Broccoli, a member of the cabbage family, is a cool season vegetable that is easily started from seed. Several broccoli varieties offer an extended harvest period by producing substantial side shoots after the main head is harvested, including Green Comet and Packman. Other varieties that Ohio State University recommends for the home gardener to start from seed are Premium Crop and Green Hornet.
Plant broccoli transplants rather than direct seeding in the garden, suggests the University of Illinois, in order to give broccoli the head start it needs to bear its crop before the onset of extreme summer heat. Start broccoli seeds indoors six to eight weeks before the first frost-free date in your area.
Sterilize transplant trays in a 10 percent bleach solution. Allow them to air dry.
Place broccoli seed in a mesh bag (or in several layers of cheesecloth secured tightly) and put it in a water that has been heated to exactly 122 degrees F. Allow the seeds to sit in the water for 20 minutes, then plunge the mesh bag of seeds into cold water.
Fill the trays with moistened potting mix. Sprinkle broccoli seeds on the potting mix and cover with about ¼ inch of potting mix.
Keep the soil moist and maintain a temperature between 70 and 75 degrees F until seedlings emerge.
Reduce the temperature slightly once seedlings emerge so that the daytime temperature is between 65 and 70 degrees F and nighttime temperature is 50 to 60 degrees F. Thin seedlings to one per plug or cell, or for undivided flats, one plant every 2 inches.
Water seedlings about twice per week. It is fine for soil to dry somewhat between waterings, but do not allow plants to wilt.
Harden broccoli plants off 10 days prior to planting in the garden. Set the trays in a cold frame and reduce the frequency of watering, but do not allow plants to wilt severely.