Plant in the spring or fall. Turnips thrive in cooler temperatures, so you should plant them when soil temperatures are still somewhat cool. For spring turnips, sow the seeds outdoors three weeks before the last expected frost. For fall turnips, sow the seeds in midsummer, roughly two months before the first expected frost of winter.
The soil temperature needs to be an average of at least 40 degrees F for the seeds to germinate, but temperatures between 50- and 70-degrees F encourage the most rapid growth.
Fall turnips are typically sweeter than spring turnips, and they are also less likely to attract root maggots.
Turnips thrive in full sun, so the area you choose should receive at least six hours of direct sunlight daily, if not a little more.
Loosen the soil with a rake or shovel to a depth of 12 to 15 inches, then mix in a 2-inch to 4-inch layer of compost.
Broadcast the seed or plant in rows in your prepared soil as evenly as possible. Cover the seed gently with 1/4 inch of soil for spring turnips or 1/2 inch of soil for fall turnips.
Gently water in.
When the seedlings reach a height of 4 inches, pull the weakest ones so that the strongest have more room and resources. If you only want to grow turnips for their greens, however, you should not thin them out.
Turnips need 1 inch of water per week. Any less will cause the roots to become tough and bitter, but too much more can cause the turnips to rot.
When the plants reach a height of 5 inches, add a 2-inch layer of mulch around the greens.
While not strictly necessary, a monthly application of mild, organic fertilizer can help strengthen the turnip root. Choose a fertilizer high in potassium and phosphorus rather than one high in nitrogen.
You should be able to harvest mature, ripe turnips after five to ten weeks depending on variety. See your seed packet for days to maturity.