The most important part of growing broccoli and cauliflower is how you prepare the soil. Cauliflower and broccoli need firm, fairly compacted and rich soil. Your soil needs to be very fertile and not loose to support the broccoli and cauliflower. You'll need to start your soil preparation the autumn prior to planting and sow the seeds in the spring after the last frost has passed. Working your soil amendments and preparing the planting bed months in advance of planting allows the soil bed to firm down and compact during the winter months.
Select a spot in your garden that's in partial sunlight and has firm, heavy and fertile soil. Ensure that you select a spot where you did not grow broccoli, cauliflower, Brussels sprouts, turnips or cabbage the previous year.
Prepare the planting bed in early autumn. Spread a 2-inch layer of organic compost and rotted manure onto the soil and work it into the soil to a depth of about 6 to 12 inches using a pitchfork or rototiller.
Firm down the soil in the planting bed using a roller or by stomping the soil down using your feet. Spread an all-purpose, slow-release fertilizer over the planting bed in the early spring, about three weeks before planting the broccoli and cauliflower, raking it gently over the soil surface.
Plant the broccoli and cauliflower seeds 1/2 inch deep into the soil, 1 inch apart and in rows that are about 3 feet apart. Water the seed bed generously to keep it evenly moist at all times.
Thin out the seedling plants when they're at least 1 inch tall, removing all of the weakest seedlings. Ensure that the remaining strongest seedling plants are spaced about 15 to 24 inches apart in the rows.