Brussels sprouts (Brassica oleracea var. gemmifera) grow slowly and are not tolerant of high temperatures, characteristics that make them a challenge to grow in climates with long, hot summers. They are biennials -- taking two years to complete their life cycle -- and are able to survive the winter in the relatively mild climates of U.S. Department of Agriculture plant hardiness zones 7 to 9. If left to grow into their second season, the plants will flower and set seed, but they are most often grown in the vegetable garden as annuals and pulled up after the harvest of their sprouts late in the season.
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Brussels sprout seeds need temperatures above 50 degrees Fahrenheit to germinate, but in most areas, sowing seeds directly in the garden in early spring doesn't produce a good crop because the plant begins to produce sprouts about the time that summer heat peaks. More often, gardeners start seeds indoors in the spring and set out transplants in the early- to mid-summer for a fall harvest.
Five to six weeks after germination, seedlings are typically about 6 inches high. They have round or heart-shaped, gray-green leaves with a thick, waxy texture.
Mature Brussels sprout plants are tall and upright, reaching a height of about 2 or 3 feet and a spread of about 2 feet. They have a thick, light gray-green main stem from which grow the large, heavily veined leaves.
The edible sprouts develop from buds at the plant's leaf axils, the point where the leaf's stem, called a petiole, meets the main stem of the plant. The sprouts themselves consist of clusters of small leaves that form a tightly packed head -- like a mini cabbage -- 1 to 2 inches in diameter; sprouts that develop during warm weather may form more loosely.
Sprouts begin to mature first at the bottom of the plant and work their way upward, and sprouts are typically ready to harvest within 90 to 100 days after the plants are transplanted into the garden.
Plants left to overwinter will only flower and set seed if they're exposed to temperatures below 45 degrees Fahrenheit for a month or two. When it does flower in the spring of its second year, the plant bears clusters of bright yellow, four-petaled flowers on a tall flower stalk at the top of the plant, and at this point any sprouts on the plant will have an unpleasant flavor.
Among commercially developed Brussels sprout cultivars, some have been bred to reduce the species' long growing season and increase its heat tolerance. 'Bubbles,' for example, matures after about 82 days and is especially tolerant of warm temperatures. 'Jade Cross E' and 'Oliver' are notable for their sprouts, which are large and relatively easy to remove from the plant. 'Rubine' is an unusual variety that produces red sprouts but is very slow-growing, taking about 105 days to mature.