Quick answer: Greenhouse tomatoes grow well at about 60–80 % RH in the day (VPD roughly 0.5–1.0 kPa), with night humidity kept below saturation so the canopy never gets wet. The risk hours are from sunset to just after sunrise, when a dense, transpiring canopy cools and vents are closed. Keep air moving through the crop, vent and heat when outside air is drier, and use refrigerant dehumidifiers where venting costs too much heat or the outside air is humid. Desiccant units are rarely needed in heated tomato houses.
Quick answer
- Day: 60–80 % RH, VPD about 0.5–1.0 kPa.
- Night: no condensation on leaves or fruit; about 89 % RH at 16 °C leaves a safe margin.
- Above about 85 % RH for long periods, leaf mold and Botrytis risk rise sharply.
Typical climate targets for greenhouse tomatoes
| Period | Air temperature | RH | VPD | Goal |
|---|---|---|---|---|
| Day, sunny | about 21–27 °C | 60–80 % | about 0.5–1.0 kPa | Active transpiration, calcium transport, pollination |
| Day, cloudy | slightly lower | up to about 80 % | at least about 0.4 kPa | Keep the crop transpiring |
| Pre-night and sunrise | ramping | lowest margin of the day | — | Avoid condensation as leaves and fruit change temperature |
| Night | about 16–18 °C | below about 89 % at 16 °C | above about 0.2 kPa | No free water on the crop |
These are typical ranges; follow your cultivar and crop advisor. VPD is a better guide to plant water loss than RH alone (MSU Extension). See our VPD chart guide.
Why tomato houses get humid so fast
A mature tomato crop has a large leaf area, and it keeps transpiring after sunset while the air cools. Dense rows and lowered leaves hold humid air in the canopy, so leaf-level humidity is higher than the climate computer's sensor above the crop shows. Fruit that is cooler than the air at sunrise can get wet even when leaves stay dry.
How humidity affects tomato problems
| Problem | Role of humidity | What helps |
|---|---|---|
| Botrytis (gray mold) | Spores germinate in free water; infections enter leaf scars, pruning wounds and stem cracks in humid conditions | NC State recommends ventilation to keep RH below 90 %, keeping leaves dry, avoiding overhead irrigation and maintaining air movement (NC State Extension) |
| Leaf mold | Favored by long periods of high humidity, typically above about 85 % RH | Air movement, lower night humidity, resistant cultivars |
| Fruit cracking | Under high humidity with moist roots, water moves into fruit faster than the skin can expand; swings in humidity and temperature add stress | Steady climate, even irrigation, avoid sudden RH drops on hot days (Purdue) |
| Blossom-end rot | A calcium shortage in developing fruit, linked to uneven water supply among other factors (UMD Extension) | Even irrigation and a transpiring crop; humidity is one factor, not the fix |
Related: NC State Extension, purdue, UMD Extension.
Crack types help diagnose the cause: radial cracks from the stem end, concentric rings around the shoulder, and fine russeting of the skin (UConn IPM). Large-fruited cultivars and heavy fruit loads are more prone (University of Delaware).
Refrigerant, desiccant or ventilation?
| Option | Fits tomato houses when | Notes |
|---|---|---|
| Vent + heat | Outside air is cold and dry | Usually the first tool; costs heat and CO₂ |
| Refrigerant dehumidifier | Night house temperature above about 15 °C; humid outside air; screens closed | Efficient at tomato night conditions; returns condensation heat to the house |
| Desiccant dehumidifier | Very cold houses, or strict dew-point targets (propagation, storage) | Needs regeneration heat and an exhaust route; seldom the economic choice in heated production houses |
A desiccant unit removes moisture by adsorption and needs heat to regenerate the material (NREL); compare both options at your night condition and include all energy before choosing. Our desiccant selection guide explains when it pays.
Sizing and placement
- Size for the night peak: crop transpiration at night plus evaporation from substrate and paths, minus what venting removes. Our 5-hectare tomato greenhouse sizing walk-through shows the inputs, and greenhouse humidity control works through a venting vs dehumidifying comparison.
- Use the house condition: a unit rated at 30 °C/80 % RH removes much less at 17 °C/88 % RH. Ask for capacity at your night setpoint.
- Distribute: several units along the house, or units connected to air tubes under the gutters, so dry air reaches every row. Pair with horizontal airflow fans.
- Control: stage the dehumidifiers with vents and heating from the climate computer, starting before sunset.
- Condensate: drain to a collection tank; some growers reuse it for irrigation after testing.
See our greenhouse dehumidifiers and how to choose a greenhouse dehumidifier.
What to send for a tomato greenhouse quote
- Greenhouse area, height, number of spans and screen system
- Crop stage, row spacing and leaf-removal practice
- Day and night temperature and humidity now, and the target
- Heating system, vent control and CO₂ strategy
- Irrigation method and timing
- Power supply and condensate drainage points
FAQ
Frequently asked questions
What humidity is best for greenhouse tomatoes?
About 60–80 % RH during the day, with a night target that keeps leaves and fruit dry, for example below about 89 % RH at 16 °C.
What VPD should tomatoes have?
Roughly 0.5–1.0 kPa in the day; keep at least about 0.2 kPa at night so the crop does not get wet.
Does high humidity cause tomato fruit cracking?
It contributes. High humidity with moist roots raises water flow into the fruit, and swings in humidity and temperature add stress. Cultivar and fruit load matter too.
Do tomato greenhouses need dehumidifiers?
Not always. They pay off when outside air is humid, when heat and CO₂ are expensive to vent, and when screens stay closed at night.
Is a desiccant dehumidifier good for tomatoes?
Only in cold houses or for very dry targets. In heated production houses, refrigerant units are usually more efficient.
Sources
- NC State Extension, Botrytis gray mold of tomato.
- Michigan State University Extension, Vapor pressure deficit for greenhouse growers.
- Purdue Vegetable Crops Hotline, Cracking tomatoes.
- UConn IPM, Tomato growth cracks; University of Delaware Cooperative Extension, Fruit cracking in tomato.
- University of Maryland Extension, Blossom-end rot of vegetables.
- NREL, Thermally activated desiccant technology.
