The term “bolt” means premature flowering in vegetables. Many cool-season vegetables, such as broccoli, cabbage, and onions, are grown and harvested before they flower. A number of these vegetables are from the Mediterranean region and grow best in temperatures between 50-70 F. Once the vegetable bolts, it is no longer good to eat.
Biannual or Winter Annual Vegetables
Many of the vegetables we plant and harvest in one season are really biannual or winter annual vegetables.
Biannual Vegetables
These vegetables put their energy into growing roots or tubers and foliage the first year, then use that energy to produce flowers and seeds the second year. Examples of these vegetables include all of the Brassicaceae or ‘Mustard’ family, including broccoli, cabbage, cauliflower, Brussels sprouts, and radishes.
Winter Annual Vegetables
These plants germinate in the fall or early spring. They are very tolerant of cold temperatures. They flower in the late spring or summer of the next year. Allums, including onions, garlic, and leeks, are an example of winter annual vegetables.
Triggers For Bolting
Bolting is caused by several factors. Here is a list of the most important.
Heat
When temperatures exceed the 70-degree point for a number of days, it can trigger bolting. In a year where the temperature goes from cold to hot with few warm days, bolting is more likely than in a year where there are months of warm temperatures between the cold and hot periods.
Day Length
In addition to heat, most of the plants need a certain day-length to flower. Depending on the cultivar, this can be 15 hours in some northern areas or 16-17 in more southern climates. Once the day-length is met, the plant is triggered and flowers.
Vernalization
After a plant has grown to a particular size, it needs a certain number of hours of temperature below 50 F in order to flower. For example, if you plant onions and have warm days with periodic freezes, then a number of cool days, they are more likely to bolt than if you have a warm spring. This is called vernalization. It is similar to chilling hours in fruit trees.
Stresses
Stresses, such as inadequate water, diseases, and few nutrients, can cause a plant to bolt. The plant reacts to these stresses by producing flowers and seeds while it still can, even if it is early in the season.
Preventing Bolting
Since bolting is triggered by multiple factors out of a grower’s control, there is little you can do to prevent it. Making sure the plants get adequate water, are free of disease, and have adequate nutrients will help control stress on the plant. The rest is up to mother nature. Many cool-season crops will grow better in the fall when temperatures and day lengths are unlikely to trigger the plant to flower.
Maintaining Crop Quality
As soon as you see evidence the plant is flowering, pinch off the flowers. You will need to harvest the vegetable soon or the flavor and texture will become undesirable. Some plants, such as lettuce, are ruined when the flower stalk begins to grow. Others, such as onions, will still be good as long as you harvest them immediately.
