Till the soil with a hoe or spade to a depth of 6 inches once the soil thaws and dries out enough to work in early spring. Alternatively, plant turnips in early fall once the soil temperature reaches 65 degrees Fahrenheit. Avoid digging in wet garden beds, because this causes the soil to clump up and harden.
Spread 1 1/2 lbs. of 5-10-10 fertilizer over every 50 square feet of turnip bed. Mix the fertilizer in with the loosened soil before planting.
Sow the turnip seeds on the soil surface in rows spaced 12 to 24 inches apart. Plant up to 20 seeds per foot of row. Cover the seeds with 1/2 inch of moistened soil.
Thin the seedlings after they sprout and once the majority reach a 4-inch height. Pluck out the extra seedlings so the remainder are spaced approximately two inches apart.
Water the turnips only when the soil begins to dry. Provide enough moisture to penetrate to a 4- to 6-inch depth but avoid overwatering that leads to soggy soil.
Harvest the green tops once they become large enough to use, usually once they grow to a 4-inch length. Cut the tops off at the base of the plant. The turnip continues to grow new tops until hot weather or a hard freeze kills the plants.