Courgette

Cucurbita pepo

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At a Glance

Overview

Courgette (Cucurbita pepo) is a bushy or trailing annual vegetable in the cucumber family (Cucurbitaceae), grown in British gardens for its prolific crop of immature fruits harvested through summer and into early autumn. The same species also produces marrows, pumpkins and some ornamental gourds; the courgette is simply a C. pepo grown for its small, tender fruits picked before they mature. The following quick-care table summarises the essentials for UK growers; the full article below covers each in detail.

Courgette (Cucurbita pepo) is widely grown in UK allotments and kitchen gardens because it is one of the most productive summer crops a small space can yield: a single bush plant can produce ten or more fruits a week at peak. It is half-hardy, sensitive to frost, and in British conditions is raised under cover in spring and planted out after the last frost. The crop has a relatively short, intensive harvest season from late June or July through to the first cold nights of September, after which plants are usually cleared. Courgette flowers are also edible and are increasingly sold as a separate ingredient, particularly in Mediterranean-style cooking. The plant is grown from seed each year and is unsuitable for overwintering outdoors anywhere in the UK. In mild, sheltered gardens, a late sowing in June can crop into October, but yield falls sharply once nights drop below about 10 °C.

Appearance

Courgette plants develop a rosette of large, lobed leaves held on thick, hollow, prickly stems. The leaves are typically 20 to 40 cm across, palmate with five shallow lobes, dark green and often marbled with silvery patches along the veins; both leaf and stem are covered in fine, stinging hairs that can irritate bare skin. The overall habit depends on the cultivar: bush forms (the most common in the UK) form a compact, open clump 45 to 90 cm tall and up to about 1.2 m across, while trailing or semi-trailing cultivars produce longer stems that can run 2 to 3 m and are usually grown up a sturdy support or allowed to sprawl.

The flowers are large, bright yellow-orange, five-petalled trumpets 6 to 10 cm across, and are borne on separate male and female plants on the same individual (the species is monoecious). Male flowers usually appear a week or two before the first females and are carried on long, thin stalks; female flowers sit on the end of a tiny, immature fruit and have a swelling just behind the petals. Both flower types open in the morning and usually wilt by afternoon. The fruits are cylindrical, slightly ridged, and dark, mid- or pale green in most cultivars, although yellow, striped and round-fruited forms exist. Fruits are normally harvested at 10 to 20 cm long while the skin is still tender and the seeds undeveloped; if left, they swell rapidly into marrows.

Growing Conditions

Courgettes need a warm, sunny, sheltered site and a deep, fertile, moisture-retentive soil that drains freely in winter. They are greedy feeders and respond strongly to soil improved with garden compost or well-rotted manure in the autumn or spring before planting. A pH in the range of about 6.0 to 7.0 suits them; very acidic ground should be limed. In cooler or exposed parts of the UK, a south- or south-west-facing border at the foot of a warm wall is ideal, as it gives the plants the extra warmth and shelter they need to crop reliably.

The species is not frost-hardy and is killed by even a light frost, so it is grown as a half-hardy annual. Courgettes dislike cold soil: growth stalls below about 13 °C, and roots will rot in cold, waterlogged ground. A fortnight of cold, wet weather after planting out can check the crop severely, which is why most UK growers raise plants indoors and wait until late May or early June to plant them out, by which time soil temperatures have usually risen above 15 °C. Once established, plants tolerate short dry spells but yield drops sharply under sustained drought; consistent moisture from flowering onwards is the single biggest factor in a heavy crop.

Courgettes are not wind-pollinated: their fruit set depends on bees and other insects transferring pollen from male to female flowers. In cold or wet summers, when pollinator activity is low, fruits can fail to set, wither on the end of the female flower, or grow unevenly. Growing nectar-rich flowers nearby, and avoiding insecticide use near the crop, helps maintain pollination.

Planting and Care

Sow seed indoors in April or early May, 1 to 2 cm deep in 7 to 9 cm pots of multipurpose compost, on a warm windowsill or in a heated propagator at about 20 to 25 °C. Seedlings appear within a week and should be grown on in good light, then hardened off for 7 to 10 days before planting out. Plant out from late May to mid-June, once all risk of frost has passed, spacing bush cultivars about 90 cm apart and trailing cultivars 1.2 to 1.5 m apart. A handful of general-purpose fertiliser in the planting hole, or a mulch of garden compost after planting, gives plants a strong start.

