Turnips and Rutabaga are cool season crops that requires temperatures between 50 and 70 degrees to grow to maturity. The soil must be at least 40 degrees for seeds to germinate which typically takes about 7-14 days. Hot weather will cause the leaves to brown and turn the vegetable root woody and bitter.
Turnips and rutabagas are fairly tough crops that can be grown in almost any type of soil, but they do best in loose, organically rich, water-retentive but well-drained soil that's been worked deeply. The pH should be between 6.5 and 7.0. Phosphorus helps with root development so an application of rock phosphate or granite dust is helpful. Too much nitrogen will produce thick leaves but underdeveloped roots, so don't over fertilize or use uncomposted manure.
Because turnips and rutabagas are light feeders, they can be rotated with heavy feeders like corn or squash. But don't plant your crop near mustard greens, which will inhibit its growth, or near other brassica cousins such as broccoli or cabbage, since all family members are susceptible to the same diseases. Turnips and rutabagas can handle partial shade, so climbing peas make a good companion plant.
Neither of these root crops transplants well, so sow your seeds where you intend to grow them. Sow a spring crop 1/4 inch deep; plant fall crops 1/2 inch deep two months for turnips and three months for rutabagas before the first expected frost. Seeds may be broadcast and later thinned to three or four inches apart, or they can be planted in rows 18 or more inches apart. Give rutabaga plants six inches in which to grow.
Keep down weeds and aerate the soil with hoeing and hand-cultivating, and never, at any stage, allow the bed to dry out. You want to keep these vegetables growing fast and continuously, and water is the secret. Constant moisture will produce a good, well-flavored, tender crop, while lack of moisture will make the roots fibrous and strong-tasting and will force the plant to send up a seed stalk. When the leaves are about five inches long, you can mulch the plants to keep weeds down and moisture in.