Broccoli is a cool-season vegetable. It thrives in early spring, producing an abundant harvest before other warmer-season vegetables begin to produce. Start your broccoli inside in late February for an even earlier harvest. Starting seeds indoors gives your plants a jump start on the growing season, so when the outdoor planting time arrives the plants are already well on their way to maturity. While broccoli tolerates some frost, it doesn't tolerate extreme freezing, so this also gives you a chance to grow the vegetable in areas with short springs and long winters.
Fill individual, 3-inch-diameter seedling pots with a sterile potting mix. Fill each container to within ¾ inch of the rim of the pot.
Sow one to two broccoli seeds per pot. Plant the seeds approximately 1/4 inch deep, or to a depth twice that of the seed's width.
Water the soil until it is evenly moist throughout but not soggy. Cover the pot in a plastic bag to help retain the moisture while the seeds are germinating.
Place the pots in a cool room (55 degrees F is ideal) to germinate. Unlike many other vegetables, broccoli requires cool temperatures to germinate. Remove the plastic bag once seeds germinate, usually within seven days of sowing.
Move the broccoli seedlings to a sunny windowsill. Water as needed to keep the soil moist but not soggy. Rotate the pots every two to three days so the plants receive equal light on all sides and grow straight.
Transplant the broccoli seedlings outside to a full-sun garden bed three to four weeks before the last expected spring frost in your area. Broccoli takes approximately 55 days to reach harvest and grows best when daytime temperatures are below 60 degrees F.