How do I grow greenhouse cucumbers properly?

Greenhouse cucumbers want conditions that tomatoes merely tolerate: more warmth, much more humidity, and shelter from direct scorching sun. That is why growing them together is a compromise. Grown on their own terms they are one of the most productive things a greenhouse can do.

What you'll need

• All-female F1 greenhouse cucumber plants or seed, true
• String or a cane and horizontal wires, true
• A rich, well-drained compost or improved border soil, true
• High-potash feed
• Soft twine

Step by step

1. Choose an all-female greenhouse variety.Modern all-female F1 greenhouse cucumbers produce no male flowers, so no pollination occurs and no bitterness results. If you grow an older variety that does produce males, they must be removed — but with an all-female type this whole issue disappears.
2. Give them warmth and start them late enough.Cucumbers want warmer conditions than tomatoes and sulk badly below about fifteen degrees. In an unheated greenhouse, sow or plant out several weeks later than tomatoes, once nights are reliably mild.
3. Plant into rich, well-drained compost, slightly proud.Plant with the top of the rootball a little above the surrounding level and water around rather than onto the stem. Stem rot at the collar is the commonest way to lose a greenhouse cucumber.
4. Train the main stem up a string or cane.Same string arrangement as tomatoes, or a stout cane. Take the single main stem up to the wire and stop it there by pinching the tip.
5. Manage the side shoots by the umbrella method.Each side shoot carries fruit. Pinch each one out at two leaves beyond the first fruit. Once the main stem reaches the wire, let two side shoots run over the wire and hang down, pinching them when they reach halfway down. This is the standard glasshouse system.
6. Remove the lowest fruits and shoots.Take off any fruit and side shoots on the bottom 60cm of stem. It keeps air moving around the base, reduces rot, and lets the plant establish before it starts cropping.
7. Keep it humid and shaded from the harshest sun.Damp down the floor daily in warm weather. Cucumbers scorch in direct midsummer sun through glass, so shading benefits them more than it does tomatoes.
8. Water generously and feed once fruiting.Cucumbers are thirsty and shallow-rooted. Keep the compost consistently moist — never let it swing dry — and feed with a balanced then high-potash feed weekly once the first fruits set.
9. Pick constantly.Cut every fruit as soon as it is a usable size, with a knife rather than by pulling. A fruit left to mature on the plant signals that the plant's job is done and cropping slows or stops.

Tips

• Cucumbers are among the first plants to show red spider mite in a hot dry greenhouse. Damping down daily is both a growing requirement and a control measure.
• One or two plants is plenty for a household. A well-grown greenhouse cucumber is extremely productive and three plants will defeat you.
• Ridge and outdoor cucumbers are a different crop with different rules — they do need pollination, they are trailed rather than trained, and they tolerate much cooler conditions.

Common mistakes

• Letting the compost dry out between waterings — Cucumbers respond to water stress with bitter, misshapen fruit and by dropping small fruits. Consistency matters more than quantity.
• Planting too deep — Buried stems rot at the collar, which is the single commonest cause of sudden collapse in a greenhouse cucumber. Plant slightly proud and keep water off the stem.
• Growing them in full unshaded sun through glass — Leaves scorch and the plant checks. Cucumbers evolved in humid shade, and shading the house benefits them substantially.
• Leaving one fruit to grow large — Once a fruit starts maturing seed, the plant slows or stops setting new ones. Pick everything, even if you cannot use it.

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