The second rainy season is ending in the forest belt of southern Cameroon, and this is the period when tunnel greenhouses start dripping at night. The decision to take now, before the sweet pepper flowers, concerns night ventilation and watering times, not a spray programme. When foliage stays wet in the morning, the cause is almost always mechanical: the air inside the greenhouse reaches its dew point late in the night because it was not exchanged in late afternoon. Three actions correct the situation: open up at the end of the day, keep the air moving continuously, and water in the early morning.
Why water settles on the foliage and not elsewhere
The mechanics are simple and can be checked with a thermometer. Warm air carries more vapour than cold air: according to the University of Massachusetts Extension fact sheet, air at 21 °C holds about twice the moisture of air at 10 °C, and every 11 °C rise doubles the capacity of the air while halving the relative humidity. In the evening the reverse happens. The air cools, its capacity drops, and the surplus vapour settles on any surface whose temperature reaches or falls below the dew point. The coldest ones wet first: the plastic cover, the structural tubes, then the foliage.
The same source gives two benchmarks that explain how narrow the margin is under a tunnel. At 85 percent relative humidity and 15.5 °C air temperature, condensation starts as soon as the leaf falls below 13 °C. At 95 percent relative humidity and the same air temperature, the leaf only has to be 0.5 °C cooler than the air. In a closed greenhouse late at night, the foliage loses heat by radiation towards the cover and ends up in exactly that situation.
The result is not merely cosmetic. The canopy stays wet for several hours, and that leaf wetness duration is the condition most foliar fungi and bacteria need to establish.
The two humidity thresholds to watch
Fact sheet PPFS-GH-01 from the University of Kentucky Extension, updated in July 2025, sets out two thresholds you can work with using a plain hygrometer.
- 70 percent relative humidity during the day. Holding the air below this level is often enough to slow disease down. Above it, the incidence of fungal disease rises, particularly if the spell is prolonged.
- 85 percent relative humidity. This is the level from which bacterial and oomycete diseases clearly increase.
These thresholds are read together with temperature, since the same amount of vapour gives very different values at 20 or 30 °C. The University of Massachusetts Extension publishes thresholds by temperature: 83 percent at 10 °C, 89 percent at 16 °C, 91 percent at 20 °C and 95 percent at 30 °C. Under a tropical tunnel, the practical reading is clear: in the middle of the day, at 30 °C, you are rarely in trouble. The problem plays out between 10 pm and 7 am, when the inside temperature falls and relative humidity climbs mechanically towards saturation.
So place the hygrometer at canopy height rather than under the ridge, and read it in the early morning rather than at midday.
The insect net, the most frequent cause of misting
A greenhouse that never dripped and starts doing so after a new net goes on does not have a plant problem. It has an air flow problem. The review by J. Martin, published by Universite Laval in 2007, puts figures on the effect: compared with a 40 mesh net, 52 and 78 mesh nets reduce the air discharge rate by 35 percent and 50 percent. The resistance also shows up as pressure: at a wind speed of 2 m/s, the pressure difference between inside and outside goes from about 2.5 Pa with no net to about 8 Pa with an anti-whitefly net, and exceeds 30 Pa with some anti-thrips nets.
That resistance is paid for in the indoor climate. According to the same review, an anti-whitefly net raises the inside temperature by 2.7 °C and an anti-thrips net by 4.7 °C, with a matching rise in humidity. The document takes a vapour pressure deficit of 0.5 kPa as the threshold not to cross in order to limit the spread of Botrytis cinerea.
The practical conclusion: the mesh is chosen according to the pest to be excluded, and the vent area is then recalculated accordingly. A fine net fitted to the original vents turns a properly ventilated structure into a sealed one. Mesh and vent selection is covered in detail in our guide to agricultural greenhouses in tropical Africa.
Symptom, cause, action
| What you see | Most likely cause | Action |
|---|---|---|
| Drops falling from the cover in the morning | Air not exchanged in late afternoon, cover colder than the air | Open the sides and the ridge at the end of the day, before the temperature drops |
| Wet leaves with no rain, until mid-morning | Foliage below the dew point, stagnant air at canopy level | Continuous air movement at night, removal of lower leaves, larger openings |
| Misting that only started after a net was fitted | Loss of air flow caused by the mesh | Increase the vent area, or go back to a mesh matched to the target pest |
| High humidity in only part of the greenhouse | An obstacle to circulation: dense canopy, stacked trays, an over-tight row | Clear the aisle, widen plant spacing, trellis and prune |
| Wet foliage in late afternoon | Overhead watering done too late in the day | Water in the early morning, or switch to drip |
| Puddles and permanently wet ground | Over-watering and poor drainage | Manage the dose, rework the drainage, mulch or cover bare soil |
What to set right this evening
Three actions need no purchase.
