How To Prevent Bolting In Vegetable Crops

Sunny background with autumn flowering green broccoli.

Why is my broccoli making flowers instead of nice round heads?  Broccoli, cauliflower, cabbage, kale, and Brussels sprouts are all Cole crops. They perform best in the cool temperatures of early spring vs other warm-weather summer garden vegetables like tomato, pepper, eggplant, squash, cucumbers, and such. Sometimes Cole crops will bolt before we can harvest. Can we prevent bolting? Read on to learn how to prevent bolting in Cole crops.

What Is Bolting?

Spinach starting to bolt
Photo by Howard F. Schwartz, Colorado State University,

Bolting is blooming. The flowering of many winter annual and biennial Cole crops is influenced by complex interactions between temperature, day length, and stresses of various kinds. Plant stress is usually the number one reason for bolting. However, periods of cool temperatures during early growth, followed by long daylight hours are a major contributing cause of unwanted bolting in most vegetable plants that are prone to bolting, like Cole crops.

Day Length Dilemma

Fresh organic leaves of spinach in the garden .

Many Cole crops are long-day plants, meaning that their transition into flowering is triggered by increasing day length. When plants like spinach exceed a critical day length level in later spring, they want to bolt. This is a totally natural process for them. You cannot do anything to stop it once it starts.

Sowing to Harvest

Gardener preparing broccoli seedlings for planting in freshly ploughed garden beds on a beautiful sunny morning. Organic gardening, healthy food, nutrition and diet, self-supply and housework concept.

In addition to day length requirements, most Cole crops like this require hours of cold temperatures to remain in their vegetative growing state before they begin to flower. This is a process called vernalization. This vegetative state, before a crop becomes responsive to vernalizing, is called its juvenile period. Not all vegetable biennials have a juvenile period, but with some Cole crops vernalization can start in seed form or it begins as soon as the seed sprouts. At this point, it’s a race, a slow race. This is why it is beneficial and critical to sow seeds of these types of plants early and harvest as soon as possible before these plants want to bolt. A string of cool days or nights can cause these types of plants to flower sooner than desired. It is important to know that this is an uncontrollable function of mother nature and how these plants themselves have evolved. It is nothing the gardener or seed does wrong.

Gardeners Have Some Control Over Bolting

Morning light with organic kale on a farm with automatic watering system.

The best way to avoid bolting of brassica or bolt susceptible crops is to plant them early in the season, during optimum temperatures of 50° to 75°F.  When temperatures increase, as they will, and day-length increases as it always does, these types of plants will naturally start the process of producing flowers to then make seeds for their own reproduction and survival. This is natural.  However, there are other environmental stresses that can trigger plants to bolt, including disease, drought, and nutrient imbalances. These are things gardeners do have some control over.

Additional Bolting Prevention Measures

PH meter digital device pricked in the soil. Cabbage plants.
  • Choosing varieties that indicate they are “bolt resistant”.
  • Keep plants consistently watered.
  • Apply an ample amount of moisture-preserving mulch.
  • Maintain healthy plants with proper soil pH and appropriate fertilization.
  • Utilize floating row covers as crops mature to help moderate ground temperature.
  • If you miss the early spring planting window, wait to sow in late summer for fall harvest.

Bolt Resistant Cole Crop Varieties

Cabbage (Brassica oleracea) in vegetable garden. Leafy green vegetable. Rich source of vitamin c and vitamin k. Fresh headed cabbage. Organic cabbage vegetable farm. Plant cultivation. Agriculture.

Other Recommended Reading

Little white cup of espresso coffee, opened book, blue semi-transparent vase with purple lilac flowers on rustic wooden table in the garden at spring morning after sunrise or at evening before sunset

Broccoli, cauliflower, cabbage, kale, and Brussels sprouts are all wonderfully flavorful vegetables. They can be a little challenging to grow but well worth the effort.

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