I love radishes, but except for the fall types, rarely plant them alone. As mentioned in the carrot section, it is not really necessary to make separate plantings of spring radishes, as the harvest from the carrot rows will likely be enough for your needs. If not, there are a few simple rules to follow and you will get great radishes.
Spring radishes 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. Harvest when the roots are one to two inches in diameter.
Fall radishes are much larger and later, and should be planted about 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. For fresh use they can be harvested at any time, as radish greens are a well-known herbalist’s purgative, and are thought to strengthen the kidneys. For storage, though, the roots 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— 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 radishes and turnips store best in damp sand or leaves in a cold basement or root cellar, where the temperature will stay between 35-40F/2-4C.
Aside from soil diseases which can be avoided by consistent crop rotation, the major pests of radishes 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.