Broccoli is a cool-season brassica (related to cauliflower and other cole crops) that prefers temperatures between 65 and 75 degrees Fahrenheit. It matures quickly, allowing gardeners to grow it in both spring and fall. To get a head start on spring planting, however, broccoli seeds are sown indoors, then transplanted into the garden after the last annual frost date, following a brief hardening-off period.
Select a sunny indoor location, such as a greenhouse or a south-facing window that receives several hours of sunlight. Alternately, purchase a grow-light. Most regular indoor lights are not sufficient to grow broccoli indoors.
Purchase or mix your own seed starting soil, or seedling soil mix. This mix should be light, well-draining, and fertile. If making your own, include compost both for fertility and to ensure the mix has a light, well-draining texture.
Fill a seedling tray with seed starter soil. If you will be growing the broccoli indoors for a long time, and are concerned about insufficient room for the roots as the broccoli ages, sow seeds in pots instead. The optimum depth is about a quarter inch, spacing about 3 inches apart.
Moisten the soil with a water spray bottle to prevent moving the seeds around or flushing them. Keep soil evenly moist until germination, and keep soil temperatures closer to 75 degrees for fastest germination. After germination, temperatures are more favorable closer to 60 degrees.
Transplant to the garden or to the final growing area when broccoli has four to five true leaves, roughly four to six weeks after planting seeds. The first two leaves that broccoli produces during germination are not true leaves.
Grow broccoli to maturity indoors only if you have a growing area that receives at least six hours of direct sunlight every day, or with grow lights timed to provide at least six hours of light per day. Each broccoli plant will need approximately three feet of growing space for a large head, less for a smaller head that is harvested twice. Ensure that broccoli has at least 6 to 12 inches of soil depth for roots to grow in, or grow broccoli hydroponically according to your hydroponic system specifications.