Watering is the most important routine task. Water deeply once or twice a week rather than little and often, directing water at the base of the plant rather than over the leaves, and increase the volume once the first fruits start to swell. A thick organic mulch laid after planting conserves moisture and keeps the roots cool; black plastic mulch is also widely used on allotments to warm the soil. Feed with a high-potash liquid fertiliser (such as a tomato feed) every 10 to 14 days from flowering onwards; switch to a balanced feed if leaves start to yellow.

Pruning in the strict sense is not required, but removing the oldest, lowest leaves once the plant is in full crop improves air circulation and reduces grey mould (Botrytis) in damp weather. Pinching out the growing tip of trailing cultivars after they have set three or four fruits encourages side shoots and a longer harvest. Seasonal care in the UK follows a clear rhythm: sow under cover in April, plant out after the last frost in late May or early June, harvest from July, and clear plants in September or October once fruiting slows.

Propagation is from seed each year, as the plant is an annual. Courgettes cross-pollinate freely with other C. pepo cultivars, but this only matters where seed is being saved; for ordinary crops, any F1 or open-pollinated packet of seed will give a reliable result. Saving seed requires isolation from other C. pepo, hand-pollination, or bagging of flowers, and is rarely worthwhile in a normal garden.

Common Problems

The most common problem in the UK is poor fruit set, where female flowers fail to develop into fruits, or tiny fruits turn yellow and rot at the end. This is almost always caused by cold weather, lack of pollinating insects, or a mix of the two. Hand-pollination (transferring pollen from a freshly opened male flower to a female flower with a soft brush) is a reliable workaround in poor summers.

Powdery mildew is the most widespread foliar disease: a white, powdery coating spreads across the leaves in late summer, especially during warm, dry days followed by cool, damp nights. It rarely kills the plant outright but reduces yield late in the season. Resistant cultivars are available; otherwise, improving air circulation, watering at the base, and removing the worst-affected leaves all help. Downy mildew, which produces yellow patches on the upper leaf surface and grey fuzz beneath, can also occur in damp seasons.

Grey mould (Botrytis cinerea) rots flowers and fruits in cool, wet weather, particularly where plants are over-crowded or end-of-season tissue is collapsing; prompt removal of affected flowers and fruits, and wider spacing between plants, reduces the risk.

Pest pressure is generally lower than on many other UK vegetables. Slugs and snails graze young seedlings and the skin of fruits resting on the ground; lifting fruits off the soil on a straw mulch or a tile prevents damage. Aphids, particularly the black bean aphid, colonise the growing tips in midsummer but are usually held in check by predators. Virus diseases such as cucumber mosaic virus cause mottled, distorted leaves and warty fruits; affected plants should be removed, and nearby weeds that may harbour the viruses (such as groundsel and chickweed) kept down. Courgettes are not recorded as toxic to humans, and the fruits, flowers and leaves are widely eaten; as with any food, individual sensitivities are possible.

Popular Varieties

'Defender' F1 is a widely planted bush courgette with mid-green, lightly speckled fruits and good resistance to cucumber mosaic virus. It is a popular UK allotment and garden choice, cropping reliably from July through September. 'Black Beauty' is a traditional open-pollinated cultivar, the standard mid- to dark-green courgette of mid-twentieth-century British vegetable gardens, with a bushy habit and a long harvest period. 'Yellow Jacket' F1 and 'Soleil' F1 are bright yellow, smooth-skinned bush forms that hold their colour well on the plant and add variety to the harvest.

'Romanesco' is an older Italian open-pollinated type, prized for its ribbed, pale-green fruits and firm texture; it is widely available in the UK and crops heavily from a single bush. 'Eight Ball' F1 is a round-fruited bush cultivar producing tennis-ball-sized fruits ideal for stuffing. 'Tromboncino d'Albenga' is a trailing, climbing Italian cultivar (technically C. pepo var. torticollis) with long, curved, pale-green fruits eaten young as a courgette or left to mature as a winter squash. 'All Green Bush' is a compact, reliable open-pollinated form well suited to small gardens and containers. Cultivar availability varies year to year, and the names above are widely offered by UK seed merchants including Mr Fothergill's, Suttons, Kings Seeds and Thompson & Morgan, but specific stock should be checked with suppliers each season.

Pests and Diseases

For step-by-step help, read Treating Powdery Mildew. Or browse the full plant problem solver to diagnose an issue by symptom.

Courgette in our guides

• Drip Irrigation Systems: Save Time and Water
• Growing Vegetables from Seed
• How to Grow Courgettes: Sowing to Harvest
• Gardening Jobs for October in the UK
• Summer Vegetable Garden: What to Grow and When
• Treating Powdery Mildew