Everyone has heard the expression “The temperature is really high, but it’s a dry heat.” A dry heat feels more comfortable to us because the human body perspires, which is an evaporative cooling process to regulate its temperature. The evaporation of perspiration makes us feel cooler. Under high humidity conditions, evaporation slows, so we perceive a higher temperature versus the same temperature in a low humidity environment. The combination of relative humidity and temperature are reflected in the heat index. For example, if it is 84 degrees Fahrenheit with 100 percent humidity, it “feels like” 104 degrees Fahrenheit.

The process of transpiration is an evaporative cooling mechanism plants employ to regulate their temperature by opening and closing their stomata. If humidity levels are too high, this process becomes inhibited and the plant may become overheated and stressed. Stressed plants are more susceptible to pathogens and pests than healthy plants.

Optimal humidity levels depend on a variety of factors including temperature, type of crop, region where it is grown and its stage of development. Generally, maximum growth occurs in the 60 percent to 80 percent humidity range. Specifically, cuttings/clones prefer the most humidity (upwards of 85 percent), while seeds germinate best around the 60 percent level of humidity. Humidity preferences usually decline throughout the growth cycle, as risks of problems associated with excess humidity (molds and mildews) increase as canopies become denser and fruit/vegetable/flower formation occurs. A 50 percent humidity target is best for plants in the final stage of development before harvest.

Not all plants follow these parameters. For example, tomatoes flourish in a high humidity environment, preferring daytime humidity levels around 80 percent to 90 percent, while preferring nighttime humidity levels of 65 percent to 75 percent.

Relative humidity

Humidity gauges (hygrometers) only measure humidity and do not consider relative humidity. As temperatures rise and humidity levels remain constant, the amount of water the air can hold increases. A greenhouse with 100 percent humidity at 68 degrees Fahrenheit contains twice the amount of moisture that a greenhouse with 100 percent humidity at 50 degrees Fahrenheit can “hold.” Therefore, it is often the result of a combination of excess heat and humidity that contribute to problems such as the slowing of transpiration, stunted plants and the formation of fungi. Fungi include bacteria, molds and mildews.

A clear indicator of humidity levels that are too high is the formation of condensation on the leaves or surfaces, especially at night. As the sun sets, or artificial or supplemental lighting is turned off, the temperature generally falls, while relative humidity levels remain constant, causing a spike in humidity. As the temperature drops, the air can no longer hold the previous level of moisture. In an outside environment, the temperature at which the air is saturated is referred to as the dew point. In an indoor operation, this inflection point leads to the formation of condensation.

The use of artificial lighting tends to decrease humidity levels during operation, because it raises the temperature while the relative humidity remains the same. Readable humidity levels drop when grow lights are turned on and increase when they are turned off. The use of an air conditioner will lower the temperature and humidity, while an evaporative cooler (swamp cooler) will lower the temperature but raise the humidity.

Preventive measures

The best way to combat molds and mildew is to prevent the conditions that contribute to their formation. In addition to temperature and relative humidity, airflow is a key factor in discouraging their presence. As well as helping to discourage pathogens, optimal ventilation will strengthen stems and mimic the wind that is found in an outside environment.

Selective pruning can also help to discourage the formation of molds and mildews because it allows more airflow throughout the areas of the plant that require it.

Finally, don’t overwater your plants. The presence of stagnant water also contributes to the presence and spreading of molds and mildews.

Reactive measures

Despite attempting to manipulate temperature, airflow, humidity and employing a selective pruning technique, fungi can still show up in your greenhouse. Don’t panic; there are many organic and synthetic fungicides that are suitable for edible crops. I can’t endorse a specific brand but will note that most products contain neem oil, garlic extracts or sulfur extracts.

Common molds/mildews

Fungus botrytis, also known as gray mold or gray blight, consists of spores that appear as a white powder, which turns gray as it spreads. Recently pruned areas are more susceptible because the mold thrives on the presence of nutrients. Spores tend to fall to lower areas, so the top level of the substrate should be replaced if the outbreak is severe. It may be wise to treat the entire growing area with a fungicide, not just the affected parts, because the spores may remain dormant and have a tendency to reappear.

Powdery mildew is similar to fungus Botrytis, but will not turn gray. It remains a fluffy white color. It is easier to wipe off, but spreads more rapidly than Botrytis.

White mold looks like wet baking soda. An affected plant will wilt and the leaves will appear bleached. If left unchecked, the white mold turns hard and black.


Although maintaining a consistent and preferred level of humidity will lead to optimized yields, there are preventive and reactive remedies available that can minimize the effects of excessive humidity (and the problems it creates) in the growing environment.