Broccoli is often planted indoors in the anticipation of being transplanted into the garden ground in the springtime. To get an early start on spring planting, growers often start their seeds indoors and transplant the seedlings, after a brief hardening-off period, into the garden after the last annual frost date in their area.
Though it is possible to grow broccoli indoors, you should attempt doing so only if you have a growing area that gets at least six hours of direct sunlight per day or grow lights timed to provide the same amount of direct exposure. Each broccoli plant should be allowed three feet of overall space for a large growing head and six to 12 inches of soil depth.
To grow broccoli indoors, select a sunny location, like a greenhouse, or a south-facing window that gets a lot of direct sunlight. Alternatively, you could purchase grow lights to provide additional light if you don’t have a location with sufficient sunlight. Next, buy some light, well-draining, and fertile seed or seedling starting mix and fill a seedling tray with seed starter soil.
If you plan on growing your broccoli indoors for a long time and want the roots trained to expand deeper as the broccoli matures, then plant your broccoli in pots instead of seedling trays. Space seeds about three inches apart, planting each seed about a quarter inch deep.
Then moisten the soil using a gentle spray from a spray bottle, keeping the soil evenly moist until germination and keep the temperature around 75 degrees F to speed up germination. After germination, preferred temperatures for broccoli drops to around 60 degrees F. When your broccoli plants have developed four or five true leaves (first two leaves are not true leaves), which generally happens around four to six weeks after sowing your seeds, it is safe to transplant them into the garden soil.