Growing broccoli in a greenhouse gives you the ability to enjoy fresh broccoli year round. Although a greenhouse offers an enclosed environment in which to grow a variety of seasonal plants, if you live in an area with wide temperature swings, you must still control the temperature inside the greenhouse. Cold nights, for instance, can frostbite plants in an unheated greenhouse, wasting your hard work.
Install adequate central heating and cooling in the greenhouse, knowing that greenhouses get only 25 percent of their heat from the sun and require additional heating-and-cooling regulation in regions with moderate to extreme swings in temperature.
Install two fans, one facing inward, serving as air input and one facing outward, serving as air output (exhaust). Fans help ventillate the air and introduce fresh amounts of carbon dioxide. For small greenhouses of 200 to 600 square feet, select fans capable of moving 5,000 cubic feet of air, and for larger greenhouses of 600 to 1,200 square feet, select fans capable of moving 12,000 cubic feet of air.
Install wooden crates measuring a minimum of 3 feet wide per row and at least 2 feet deep.
Spread fertilized soil into the crates to a depth of 23 inches to 24 inches. The soil settles a little, so you should fill the crates as full as possible.
Plant the broccoli seeds 1 inch deep and about 15 inches to 24 inches apart in a layer of starter fertilizer that offers the broccoli seeds the required nutrients they need to encourage fast growth.
Water the soil as the soil becomes dry to the touch, but when the broccoli sprouts, do not water the actual "heads"; water the stalks at their bases. Watering the brocolli heads encourages inedible blossoms.
Allow the plant to grow 2 1/2 to 3 1/2 months.
Harvest the plants by snipping them off beneath the head allowing for 5 inches to 6 inches of stalk.