When to sow courgettes

Courgettes are fast, frost-tender plants. Start them indoors from late April to mid-May; sown earlier, they outgrow their pots before it is warm enough outside. The seed germinates in 7–14 days at 64–72°F.

Plant out in late May or early June, once the last frost has passed. In Germany, Austria and Switzerland that means after the Ice Saints cold spell in mid-May. Young plants are best set out while still small, with a well-rooted pot and no more than three or four true leaves.

You can also sow directly where they are to grow from late May or early June: two or three seeds per spot, thinned to the strongest.

Indoors, push one seed on its edge about 0.4–0.8 in deep into a small pot of seed compost. Outdoors, sow 0.8–1.2 in deep.

Courgettes are hungry plants. Choose a warm, sunny spot with humus-rich soil that holds water but doesn't stay waterlogged, and dig in plenty of compost or well-rotted manure a few weeks before planting.

Give each plant about a square metre, or 35–39 in each way. That is why a 120 × 80 cm raised bed really only holds one plant. For most households one or two plants are plenty: a healthy plant in good weather gives three or four fruits a week.

Courgettes need steady moisture, especially from June to mid-August while they fruit. Water generously at the base of the plant rather than over the leaves, and no later than midday, so the leaves dry before night. A thick mulch around the plant, kept off the stem, holds moisture in.

Uneven watering and drought stress the plant: fruits can turn misshapen, and stress is one of the causes of bitterness (see below).

Young plants are a favourite of slugs, so protect them for the first weeks after planting out.

The first fruits are usually ready five to six weeks after planting out, from July. Pick them young, at about 4–8 in long; they taste best and the plant keeps producing. Check plants several times a week, because courgettes grow astonishingly fast.

Regular picking keeps the plant cropping over a long season. If a fruit hides under the leaves and grows into a marrow, cut it anyway.

Courgettes can very occasionally contain cucurbitacins, bitter substances that cause serious stomach and gut upsets. Cooking doesn't destroy them. The rule is simple: if a courgette tastes bitter, spit it out and throw the fruit away.

In home gardens there are two usual causes. Seed saved from your own plants may have crossed with ornamental gourds and brought the bitterness back, so buy fresh seed rather than saving your own. And stress, heat and drought especially, can raise levels, which is one more reason to water evenly.

Powdery mildew coats the leaves in white dust. Plants that repeatedly dry out are more prone to it, and still, humid air helps it spread. Keep the soil evenly moist, give plants space and sun, and remove badly affected leaves.

Small fruits that yellow and rot from the tip were usually not pollinated. That happens in cool weather, when few insects fly; it normally sorts itself out once it warms up.

Courgettes belong to the cucurbit family with cucumbers, pumpkins, squash and melons. Leave three to four years before courgettes grow in the same bed again, and treat the other cucurbits as part of the same group.

Traditional planting tables pair courgettes with onions, beans, beetroot, carrots, fennel and dill and keep them away from cabbages. We found no trials behind these pairings; they are tradition. In practice the big leaves shade anything low nearby, so give neighbours enough room.