Building a Simple Greenhouse for Melons and Cucumbers

By Summer Care for Thirsty Melon Plants
Comments 35
16 Jul 2026

Building a Simple Greenhouse for Melons and Cucumbers

Why a Roof Changes Everything

Melons, cucumbers, pumpkins and squash all trace their ancestry to warmer places than ours. Give them a sheltered, sun-soaked roof over their heads and the season stretches by six to eight weeks at each end: sow in March, plant out in late May, and you can still be cutting cucumbers in early October. The difference between a melon ripened on the vine under cover and one shipped in from a supermarket shelf is not subtle — it is the difference between perfume and water.

You do not need anything grand. A small polytunnel or a modest glasshouse of two or three square metres is enough for a couple of melon plants and half a dozen cucumbers, with room to start your pumpkins and squash off early before they go outside.

Choosing the Structure and Siting It

Site is everything. Choose a spot in full sun that receives at least six hours of direct light in midsummer — a position that gets less will give you plenty of leaf and no fruit. Look for shelter from the prevailing westerly wind, avoid the rain shadow of a large tree or hedge, and keep clear of overhanging branches that will drop debris onto the covering.

Polytunnel: cheaper, quick to erect, and warms up fast. A 3m x 6m tunnel is generous; even 2m x 4m works well.
Glasshouse: longer lasting and better light transmission, but pricier and slower to warm on a cold morning.
Location practicalities: firm, level ground, within reach of a water butt or tap, and not so far from the house that you neglect the daily door-opening.

If you rent, check your agreement before putting up anything permanent. A polytunnel on a timber base is easier to move than a brick-based glasshouse.

Raising the Frame

Most polytunnel kits use galvanised steel hoops set about 2m apart, dropped into base plates or driven into the ground. Set the hoops first and check they are upright and evenly spaced before you do anything else — a twisted frame will leave you fighting the cover forever.

Dig a trench about 20–30cm deep around the perimeter. Pull the polythene over on a still, mild day (cold plastic is brittle and unforgiving), then bury the skirt in the trench and backfill firmly. Fit a timber base rail to the inside for tying strings and anchoring staging. Add a door at each end, and if your budget stretches, roll-up sides — they transform ventilation on hot afternoons.

Ventilation and Watering

This is where most small greenhouses go wrong. Melons and cucumbers sulk above about 30°C, and pollen fails in the heat, so fruit sets poorly and bitter cucumbers appear. Aim to hold daytime temperatures between 21°C and 27°C.

Open doors and vents by mid-morning on sunny days, and close them by late afternoon to trap the evening warmth.
Hang a max-min thermometer at plant height and check it daily — you will learn your structure's habits within a fortnight.
Damp down the path on hot days to raise humidity, which cucumbers love and red spider mite hates.
Water little and often at the base, never over the leaves, ideally in the morning. Drip lines on a timer are worth every penny.

What to Grow, and How to Train It

Melons want rich soil and space. Plant on a low mound, train the main stem up a vertical string, and pinch out the growing tip at about five leaves to encourage side shoots. Let one or two strong sides develop, pinch those at two leaves beyond a fruit, and hand-pollinate female flowers with a soft brush at midday. Four fruits per plant is a realistic target — more will just be small and tasteless.

Cucumbers are the easiest win. Choose all-female varieties to avoid bitter, seedy fruit, grow two per growbag, and wind them up a string or cane. Remove the lower laterals and any fruit that forms in the first 30cm so the plant puts energy into climbing.

Pumpkins and squash prefer to finish outside, but starting them in the tunnel in April gives them a huge head start. Harden them off carefully in late May and plant out in early June once the risk of frost has passed. Bush varieties of squash will crop happily in a large pot or bed inside the tunnel, trained around a central cane.

Harvesting Well

Melons: they tell you when they are ready. The fruit slips from the vine with gentle pressure, the skin smells sweet at the blossom end, and the tendril nearest the stem turns brown and shrivelled.
Cucumbers: cut at 15–20cm, little and often — daily in high summer. Leave one to yellow and the plant slows down.
Pumpkins and squash: ready when the skin resists a thumbnail, the stem is corky and dry, and the fruit sounds hollow when tapped. Cure them in the sun for ten days before storing somewhere cool and airy.

On the Plate

A ripe melon needs nothing but a knife and a cold plate — a squeeze of lime, torn mint and a crumble of feta turn it into a proper lunch. Cucumbers shine as a quick pickle with rice vinegar, dill and a pinch of sugar, made in the morning and eaten by evening. Roast wedges of pumpkin or squash with sage, garlic and olive oil until the edges caramelise, then blitz what is left into a soup with nutmeg and cream. And do not throw the seeds away: rinse them, toss with salt and paprika, and toast in a low oven for twenty minutes. Nothing from that tunnel need go to waste.

Greenhouse

Roasted Pumpkin Soup with Warming Spices

High humidity encourages botrytis and mildew. Open vents on warm days, water in the morning, and avoid crowding plants together.

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5 Comments

Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply
- John Abraham says Jul 21, 2024 at 10:00am
  
  I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
  Reply
  - Robert Sonnysays Jul 21, 2024 at 10:00am
    
    I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
    Reply
Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply

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