Ventilate in late afternoon. The Kentucky fact sheet explicitly recommends ventilating in late afternoon, to exhaust warm, moisture-laden air before the night cooling. It is the highest-return action on the list: you take the vapour out while it is still carried by warm air, so in large quantity for the same volume exchanged.
Keep the air moving at all times. The same fact sheet advises running fans continuously during high-humidity periods, cool nights, cloudy days and rainy days. The University of Massachusetts Extension gives the order of magnitude to aim for: one air volume is renewed in 2 to 3 minutes in a greenhouse fitted with exhaust fans, against 30 minutes or more under natural ventilation alone. For air circulation it describes small fans of 1/15 hp and 40 to 50 cm diameter, placed along the walls and blowing in opposite directions on each side, running continuously, with a simple visual setting: the leaves should move slightly.
Move watering to the morning. The Kentucky fact sheet recommends watering early in the day where overhead irrigation is the only option available, so the foliage dries quickly, and prefers drip in order to keep the canopy dry. On sweet pepper grown under cover, drip settles both the leaf wetness and the consistency of the water supply.
One point worth clarifying for the tropical zone: temperate-climate literature often pairs night venting with heating, to warm the air before exhausting it. Under a tunnel in Central Africa there is no heater, and the transferable part of the method is the air exchange, not the warming. So you work on the volume exchanged and on the timing, not on temperature.
The habits that make misting worse
- Closing the greenhouse at the end of the day to keep the heat in. You are shutting in the most vapour-laden air of the whole day.
- Watering overhead in late afternoon. Leaf wetness then carries on all night.
- Tightening the planting to gain plants. The Kentucky fact sheet notes that crowded plantings are the first to develop disease, and recommends spacing that lets air move at every growth stage.
- Fitting the finest net available as a precaution. The gain against the pest is paid for in temperature and humidity, as the Universite Laval review quantifies.
- Leaving the ground bare and soaked between the rows. Soil evaporation feeds the greenhouse air all day long.
- Leaving the cover dirty. A soiled cover loses light transmission and slows the morning drying.
The equipment that fixes it for good
When adjustments are no longer enough, the problem is dimensional and is dealt with on the structure itself: roll-up side vents along the full length, a ridge opening, a net chosen at the coarsest adequate mesh, air circulation fans, a cover film with anti-condensation treatment, and a switch from overhead watering to drip. On an existing greenhouse, enlarging the side vents generally gives the best result per unit invested, above all where a net was fitted after construction.
Structures run as soilless systems call for the same rigour, with the added question of the growing medium, covered in our article on choosing a substrate under cover. For the nursery phase, shading and tray management are set out in our guide to nurseries under shade net.
Have JOGOO rework your greenhouse ventilation
JOGOO Agriculture designs, supplies and installs ventilated tunnel greenhouses for hot, humid climates, and reworks existing structures whose ventilation falls short. See the Agricultural Greenhouses page for the available solutions, then write to us from the Contact page for a quote based on the dimensions of your structure and the crop you grow.
Frequently asked questions
At what relative humidity should you be concerned under cover?
The University of Kentucky Extension gives two benchmarks: above 70 percent during the day the incidence of fungal disease rises, and from 85 percent bacterial and oomycete diseases clearly increase.
Why does condensation appear at night and not during the day?
Because the capacity of air to carry vapour falls with temperature. At 85 percent relative humidity and 15.5 °C air temperature, condensation begins as soon as the leaf falls below 13 °C, according to the University of Massachusetts Extension.
Should the greenhouse be open at night?
The essential step is exhausting the laden air in late afternoon, as the Kentucky fact sheet recommends, then keeping air circulating through the night. Whether to leave vents open overnight depends on the wind and rain that night.
Is my insect net to blame?
It may be. According to the review published by Universite Laval, 52 and 78 mesh nets reduce the air discharge rate by 35 percent and 50 percent compared with a 40 mesh net, and an anti-thrips net raises the inside temperature by 4.7 °C.
Should air circulation fans run continuously?
Yes during humid spells, cool nights and cloudy or rainy days, according to the Kentucky fact sheet. The setting is checked by eye: the leaves should move slightly.
Does drip irrigation remove the problem?
It removes the leaf wetness caused by watering, which is one part of the problem. Night condensation depends on ventilation and on the temperature gap between the air and the leaf.
Sources
- University of Kentucky, Extension Plant Pathology, Managing Greenhouse and High Tunnel Environmental Conditions to Reduce Plant Diseases, PPFS-GH-01, July 2025
- UMass Extension, Greenhouse Crops and Floriculture Program, Reducing Humidity in the Greenhouse, John W. Bartok Jr.
- J. Martin, Universite Laval, 2007, use of agronomic nets on naturally ventilated greenhouses, impact on climate and ventilation
