Bolting of crops overwintered in high tunnels is common in the spring. ‘Bolting’ refers to lengthening and blooming of the flowering stalk. Bolting is often a problem because the quality of the marketable part of the plant declines. Also, plants subject to bolting are programmed to die once they complete flowering and seed production so yield will decline in quantity as well as quality. Sometimes bolting is not a problem because the stalk, buds, and flowers can be sold as a new product while they last; this is often the case with kale, mustards and related crops.
Crops susceptible to bolting include those in the mustard family such as kale, mustards, tatsoi, bok choy (pac choi), mizuna, turnip, radish, etc.; carrots; beets and in some cases Swiss chard; onions; lettuce; and spinach. (Figure 1)
Figure 1. A variety of crops in the mustard family bolting in mid-March in a high tunnel.
Bolting is triggered by environmental conditions. Some plant types are triggered to develop flowers by extended periods of cool temperatures. This is called “vernalization.” In high tunnels vernalization might occur from late fall to early spring, depending on when a crop is planted and temperatures in the structure. After vernalization, when temperatures warm, the flowering stem lengthens, flowers continue to develop, and eventually bloom. Generally the warmer it is after vernalization has occurred, the faster the flowering process proceeds. All of the crops listed above except lettuce and spinach are triggered to flower by cool temperatures; for mustard family crops (e.g. radish) lengthening days further promote flowering. The wild relatives of crops with these traits are typically winter annuals or short-lived biennials: they emerge from seed one season, bloom the following spring, and then die–think of yellow rocket or Queen Anne’s Lace (wild carrot).
Lettuce and spinach also bloom in the spring, but their flowering is not triggered by cool temperatures. Long days trigger the switch to flowering in spinach, and high temperature combined with longer days trigger flowering in lettuce. These crops may not bolt until after the high tunnel has been rotated into summer crops.
Bolting is often predictable, so the first step in dealing with it is knowing what to expect for the crops you grow. With fall-planted crops it may be difficult to manage the high tunnel environment to eliminate spring bolting in crops where cool temperatures trigger flowering. However, it might be possible to slow development of the flowering stalk in the spring by keeping temperatures cool in the tunnel with frequent venting. For crops triggered to bloom by cool temperatures that are planted in the spring, it might be possible to reduce bolting by planting them a little later and managing for warmer temperatures inside the high tunnel. This is an area in which more research would likely lead to more useful recommendations.
Another way to deal with bolting is to look for existing crops and varieties of crops that are less susceptible to bolting, and over time, develop new ones through selection and breeding. This can be done on an individual farm, at research farms, and by those who breed new varieties. There are plenty of lettuce and spinach varieties already available that have been bred for delayed bolting in field production. Figures 2, 3, and 4 below illustrate some of the existing differences in bolting among kinds, types, and varieties of crops in the mustard family. In Fig. 2, tatsoi, mizuna and pac choi are all bolting by March 8. In Fig. 3, mustard ‘Ruby Streaks’ has completely bolted by March 30, but ‘Golden Frills’, ‘Giant Red’, and ‘Green Wave’ mustards are still mainly vegetative. In Fig. 4, kale varieties haven’t bolted by March 8, but by April 10, ‘Red Russian’, ‘Ripbor’, and ‘Vates’ are all flowering, while only a few plants of ‘Lacinato’ have bolted. With continuing increases in high tunnel production we can expect more knowledge about and development of crops and varieties that resist bolting in those environments.
Figure 2. Tatsoi, mizuna, and pac choi seeded in a high tunnel in late Sept. – Oct. bolted by early March the following year.
Figure 3. Mustards seeded in a high tunnel in Oct. varied in how early they bolted the following year.
Figure 4. Kale varieties seeded in a high tunnel in Oct. varied in how soon they bolted the following spring.