden frames to drape the shade cloth.
A study conducted by Kansas State University and the Organic Farming Research Foundation determined that lettuce and leafy green yield and quality increased tremendously through the use of simple tunnel frames covered with 39 percent shade cloth. A control group of lettuce and leafy greens planted in regular field fared much more poorly. "Shade-grown lettuce and greens grew larger, faster than their open-field-grown counterparts, and appeared to have consistently higher quality, being consistently cleaner (due to lack of rain splashing) and more succulent," the study concluded. The research noted that both head-type lettuce and the varieties that can be harvested throughout the season benefited from shade cloth.
Mother Earth News points out that a homemade version of shade cloth, using two sheets of black nylon netting, provides about 30 percent shade. Adding lightweight cotton provides even more protection during very hot weather.
Sorrel, with its large leaves and lemony zest, lends itself well to the kinds of pureed soups and sauces that taste especially refreshing on hot summer nights. And sorrel plants continue to produce those leaves from very early spring to late fall. The only problem? Sorrel tends to go to seed when exposed to too much heat. Use shade cloth much the same way you would with lettuce crops. But because sorrel is a perennial vegetable, you might want to take more care setting up a tunnel or raised-bed system. Choose Rumex acestosa, the classic garden sorrel that grows about 3 feet tall and has large leaves, or low-growing Rumex scutatus (French sorrel), whose smaller leaves work best in salads rather than as a cooked green.
Along with sorrel, many other cooking greens tend to suffer in high heat, including spinach, mustard greens, kale and collard greens. Even sturdy Swiss chard appreciates shade cloth in excessive heat. Extend their seasons by using 39 percent shade cloth--more in extremely hot regions or during unusually hot weather. Continually plant new seeds in adjacent beds to enable whole-plant harvesting for several months in a row.
Broccoli treats gardeners to gratifyingly early harvest seasons, but will bolt if not harvested quickly enough. This is a shame, because broccoli's season can be extended by simply collecting its side shoots, rather than going the "off with their heads" whole-plant harvest routine. The former method only work when the plants are prevented from bolting. Use 30 to 45 percent shade cloth to keep yourself in broccoli shoots for months.
Other cool-season crops that may benefit from shade cloth include cauliflower, peas, celery and celeriac.