Broccoli is cool-season crop, so it should be started in late winter or early spring (depending on your climate) for an early summer crop, or in mid- to late summer for a fall crop. High temperatures will affect development of the broccoli head (the harvestable part), so the goal is to get broccoli to mature before or after high temperatures are expected.
Broccoli seeds are capable of germinating in soil temperatures as low as 40°F (4°C), but warmer soil is preferred and will greatly speed up development.
For spring plantings, broccoli may be started indoors or outdoors a few weeks ahead of your last spring frost date. Consult our Planting Calendar to see recommended dates for your area. Generally speaking:
Start seeds indoors 6 to 8 weeks before your last frost date.
Sow seeds outdoors 2 to 3 weeks before your last frost date, or as soon as the soil can be worked in the spring.
For fall plantings (best in warm climates), sow seeds outdoors 85 to 100 days before the first fall frost, when soil and ambient temperatures are high.
HOW TO PLANT BROCCOLI
Broccoli requires a site with exposure to full sun (6 to 8 hours per day). Lack of sunlight may produce thin, leggy plants and subpar heads.
Plant in a bed of moist, fertile soil that drains well.
Soil pH should be slightly acidic, between 6.0 and 7.0.
If starting seeds outdoors, sow seeds ½-inch deep and 3 inches apart.
Once seedlings reach a height of 2 to 3 inches, thin them so that plants are 12 to 20 inches apart.
If you started seeds indoors, plant transplants that are 4 to 6 weeks old (and have 4 or 5 leaves) outdoors, 12 to 20 inches apart, in holes slightly deeper than their container depth.
Space rows of broccoli 3 feet apart. (Closer spacing yields smaller main heads, but more secondary heads.)