"Bolting," the horticultural term for a plant going to seed, results in tough, bitter vegetables. Cool-weather crops like lettuce, broccoli, spinach and sorrel especially dislike the dog days of summer. Bolting plants can be so exasperating that you might be tempted to bolt---right out of the garden! You can, however, reduce the likelihood of watching your nutritious edibles bolt, with just a little forethought.
Making the Bolt-Proof Garden
Respect your climate. In southern regions, some things just won't grow in July, no matter how much shade cloth you put up. If, however, bringing a head of iceberg lettuce to the table on a steamy Georgia night is your version of climbing Mount Everest, then by all means use your ingenuity to prove the naysayers wrong.
Know which plants are most likely to bolt. Leafy plants like spinach, lettuce--especially the crisp-heads-- and beets, onions, sorrel and broccoli lead the bolt risk list, so you'll want to pay special attention to them.
Learn to recognize the danger signs. "A good plant gone bad"--one that's bolted--has a characteristic look. Your previously low-growing, tightly budded green broccoli will suddenly sprout a long arm, seemingly, on which will soon appear the dreaded yellow seed heads. Sorrel, lettuce and spinach all send up similar stalks.
Choose bolt-resistant cultivars. Most breeders offer varieties that address the issue of keeping sensitive plants going during summer's height. Look for the phrase "bolt-resistant" or "heat tolerant" on your seed packet. As a rule of thumb, the varieties that take the shortest number of days to grow are the best because they won't be coming into their own just when the weather turns hot.
Sow seeds successively for spring and fall crops. If you're absolutely determined to grow crisp-headed lettuce, begin seedlings in the very early spring for spring harvesting, and late summer for a fall crop.
If buying seedlings, choose the smaller plants. The Oregon State University Extension Service has found that new transplants bolt less often when the stem's diameter measures less than that of a pencil.
Position your cool-season plants in the shade of buildings, taller crops or special screening systems. This practice reduces the risk of plants going to seed in the dog days of summer. Lettuce or spinach tucked in among the pole beans, corn or staked tomatoes stand a better chance of delivering succulent greens than those left unprotected.
Pick, pick, pick! Some plants bolt not just because of the weather, but because they need harvesting. Home gardeners can keep their broccoli going for months by cutting off the central head (the one that looks like what the supermarket sells) at the base of the plant, then harvesting the smaller side shoots that continue to form all season. Fail to keep up with those side shoot and you'll eventually get "riced" broccoli--those yellow flowers that appear when the shoot buds blossom. Lettuce, spinach and other cooking greens also bolt less frequently if you keep harvesting the outer leaves from the plants, leaving the center to regrow more greens.