Brassica oleracea var. italica Seeds | Broccoli
LATIN NAME: Brassica oleracea var. Italica
COMMON NAME: Broccoli
Plant Profile
Broccoli belongs to Brassica oleracea, the same species as cabbage, kale, cauliflower and kohlrabi, all developed from a coastal wild cabbage with thick, waxy, blue-green leaves. Because the crops differ by which part is enlarged rather than by species, the group is classified by cultivar group: italica for broccoli, capitata for cabbage, botrytis for cauliflower, acephala for kale and gongylodes for kohlrabi. Crossing between groups is easy, which is why a wild relative can be used to improve a market crop.
Two quite different vegetables share the name. In British usage broccoli once meant the overwintering types that throw many small purple or white spears in spring, while the single large green head is calabrese, a name taken from Calabria in southern Italy. The heading type dominates world trade because a whole head can be cut and packed in one movement, whereas sprouting types are harvested spear by spear over an extended period and are grown mainly where a cool spring follows a mild winter.
Broccoli is an outbreeder with a sporophytic self-incompatibility system, which is exactly what a breeder wants when producing hybrid seed: cross-pollination is enforced and inbreeding limited. Modern hybrid seed is also made with cytoplasmic male sterility, the best known source in this genus being the Ogura sterility cytoplasm brought in from wild radish, so one parent produces no viable pollen and all seed on it is hybrid. Doubled haploid lines, produced by culturing pollen, give completely uniform parents in a single step.
A broccoli head is living tissue that continues to develop after cutting. Warmth and the plant hormone ethylene accelerate the small buds toward opening, chlorophyll breaks down, and the head turns yellow from the outer beads inward. The response measures how much the vegetable is still a plant rather than a product, and it explains the post-harvest chain built around the crop: cutting at the button stage while the beads are tight and no petals show, cooling the head immediately, holding it near freezing in high humidity, and, in long-distance trade, lowering the oxygen and raising the carbon dioxide around the head to slow the change. Breeding contributes late-heading, heat-tolerant lines that keep the buds compact in warmer weather.
Science & Biotechnology
Brassica tissue separates two things that meet only when the plant is damaged: glucosinolates, stored in the cell vacuole, and the enzyme myrosinase, held apart from them in specialised cells. Chopping or chewing breaks the cells, myrosinase hydrolyses the glucosinolate, and the unstable intermediate that forms rearranges into an isothiocyanate. In broccoli the dominant glucosinolate is glucoraphanin, whose isothiocyanate is sulforaphane, the compound at the centre of most of the crop’s biomedical research and of work on the transcription factor Nrf2 and antioxidant response pathways. Heat destroys myrosinase but not the glucosinolate, so boiling a head without any further enzyme leaves much of the glucoraphanin intact but unavailable, while chopping and waiting before cooking, or adding a pinch of mustard powder at the end, restores the reaction.
Uses & Benefits
The edible head is a dome of meristems, each bud at the same stage of development, held on a stalk that thickens as the buds enlarge. Cut a head through the middle and the architecture is plain: a main axis, side branches and the buds themselves, all on a plant that carries large waxy grey-green leaves and a taproot with a dense mat of fine feeding roots. Left alone, the plant sends up a tall stem in its second year, opening pale yellow flowers with four petals arranged as a cross, followed by long slender pods that split when ripe — the reason commercial seed crops must be harvested on time.
Growing from Seed
Sow into a seed tray of loam-based compost with grit through it, covering the seed with about a centimetre of the mix, and prick the seedlings out into deep modules once they have a pair of true leaves; the plant builds a large root system quickly and resents being moved late. Set plants about forty-five centimetres apart in rows half a metre apart, burying the stem to the first true leaves so the plant is firm. Water steadily rather than in sudden floods, since an irregular supply followed by heavy rain splits heads, and keep the bed weed-free. Feed with a nitrogen-rich top dressing once plants are established and a light potassium feed as the head begins to form, and cut the central head while the beads are tight; on calabrese types the side shoots then develop and can be cut the same way.
Clubroot decides where this crop can be grown: the soil-borne organism distorts and swells the roots and survives in the ground for many seasons, so affected land should be given a break from brassicas and the pH raised with lime, which suppresses it. Cabbage white caterpillars and diamondback moth larvae are the main leaf feeders, and the standard defences are insect mesh over the crop and hand-picking of eggs and larvae. Cabbage root fly lays at the base of the stem, so firm planting, a collar and mesh all help, while cabbage aphid should be dealt with while it is still on the undersides of the lower leaves. Flea beetle shot-holes are worst on seedlings in dry weather.
Our Take
Treat broccoli as a cool-season crop and give it steady moisture, and the heads will stay tight enough to snap. I sow it alongside the other cultivars of Brassica oleracea so that cabbage, kale and broccoli are cut over the same long season, and I keep black kale in the same plot for picking through the winter. Where you want the same mustard bite without waiting for a head, rapini gives it from a much quicker crop.
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