How Does Broccoli Grow

If you have never grown broccoli in the garden, you might be surprised to learn that the part of the plant we eat is a flower, or more precisely, a cluster of immature flower buds harvested before they open. Broccoli has an interesting growth habit, one developed through thousands of years of cultivation. To understand how to grow broccoli in the garden, it is helpful to understand how broccoli grows and develops.

The Origins of Broccoli

Broccoli and its relatives—cabbage, mustard, Brussels sprouts, cauliflower, kale, kohlrabi, and others—all began as the same plant: a wild mustard. Through thousands of years of human selection and breeding, different cultivars were developed from these wild plants. Plants that produced desirable foliage were selected and bred into mustard and kale, while those with large terminal buds led to the many types of cabbages available today. Brussels sprouts were bred from plants that produced large lateral buds along the shoots. Broccoli plants, as well as cauliflower, were selected for their dense flower heads.

How Broccoli Heads Grow

The earliest broccoli plants looked quite different from those grown today, but over time and through human selection plants came to produce larger and larger flower heads. Most varieties grown today produce a large dome-shaped flower head on a thick stem rising above a rosette of large basal leaves. Producing those large heads requires large amounts of water and nutrients to feed the basal leaves which produce energy necessary for flowering.

Broccoli plants are typically transplanted into the garden when they have just 4-6 small leaves. These leaves capture the sun’s energy and, through photosynthesis, produce food for the growing roots, stems, and foliage. With each new set of leaves, the plant’s ability to capture sunlight and turn it into energy increases, making the plants grow more quickly. This vegetative growth continues, with plants growing upwards and outwards, while also storing energy for future flowers. 

Once the plant has stored enough energy to bloom, it shifts resources from vegetative growth into flowering. For most broccoli varieties, that energy is directed to the flower bud at the top the main stalk. The flower bud expands, its stem thickening as hundreds of buds develop into what we commonly call a crown or head. These heads are harvested while the flower buds are small and still tightly closed, and heads are four to six inches across. If left to grow too long, the flower buds will elongate and open, revealing hundreds of yellow blooms. Following harvest of the main head, smaller heads will form along remaining side branches.

How To Grow Broccoli

Broccoli plants are cool-season vegetables, meaning they require cool air and soil temperatures to grow.  Plants are very cold hardy, but do not tolerate heat well. As such, broccoli is planted in spring for an early summer harvest. Plant quality will decline as temperatures reach 80˚F. 

If you live in a hot climate, look for fast maturing varieties to ensure a crop before high temperatures arrive. Another option is to grow broccoli relatives like raab and gailon, which have relatively short maturation periods.  

Exact planting time varies depending on location but can be estimated based on the last spring frost date for your area. Broccoli seeds are typically started indoors, and the seedlings are transplanted into the garden two weeks before the last spring frost date. Seedlings can be purchased at your local garden center. If you prefer to start your own plants from seed, sow seeds indoors six to eight weeks before your last frost date. Broccoli needs about 4 to 6 weeks from seeding until the plants are ready to transplant.  

Like most vegetable crops, broccoli plants require full sun and rich, well-draining soils. Broccoli plants are heavy feeders and will benefit from incorporation of organic matter and nitrogen into the soil at planting time, and regular fertilizing through the growing season. Space plants 18 inches apart and water well. Maintain even moisture throughout plant development and fertilize about three weeks after transplanting to feed those growing plants.

Protect Against Common Broccoli Pests

A simple trick at planting time can save plants from a devastating pest called cutworms, which can wipe out an entire row of plants overnight. Cutworms are moth larvae that feed on the plant stems at the soil surface, often chewing through the entire stem and removing the plant tops.  Protect transplants by placing a stick, pencil, or straw alongside (touching) the plant stem. The cutworms need to encircle the stem completely to feed and the stick disrupts this behavior.

Another option is to plant seedlings inside rings made from toilet paper or paper towel tubes sunk one to two inches into the soil. Cutworms feed right at ground level and these rings (often called collars) create a barrier between the worms and the plants.  

One of the main pests of broccoli plants are cabbage worms, a complex of three butterfly and moth species: the cabbage looper, diamondback moth, and imported cabbageworm. Row covers will keep these pests at bay by preventing adults from laying eggs on the plants. 

Broccoli is an enjoyable crop to grow and great for gardeners looking to try something new. The heads are incredibly nutritious, with substantial quantities of protein, carbohydrates, calcium, and iron. They are also packed with vitamins including thiamine, riboflavin, niacin, carotene (vitamin A) and ascorbic acid (vitamin C).