Turnips can both be grown spring and fall, and there are cultivars appropriate to each season. We put them in the third section of our rotation, even though they are brassica relatives. This way they get the fertility appropriate to their status as root crops, yet are still not planted in a bed that has had or will have brassicas in a successive year.
Spring turnips should be planted in succession from the moment the ground can be worked in spring until a month or so before the onset of hot, dry summer weather. Sow the seed 1/4 inch deep in rows only six inches apart in the bed, as the plants will never have a chance to get very large. As soon as they have their first true leaves thin them to two inches apart in the rows. Fall turnips are much larger and later, and should be planted ninety days before the first fall frost, 1/4 to 1/2 inch deep in rows eight to twelve inches apart. As soon as they have their first leaves, thin them to three inches apart in the row.
Pests & Diseases Of Turnips
Aside from soil diseases which can be avoided by consistent crop rotation, the major pests of both turnips are the same root maggots that bother other brassica crops, and the control is the same: cover the rows immediately after planting with a floating row cover, and don’t remove it during the active laying season of the fly. Remember the principle of trap crops: it is often worthwhile to set out a few leftover, susceptible plants in an out-of-the-way part of the garden to attract particular pests, which can then be easily found and destroyed.
Harvest spring when the roots are one to two inches in diameter. Fall turnips can be harvested for fresh use they can be harvested at any time, and many Southern gardeners relish turnip leaves for boiling greens. For storage, fall turnips (and rutabagas) should be left until the tops have been nipped by frost. But be sure to harvest before fall frosts are hard enough to damage the crown of the plant, or else they will rot in storage — usually about 25F/-4C is the critical temperature to avoid. The roots can be protected from an early frost if necessary by hilling soil over the crowns. Both fall turnips and fall radishes should be stored in damp sand or leaves in a cold basement or root cellar, where the temperature will stay 35-40F/2-4C.