What Causes Radishes to Bolt?

Genetics

• Fewer days to flowering -- and, thus, to bolting -- is a dominant trait in radishes, according to biologists Allison A. Snow and Lesley G. Campbell in their book chapter, "Can Feral Radishes Become Weeds?" Many radish varieties, especially spring varieties, have very short growing seasons so timing and planting conditions are critical to a quality harvest. Repeat plantings of radish seeds every week will extend the harvest if conditions are good, or ensure at least some harvest when conditions are problematic.

Day Length

• Most radishes are sensitive to day length. As days get longer, radishes start to flower. Planting radishes too late in the growing season or planting a variety at the wrong time of year are among the reasons for bolting. For spring radishes to mature, you must plant them in late winter, no less than about three weeks before the last frost date. Fall or winter radishes, which mature about the time of the first frost date, require about twice as long to reach maturity and typically produce much larger roots.

Temperature

• There is typically a close relationship between temperature and day length, with increasing temperatures accompanying increasing day length. An extended cool period before temperatures rise is necessary for radishes to flower, though. Unseasonable temperatures may cause radishes to bolt, however, even if you plant them at the proper time. Spring and fall temperatures ranging from 50 to 60 degrees produce the best radishes, according to the University of Massachusetts Amherst Extension. For fall and winter radishes, cold temperatures in the form of the first frosts improve both taste and texture, note Cornell University horticulturalists.

Considerations

• Know not only your radish variety and its requirements, but use your U.S. Department of Agriculture plant hardiness zone and typical first and last frost dates to determine planting dates to minimize the likelihood of bolting.

  One consolation if your radishes do bolt is that for open-pollinated varieties, bolting will produce seeds to plant for next year's crop. There are even some varieties grown for their immature seedpods rather than their roots, so bolting isn't always a problem.