Managing Greenhouse Humidity to Avoid Mould

By Roasted Pumpkin Soup with Warming Spices
Comments 35
10 Jul 2026

Managing Greenhouse Humidity to Avoid Mould

Why British greenhouses become mould factories (and why your curcubits suffer most)

Our maritime climate is a blessing and a curse. Cool nights, damp mornings and sudden showers mean that a closed greenhouse traps moisture like a sealed jam jar. Melons, pumpkins and squash – all members of the curcubit family – have large, soft leaves and grow rampantly. When you pack them in, humidity can easily climb above 85%. That is the perfect nursery for botrytis (grey mould) and powdery or downy mildew. I have lost more fruits to a fuggy August than to any pest. The good news? You can beat mould with a few simple habits, starting the moment you plant out.

Ventilation: open up on warm days, but be canny

Still air is the enemy. On any day when the temperature outside reaches 15°C, crack open roof vents and louvres first thing in the morning. Even a 10‑minute air exchange shifts the dew point and stops condensation forming on leaves. On warmer days (20°C and above), open vents fully by mid‑morning. Then close them mid‑afternoon to trap warmth for the night – but if the evening is humid, leave a small gap. Automatic vent openers are worth every penny. And do not be afraid of a little breeze: curcubits are tough, and a gentle draught through the greenhouse is far better than a stagnant soup.

Water in the morning, at the base, never on the leaves

Watering is where many growers go wrong. An evening soak leaves droplets on foliage overnight – an open invitation to fungal spores. Instead, water early, ideally before 10am. Use a watering can or drip line to deliver water straight to the roots. Never wet the leaves, and avoid splashing soil up onto stems. Morning watering also gives any accidental splashes time to dry before the cooler evening. While you are at it, check the soil with your finger: if it is soggy, skip a day. Overwatering raises humidity through evaporation. A layer of straw or bark mulch on the soil surface will lock in moisture and cut down on airborne damp.

Don't crowd your plants – spacing and pruning for airflow

Melons, pumpkins and squash are thugs. In a greenhouse, they will swallow every inch unless you tame them. Train vines vertically up strings or wires. Remove lower leaves that touch the soil, and pinch out side shoots that block light and air. Space plants at least 60–90 cm apart. For pumpkins, you may need to grow just one or two plants in a small greenhouse. Lift developing fruits off the ground using tiles, slings or old netting. Prune out any dense, yellowing foliage. The goal is simple: you should be able to see through the canopy. Good airflow around flowers and fruit stops rot before it starts.

Monitor humidity and use simple tricks to keep it down

Buy an inexpensive hygrometer – they cost less than a packet of seeds. Aim for 60–70% relative humidity during the day, and slightly lower at night. If it creeps higher, increase ventilation or run a small fan to move the air (do not point it directly at plants; aim it at the roof). Avoid wet paths: use gravel, bark or even old paving slabs. Do not store open water butts or trays inside. A dehumidifier is overkill for most British greenhouses, but a bowl of rock salt near the door will absorb a little moisture. Most importantly, check your greenhouse twice a day – morning and late afternoon – and act on what you see.

Harvesting and cooking: reaping a mould-free crop

Harvest melons when they slip easily from the vine and smell sweet at the blossom end. Pumpkins and winter squash are ready when the skin is hard enough to resist a thumbnail and the stem has turned corky. Cut with a clean knife, leaving a 5 cm stem. Cure them in a warm, airy spot for a week – a sunny windowsill or dry shed is ideal – then store in a cool, dark place. Check stored fruits weekly and remove any with soft spots immediately. In the kitchen, your efforts pay off: roast wedges of pumpkin and squash with rosemary and garlic, fold chunks into risotto, or blitz melon with lime for a sorbet. Because you kept humidity in check, the flavours are clean and intense – no musty tang. And a final tip: if you spot a touch of botrytis on a leaf, cut it off and bin it. Do not compost it. A little vigilance now means a long, delicious harvest.

Greenhouse

Fresh Melon Salad with Mint and Feta

As nights cool, remove faded leaves, harvest mature pumpkins, and clear spent vines. Add compost to beds before winter arrives.

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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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