Keep broccoli from bolting with steady watering and shade
A bolt of lightning is a sudden burst, and a horse that is spooked will bolt or run suddenly. And when we are scared in bed, we sit bolt upright.
Sometimes the garden is bolting, not from lightning or scary noises, but from stress. Broccoli (Brassica oleracea var. italica) is famous for bolting, or going to seed, when the weather gets hot. The Romans called broccoli “the five green fingers of Jupiter.”
Bolting is the broccoli plant's response to stress – it is attempting to quickly produce seeds before dying. Not only does hot weather cause bolting, but also conditions such as longer daylight hours and warmer soil temperature can cause the response. Broccoli is a cool-season vegetable that grows best with a soil temperature from 65°F to 75°F.
The best way to prevent it from bolting is to keep the roots cool and moist. Use a deep layer of organic mulch, such as grass clippings, leaves or straw. These mulches will insulate the ground, keep moisture in the soil and block out direct sunlight. Because sunlight can be a major trigger causing bolting, try shading your broccoli plants using row covers or shade cloth. You can also plant taller vegetables such as corn and tomatoes next to the broccoli.
Try setting out new broccoli plants every few weeks instead of all at once. This way your plants will be at all different maturities, so when hot weather does hit, each plant will respond differently, and you may be able to rescue some.
Broccoli grows best in moist, loamy, well-drained soil with a pH between 6.0 and 6.8, although it will do fine in almost any soil as long as it drains well. Work in compost to lighten heavy soils such as clay or rocky dirt. Fertilize your plants about three weeks after transplanting, using a fertilizer that is low in nitrogen, such as a 5-10-10 formula.
Keeping broccoli steadily watered will prevent stress and help avoid bolting. Water at least 1 to 1 1/2 inches every week.
You can also plant broccoli varieties that are resistant to summer heat such as Flash and Green Magic. The hybrid Destiny is exceptionally tolerant to hot weather, producing small to medium heads with tinges of purple.
Try planting heat-resistant broccoli varieties, keep your plants well watered and give them some shade, and you just might be able to keep them from bolting to seed. Otherwise, it is simply fate whether you get a summer harvest or instead get shot down by a “bolt from the blue.”