A wet inner cover on a chilly spring morning can be worrying, particularly when droplets are falling onto newly sown seedlings. Understanding why polytunnels get damp makes the problem much easier to manage. In most cases, it is condensation rather than a fault with the cover, and a few practical changes can make a noticeable difference.
A polytunnel creates a warmer, more sheltered growing space than the open garden. That is exactly why it helps crops get started earlier and carry on later into autumn. However, that enclosed shelter also traps humid air. When that warm air touches cold polythene, the moisture in it turns back into water droplets.
Why do polytunnels get damp inside?
Every polytunnel has some moisture in the air. Plants release water vapour through their leaves, damp soil evaporates after watering, and wet pots, compost and paths all add to the humidity. On a cold night or a grey morning, the polythene surface is often much colder than the air inside. Condensation then forms on the inside of the cover.
A little condensation is normal, especially from late autumn through early spring. The issue is when it stays wet for long periods, runs down the sides, drips heavily onto plants or encourages mould and fungal disease.
The main causes are usually working together rather than appearing alone:
- Limited ventilation means humid air cannot escape.
- Watering late in the day leaves wet soil and moist air overnight.
- Closely packed plants restrict air movement around leaves and pots.
- Bare, damp ground releases moisture steadily into the tunnel.
- A shaded or poorly positioned tunnel stays cold for longer.
- Small holes, loose fittings or worn seams can allow rainwater in.
Condensation or a leak: how to tell the difference
Before changing your growing routine, inspect the tunnel. Condensation tends to appear as a fine mist or evenly spread droplets across large areas of the inside cover. It is most obvious early in the morning, after a cold night, and may reduce once the doors and vents are opened.
A leak is more localised. Look for a wet patch beneath a particular point, a repeated drip in the same place, visible splits, loose clips or tears near the frame. Water entering around a door edge, base rail or seam may also point to a fitting issue rather than ordinary condensation.
Check after rainfall as well as on a dry but cold morning. If the inside is damp only after rain, inspect the cover tension and fixings. A cover that has become loose can collect water in folds, increasing the chance of drips and wear. Small tears should be repaired promptly, as wind can turn a minor problem into a larger one.
Improve airflow without chilling your plants
Ventilation is the most effective answer to persistent dampness. It replaces warm, humid air with fresher air before it can settle on the cover and plants. In mild weather, open both doors or roll up the side vents during the day. Creating airflow from one end of the polytunnel to the other works better than opening a single small entrance.
The right amount depends on the season and the crop. Tomatoes, cucumbers and peppers benefit from regular ventilation, particularly once the weather warms up. Hardy salad leaves and brassicas can tolerate a cooler tunnel, so it is often worth airing these even on a dull day. Tender seedlings need more care: open up gradually and close before the temperature drops sharply in late afternoon.
Do not wait for bright sunshine. A still, overcast day can be humid enough to cause trouble. Even a short period of airing in the morning can clear moisture that has gathered overnight.
Avoid blocking vents with stored items
It is tempting to use spare tunnel space for tools, compost bags and pots, but bulky items near doors and vents stop air moving through. Keep access routes clear and leave a small gap between benches, grow bags and the side walls. This also makes it easier to spot pests, water accurately and harvest crops.
If you are choosing a new structure, consider ventilation as a buying feature, not an extra. A larger walk-in polytunnel with accessible doors at both ends is often easier to manage than a very compact model filled to capacity. For a small garden, a mini greenhouse may be more practical, but it will need regular opening because its air volume is lower and humidity builds quickly.
Water in the morning, not at night
Late watering is one of the most common reasons a tunnel feels wet by dawn. Water has little chance to evaporate before temperatures fall, so it remains in the compost and raises humidity overnight. Water plants in the morning where possible, aiming at the root area rather than spraying leaves.
Use a watering can or hose carefully to avoid soaking paths and unused areas. Tomatoes and other thirsty crops still need consistent watering in hot weather, but a deep, targeted soak is better than frequent light watering across the whole tunnel. Letting compost dry slightly at the surface between watering also helps, provided the crop is suited to it.
