How do You Get Rid of Humidity in a Greenhouse?
High humidity in a greenhouse can lead to mold, mildew, and plant diseases. Getting rid of excess moisture means improving ventilation, controlling watering, and sometimes using a dehumidifier or heater. The key is to keep relative humidity between 50% and 70% for most plants.
Why is high humidity a problem in a greenhouse?
When humidity stays above 85% for long periods, water condenses on leaves and surfaces. This creates the perfect environment for fungal diseases like botrytis and powdery mildew. High humidity also slows down transpiration, which plants need to take up nutrients. In extreme cases, leaves may rot or drop off. Even if plants survive, the yield and quality of fruits and flowers often drop.
Another issue is that warm, humid air can cause heat stress when temperatures rise. Plants cannot cool themselves efficiently if the air is already full of moisture. That is why controlling humidity is just as important as controlling temperature inside a greenhouse.
How can I reduce humidity in a greenhouse without a dehumidifier?
You can lower humidity using simple methods before buying any electric device. Here are the most effective ways:
- Open vents and doors – Natural ventilation exchanges humid inside air with drier outside air. Even a small crack can help.
- Use exhaust fans – Position fans to pull moist air out at the top of the greenhouse, where hot, humid air collects.
- Water in the morning – Watering early gives plants time to absorb moisture before night when humidity rises.
- Avoid wetting leaves – Use drip irrigation or water at soil level instead of overhead sprinklers.
- Space plants out – Crowded plants trap moisture. Allow air to flow between them.
- Use shade cloth – Too much heat can cause rapid evaporation and spike humidity. Light shading balances temperature and moisture.
These steps cost very little and work for most small greenhouses. However, if you live in a very humid climate or have a sealed greenhouse, you may need a stronger solution.
What type of dehumidifier works best for a greenhouse?
If natural methods are not enough, a dehumidifier can remove large amounts of water from the air. For greenhouses, the best type is a compressor-based dehumidifier that can operate at cool temperatures (50–60°F). These units pull air over cold coils, condense moisture into a tank or drain line, and release drier air.
Look for a model with a built-in humidistat so you can set the humidity level automatically. The capacity is measured in pints per day. For a small hobby greenhouse (100–200 square feet), a 30-pint unit is usually enough. For larger greenhouses (300+ square feet), consider a 50-pint or higher model. Placement matters: put the dehumidifier in the center of the greenhouse, away from direct sunlight and water spray.
If your greenhouse is connected to a house or garage, a portable dehumidifier with a hose drain is convenient. For standalone greenhouses, a battery-powered or solar-compatible model can work, but most need an electrical outlet. A good option is the top-rated compressor dehumidifier for greenhouses that includes continuous drainage.
How does air circulation help lower humidity?
Moving air prevents pockets of moisture from building up around plants. Even if the overall humidity in the greenhouse is high, good air circulation reduces the risk of condensation on leaves. Circulation fans (also called horizontal airflow fans) mix the air so that temperature and humidity are even throughout the space.
For best results, install fans at opposite ends of the greenhouse, slightly above plant height. Aim them to push air along the length of the greenhouse. A single fan can cover about 30 feet. In a small greenhouse (8×10 feet), one fan is enough. In a larger one, use two fans pointing in opposite directions. Run fans 24 hours a day during humid periods, but at a low speed so plants are not stressed.
Consider a quiet circulation fan designed for greenhouses with a built-in thermostat and humidity controller.
Can watering techniques really lower greenhouse humidity?
Yes, because most greenhouse humidity comes from water evaporating from soil and plant leaves. Changing how and when you water can cut humidity by 10–20% or more. Here is a comparison of common watering methods and their effect on humidity:
| Watering method | Humidity impact | Best use |
|---|---|---|
| Overhead sprinklers | High – wets leaves and soil surface | Only for hardy plants in low-humidity periods |
| Drip irrigation | Low – water goes to roots only | Ideal for most greenhouses |
| Hand watering with a wand | Medium – can avoid leaves if careful | Small greenhouses, targeted watering |
| Flood floor (ebb and flow) | Medium to high – water sits on floor | Only with good ventilation |
Switch to drip irrigation or soaker hoses to keep leaves dry. Water only when the top inch of soil is dry. If you use a timer, set it early in the morning so the sun can dry any excess moisture. Also, avoid leaving puddles on the floor; they raise humidity for hours.
How do I use a hygrometer to monitor humidity?
A hygrometer measures relative humidity. Without one, you are guessing. Place a hygrometer at plant height in the center of the greenhouse. For accuracy, get a digital model that also shows temperature. Many have min/max records so you can see the highest and lowest humidity over a day.
Here is a simple checklist to guide your actions based on readings:
- 50–70% – Perfect for most vegetables, herbs, and flowers. No action needed.
- 70–85% – Moderate risk. Increase ventilation or run fans for a few hours.
- Above 85% – High risk of disease. Open vents, run fans continuously, and consider a dehumidifier. Check for leaks or overwatering.
If you notice condensation on leaves or the plastic covering, humidity is too high even if the hygrometer shows a lower number. In that case, act immediately. A reliable hygrometer is a small investment that pays off. Find a digital hygrometer with remote sensor for greenhouse use so you can check readings without opening the door.
Are there natural ways to reduce humidity in a greenhouse?
Yes, a few natural methods can help, though they are usually less powerful than mechanical solutions:
- Desiccant materials – Place buckets of silica gel, calcium chloride, or activated charcoal inside the greenhouse. They absorb moisture from the air, but need to be replaced or dried out regularly. Work best in small, enclosed spaces like a cold frame.
- Certain plants – Some plants like ferns, peace lilies, and spider plants absorb moisture through their leaves and can help reduce humidity slightly. However, they also release some water through transpiration, so the effect is modest.
- Bare soil covering – Cover the soil surface with a plastic mulch or landscape fabric. This prevents evaporation from the soil. White or reflective covers also help keep the greenhouse cooler.
- Charcoal or chalk – Crushed charcoal or chalk powder can absorb surface moisture if spread on a tray, but it is not a long-term solution.
These natural methods are best as supplements, not the main strategy. For serious humidity control, combine them with ventilation and possibly a dehumidifier.
When should I use a heater to reduce humidity?
Heating the air lowers its relative humidity even if the amount of water stays the same. For every 10°F increase, relative humidity drops by about 10%. So sometimes turning on a heater can push humidity down below the danger zone. This works best in cool weather when outside air is cold and dry.
But heating alone without ventilation will just trap moist air. The real trick is to heat and ventilate simultaneously. For example, you can run a small heater while opening a roof vent. The warm air rises and escapes, pulling in drier, cooler outside air. The heater keeps the plants from getting too cold.
Use this method only when outdoor humidity is below 60%. If it is raining or foggy outside, heating will not help because the incoming air is already saturated. Also, never use a propane or gas heater without proper venting—they produce water vapor as a byproduct, making humidity worse.
For small greenhouses, a thermostat-controlled electric heater with a built-in fan is a good choice. It can be combined with a timer or humidity controller to run only when needed.
Reducing greenhouse humidity is a matter of combining ventilation, smart watering, and the right tools. Start with free or low-cost changes like opening vents and watering in the morning. If humidity stays high, add a circulation fan, then a dehumidifier if necessary. Monitor with a hygrometer so you know exactly what your greenhouse needs. With steady adjustments, you can keep humidity in the safe zone and grow healthy, productive plants year-round.