Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Bolted spinach plant. This variety is not well adapted to spring planting.
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 (daylength effect). High temperatures will accelerate bolting in spinach and lettuce. Lettuce may also be induced to bold just by high temperature stress.
Seedlings exposed to low temperatures early in the season may also be induced to bolt. This is called vernalization. Many mustard family plants need a cold period (vernalization of seedlings) 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 a greater amount of cold to initiate the bolting response.
Overwintered Brassica crops such as mustards, turnips, kale, and collards will be fully vernalized and will bolt in the spring. They should be harvested before bolting occurs.
In the cole crop group, cabbage planted very early in cold springs may bolt. 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 must be of a certain physiological 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).
Use of transplants can also reduce bolting in Brassica crops. Transplants are produced from non-vernalized seed in greenhouse conditions and are less susceptible to bolting.
Select varieties that are adapted to the spring planting season (an example would be Savannah mustard). Choose 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.