Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Bolting is the term used for flower stalk formation in vegetables. Bolting response may be related to temperature, daylength, or a combination.
Bolting in spinach, lettuce, and some radishes (oriental types) will occur naturally as days get longer. High temperatures will accelerate bolting in spinach and lettuce.
Many mustard family plants need a cold period along with lengthening days to flower. The amount of cold needed depends on the species and variety. Mustards are very prone to cold initiated spring bolting; turnips, Chinese cabbage, and salad radishes require more cold to initiate the bolting response.
In the cole crop group, cabbage planted very early in cold springs may bolt and premature flowering in broccoli, cauliflower, kale, and collards also occurs when planted too early, or if the spring is abnormally cold. However, cole crop transplants have to be of a certain age to be susceptible to this cold-initiated bolting.
Other biennial vegetables such as beets, carrots, and onions also can be induced to bolt but only once plants have reached a certain size (they are past the juvenile growth stage). This is uncommon in our region.
Controlling bolting starts with planting during the recommended planting window. Early planting will contribute to bolting in some crops (such as cabbage), late planting in others (such as lettuce).
Select varieties that are adapted to the spring planting season (an example would be Savannah mustard). Chose slow bolting varieties of spinach and lettuce. Choose spring adapted varieties of oriental radishes and Chinese cabbage.
One issue that complicates this is the use of high tunnels for early production. High tunnels allow for earlier planting but cold snaps still may drop temperatures enough to cause the cold induced flowering response in many of these crops.