How to Grow Broccoli
Broccoli can be direct-seeded in the garden but the chances of growing high quality plants are better if you start broccoli in transplant trays. Temperature is a more critical factor than day length. You want to hold your seedlings 4-5 weeks and then transplant to the garden when they can grow another 5-6 weeks without temperatures rising above 80 degrees F. The best flavor is achieved when broccoli matures during cool weather. As a general rule target two weeks before your last frost date to transplant in the spring. That means you should start your seeds 6-7 weeks before the last frost date. Broccoli can withstand light frosts if they are not prolonged, but exposure to a lengthy cold snap with temperatures below 40 degrees F will injure the plant and initiate early flower development.
For fall broccoli, calculate your seed sowing date so your plants head a month in advance of killing frost. This may mean sowing seeds as early as July, but beware, high mid-summer temperatures result in a higher percentage of plants that bolt and heads of inferior quality.
Plant your transplants 12-24" apart in the row with 18-36" between rows.
For the tenderest broccoli, harvest the heads when they are still compact, 4-6" in diameter. If you see the florets beginning to separate, you can still harvest the broccoli but the quality will be diminished. A day or two can make a difference between broccoli at its best and broccoli that is overly mature, so monitor closely once the heads begin to form.
To encourage the development of side shoots, harvest the main head by cutting 10-12" down the stalk. Cutting this low eliminates some of the nodes along the stem that will generate side shoots. By eliminating some of these nodes fewer shoots develop, but the ones that do will concentrate the plant's energy and produce larger heads.
Aphids are a common pest of broccoli, concentrating on the leaves, sometimes in seemingly overwhelming numbers. These can be washed off with a jet of water accompanied by rubbing gently the surface of the leaf between your thumb and fingers. Broccoli is also a favorite host plant for cabbage loopers and other worms. These will shred your plants quickly. The best way to control caterpillars and other worms is by spraying with the biological control, Bt, at the first sign of damage.
Seeds can be kept for three years under good storage conditions.