Why is my courgette plant flowering but not producing fruit?

Usually pollination, and often simply timing. Courgettes, along with cucumbers, pumpkins and squash, carry separate male and female flowers on the same plant, and both need to be open at once for fruit to set.

You can tell them apart easily. A female flower has a miniature fruit already formed behind the petals; a male flower sits on a plain thin stem. Plants typically produce a flush of male flowers first, sometimes for a week or two before any females appear, which alarms new growers every season and resolves itself.

The classic failure is a female flower that forms a small fruit, grows a few centimetres, then yellows, softens and drops. That is an unpollinated fruit being aborted. The usual causes are too few pollinating insects — common in cold, wet or windy weather, and in enclosed greenhouses — or a mismatch where no male flower happened to be open on the day the female was receptive.

Hand pollination is reliable and takes seconds. Pick a freshly opened male flower in the morning, strip the petals back, and brush the pollen-covered centre directly onto the centre of an open female flower. One male will serve two or three females.

Other causes are worth ruling out. Very high temperatures, above roughly 32°C, can make pollen non-viable. Excess nitrogen produces enormous leafy plants with few flowers, so check what you are feeding — a high-potash tomato feed suits fruiting far better than a general-purpose one. Severe drought stress causes flower drop. And plants crammed too closely shade each other and discourage insects from finding the flowers.