Growing Cucumbers in a Greenhouse Border

Why Grow Cucumbers in the Border?

Cucumbers are thirsty, hungry plants, and a greenhouse border suits them far better than most people expect. A growbag dries out in a single hot afternoon and its nutrient reserves are gone by mid-July; a well-prepared border holds moisture and food for months, and the roots can roam. The result is stronger growth, fewer bitter fruits and a crop that keeps going into September. Best of all, a border crop is almost self-supporting once the vines are strung up — no daily fiddling with drip trays and canes.

If your greenhouse has a soil border on only one side, that is plenty. Two or three plants will feed a household generously, with plenty of spare cucumbers for chutney, pickling and neighbours. Choose an all-female F1 hybrid such as 'Carmen', 'Diva' or 'Telegraph' — they crop heavily, produce straight fruits and save you the job of pinching out male flowers on older varieties.

Preparing the Border and Planting Out

Start a fortnight before you intend to plant. Fork the border over to a spade's depth and work in two bucketfuls of well-rotted manure or garden compost per square metre, plus a handful of blood, fish and bone. Cucumbers hate cold, wet roots, so if your border sits low, add grit or coarse sand to improve drainage. It is worth warming the soil with a sheet of black polythene for a week or two before planting.

Raise your plants from seed in late April or early May, sowing two seeds per 9cm pot at 20°C and thinning to the strongest. Plant out only when the soil has reached at least 12°C and night temperatures are holding steady — usually late May in the south, early June further north. Space plants 45cm apart and set them on a slight mound so water runs away from the stem rather than pooling against it. Keep the rootball level with the soil surface; planting deeply invites collar rot. Water in with tepid water, then leave them for a few days to settle.

Training Vines Up Their Supports

Vertical training keeps fruits clean, improves airflow and makes picking easy. Fix horizontal wires 30cm apart up the glazing bars, or run strong twine from a wire at the ridge down to a peg at each plant. Twist the vine gently around its support as it grows, checking every few days — the stems are brittle and soon resent being bent.

• Remove all sideshoots and flowers from the bottom 60cm of stem so air moves freely around the base.
• Train sideshoots out along the wires and pinch them two leaves beyond the first female flower.
• Stop the main stem two leaves above the ridge wire, or when it reaches the roof, so energy goes into fruit rather than endless vine.
• Cut off tired lower leaves as they yellow, but never strip more than a couple at a time.

Watering and Feeding for Crisp, Straight Fruits

Inconsistent watering is the main cause of bitter, misshapen cucumbers. Little and often is the rule: in summer, a well-grown plant in a border may want two or three litres a day, and more in a heatwave. Dribble water slowly around the root zone rather than flooding it, and always use water that has stood in a can overnight, so it is not shockingly cold.

Begin feeding once the first fruits have set. A high-potash tomato feed at half strength twice a week works well; once plants are cropping hard, move to full strength. A mulch of grass clippings or compost over the border surface slows evaporation and keeps the roots cool. If growth looks pale and soft with plenty of leaf but few fruits, you are overdoing the nitrogen — switch to a potash-rich feed and ease off.

Ventilation, Shading and Mildew Watch

Humidity and stagnant air are what bring down greenhouse cucumbers. Open vents and doors early on a sunny morning rather than waiting until the house is baking, and aim to keep daytime temperatures between 21°C and 27°C. Close up again in late afternoon to trap warmth for the night, but keep a little vent open if the forecast is mild and damp. Sudden swings from hot, humid days to cold nights are an open invitation to powdery mildew and botrytis.

Damp down the paths on hot mornings to raise humidity without wetting the foliage. Water at the root only, and remove any mildewed leaves promptly, cutting back to clean growth rather than tugging. Watch for red spider mite in hot, dry spells — fine webbing on the undersides of leaves is the first sign — and for whitefly, which clouds up when plants are disturbed.

Harvesting and Keeping the Crop Coming

Cut fruits while they are young and slim, using a sharp knife or secateurs rather than pulling, which can snap the vine. Pick every two or three days; a cucumber left to swell to marrow size tells the plant to slow down, and cropping drops away quickly. Regular picking is the single best way to keep cucumbers coming right through to early autumn, when a final tidy-up and a last feed will often give you a few more weeks of fruit before the vines finally give in.