Broccoli grows best in cool spring and fall temperatures. It is one of the cole crops, the family of Brassica oleracea that includes Brussels sprouts, cabbage, cauliflower, collards, kale, and kohlrabi.
Warm climates may get three harvests of broccoli by planting fast-maturing types in spring, fall, and winter.
In regions with spring and fall frosts, time the plantings so you put broccoli plants in the ground in early spring and early fall. Some varieties have been bred for heat tolerance and grow through the summer, but most grow best when temperatures are between 65 and 80 degrees F.
If you plant too early in the spring and broccoli plants are exposed to 30-degree nights and 50-degree days, the broccoli may think it's about to die and start prematurely producing tiny florets. This condition is called buttoning, which sounds cute, but the plants never produce larger heads.
Don't be surprised if your broccoli heads don't reach the same large size as the ones you buy in the supermarket. Because you're picking the heads fresh and small, the broccoli should be very tender.
How to Plant Broccoli
You can grow broccoli from seed by starting it inside about four or five weeks before your last expected spring frost. Check with your local Extension Service office for the date for your region. Plant seeds 1/2 inch deep in a seed-starting mix. Keep the mix evenly moist and growing in bright light.
Broccoli is easier to grow from transplant seedlings. Whether they've been grown in your house or inside a greenhouse (find them at garden centers), broccoli seedlings need to be hardened off by gradually exposing them to longer periods of sunlight over several days. Start by bringing the seedlings outside to a shady spot for 30 minutes, and slowly increase the amount of exposure outside over a week or two. Don't immediately put them in direct sun or they'll burn.
You can direct-sow seeds 1/2 inch deep into the ground as soon as you can work the soil and are sure the temperatures won't be too cold for growing.
At planting time, add compost to the soil or scratch in a balanced fertilizer (such as 10-10-10) according to label directions.
Space broccoli seedlings 18 to 24 inches apart. Place them in well-drained soil that receives at least eight hours of sun per day. If you live in a warm climate, consider spring-planting broccoli in partial shade to keep the broccoli from bolting, or going to seed when it gets warmer.
Broccoli plants need 1 to 1-1/2 inches of moisture each week. If you water, it's better to water deeply less frequently. Light, frequent watering can lead to roots clustered near the soil's surface, and broccoli's root system is already very shallow. Too little water can result in tough stems.
A 1- to 3-inch layer of organic mulch around the plants helps conserve moisture and keep weeds at bay.
Try several types of broccoli, and make note of how they taste. Some people believe the flavor is sweetest in the fall after a light frost.
Broccoli is ready to pick when the tops are dark green and full. Use a sharp knife to cut it straight across the stem. If you cut off the first large flower head that develops but leave the rest of the plant to grow, new side florets will grow. They'll be smaller but still taste delicious.
If you wait too long to harvest broccoli, each individual green bud turns into a small yellow flower that forms seeds if left to grow.