Grow bags, pots and trays should have drainage holes and should not sit in standing water. Raising pots slightly on slatted staging can improve drainage and air circulation underneath. If you use trays to catch water, empty them after plants have had time to drink.
Make the floor work harder
A bare soil floor can be useful for planting directly into the ground, but it can also contribute considerable moisture. If the surface is muddy, uneven or regularly soaked, improve drainage around the outside of the tunnel first. Rainwater should run away from the structure rather than collecting along its base.
For paths, a permeable membrane topped with gravel can reduce mud while allowing water to drain. Paving slabs are another tidy option for a central walkway, although they can feel colder in early spring. The best choice depends on your site, budget and whether you intend to grow in borders, raised beds or containers.
Avoid fully sealing the floor with non-breathable material if the ground beneath is already wet. That can simply trap moisture elsewhere and make cleaning harder. A dry, well-drained site will always be easier to manage than a low point in the garden where water naturally gathers.
Give plants enough room
Overcrowding creates a humid pocket around foliage. Leaves touch, airflow stops and damp conditions can lead to grey mould, mildew or blight problems. This is especially common with tomatoes, cucumbers, peppers and young plants waiting to be planted out.
Follow the recommended spacing for each crop, even when the tunnel looks as though it could hold more. Remove yellowing leaves and any plant material that is lying on damp soil. For cordon tomatoes, tie stems securely and remove side shoots where appropriate, so the plant remains open rather than becoming a dense tangle.
A small circulating fan can help in a larger tunnel with a suitable safe power supply, but it is not a substitute for proper ventilation. For most home gardeners, good spacing, clear paths and regularly opened doors provide the best value solution.
Check the cover, frame and position
Polythene covers do not last forever. Sunlight, wind and winter weather gradually affect the material, so make seasonal checks part of your routine. The cover should be taut enough to shed rainwater but not stretched so tightly that it tears around the frame. Check that ground anchors, clips and tensioning points are secure after strong winds.
Position also matters. A tunnel in full or partial sun will warm and dry more quickly than one tucked behind a dense hedge or against a north-facing wall. At the same time, an exposed site may need extra anchoring. The ideal location balances sunlight, drainage and protection from the strongest prevailing winds.
When comparing polytunnels and greenhouses at AnyDayDirect, think about how much room you need to walk, vent and space plants rather than choosing by footprint alone. A little extra growing space can make day-to-day moisture control much simpler.
When dampness is a real problem
Take action quickly if condensation is dripping persistently onto seedlings, foliage is staying wet all day, or you notice fuzzy mould on compost, leaves or stems. Remove affected material, improve airflow and avoid overwatering. Clean benches, pots and tools before reusing them, particularly if disease has been present.
If the tunnel remains very damp despite regular ventilation, assess the site after heavy rain. Poor drainage, a damaged cover or water running in from nearby ground may be the underlying cause. In that situation, changing the planting layout alone will not solve the problem.
Frequently asked questions
Should I leave my polytunnel door open at night?
Usually not in cool weather, as this can lower temperatures too much for tender plants. Open the tunnel during the day and close it before evening. In warm summer conditions, leaving doors or vents partly open overnight can be useful, provided the forecast is mild and the structure is secure.
Is condensation bad for plants?
A small amount is normal and not usually harmful. Problems begin when leaves remain wet for prolonged periods or droplets repeatedly fall on seedlings. Regular ventilation and sensible watering greatly reduce the risk.
Will a larger polytunnel be less damp?
Often, yes. A larger volume of air changes temperature more slowly and gives plants more room, but it still needs ventilation. A large tunnel packed tightly with plants can be more humid than a smaller, well-managed one.
A dry, productive polytunnel is not one with no moisture at all. It is one where humidity has somewhere to go, water reaches plants without soaking everything around them, and the growing space is checked little and often. Those small habits protect your crops and make the tunnel a more reliable place to grow throughout the UK season.