Bolting - Radish and Brassicas

Bolting - Radish and Brassicas - Solution Guide

Bolting - Radish and Brassicas

Flowering in most winter annual and biennial plants, such as radish or Brassicas, is influenced by complex interactions between temperature, day length, and stresses of various kinds. One of the leading causes of bolting vegetables is a period of cool temperatures during early growth, followed by long daylight hours.

Historical Origins

Biennial and winter annual vegetable crops are cool-season crops. They originate in the Mediterranean region and grow best at temperatures between 50 and 70°F. Many are also long-day plants, meaning their transition to flowering or bulbing is stimulated only after day lengths exceed the critical amount of day length in late spring or early summer.


In addition to day length requirements, most winter-hardy biennial and perennial crops in our growing region need a certain number of hours at cool temperatures after growing to a specific size before they will flower. This is called vernalization. The time before a crop becomes responsive to vernalizing temperatures is called its juvenile period or leaf stage. Not all vegetable biennials have a juvenile period; some can even vernalize as a seed or begin as soon as they sprout.

For these spring-planted crops, a string of cool days or dipping night temperatures can precondition those plants to flower sooner than desired. This unwanted, premature flowering is what gardeners refer to as bolting. While there are some steps that gardeners can take to help avoid this issue, some factors can still lead to plant bolting.

Environmental Stresses

Several other environmental stresses can lead to plant bolting.

  • Disease or pest pressures.
  • Drought or excess moisture.
  • Nutrient imbalances.

Steps to Avoiding Bolting

  • Plant cool season; bolt susceptible crops early, but not too early.
  • Plant successive crops at two-week intervals to limit losses due to unstable, unpredictable weather.
  • Plant according to the weather patterns, not the calendar.
  • Choose bolt-resistant varieties.
  • Maintain consistent soil temps by mulching appropriately.
  • Water consistently.
  • Don't over-fertilize. Especially limit nitrogen applications & animal manures high in nitrogen.