What is the ideal humidity in a greenhouse?

The ideal humidity level in a greenhouse depends on several factors, including the plants being grown, climate, and time of year. Generally speaking, most plants thrive in a relative humidity range between 40-60%. However, humidity requirements can vary significantly for different plants. Understanding optimal humidity levels and managing greenhouse humidity effectively is crucial for healthy plant growth.

Why is greenhouse humidity important?

Humidity directly impacts plant transpiration and photosynthesis. When humidity is too low, plants lose moisture faster through transpiration. This causes stress, wilting, and poor growth. Low humidity also makes it difficult for plants to pull water up from their roots. On the other hand, very high humidity reduces transpiration rates and can encourage disease development from excess moisture on leaves and stems. Managing humidity is essential for photosynthesis as well. When humidity is too low, plants cannot take in CO2 efficiently. If humidity is too high, oxygen exchange is limited. An optimal humidity range maximizes photosynthesis and plant health.

Ideal greenhouse humidity levels

Most plants grow best with a relative humidity range between 40-60%. This minimizes water loss through transpiration while still allowing adequate gas exchange. Some general greenhouse humidity guidelines by crop:

• Vegetables and herbs: 45-60% RH
• Flowers and flowering plants: 40-50% RH
• Cacti and succulents: 40-50% RH
• Orchids and tropicals: 50-70% RH
• Propagation and seedlings: 65-70% RH

These ranges represent optimal humidity for most cases. However, specific cultivars or growth stages may require adjustments. Growers need to observe plants and adjust as needed.

Ideal greenhouse humidity by season

In addition to crop type, optimal greenhouse humidity fluctuates with the seasons. In winter, most greenhouses need added humidity. Heating systems tend to dry out the air. Plants also transpire less during shorter winter days. Added humidity, around 60%, prevents winter dryness. During warmer months, ventilation and fans help reduce humidity. Ideal seasonal greenhouse humidity ranges are:

• Winter: 50-60% RH
• Spring: 50-70% RH
• Summer: 40-60% RH
• Fall: 40-60% RH

Monitoring humidity daily helps detect seasonal shifts. Increase or reduce humidity as needed to stay within optimal ranges.

Methods for controlling greenhouse humidity

A greenhouse humidification and dehumidification system helps maintain ideal humidity levels. Some common methods include:

• Evaporative coolers – Increase humidity as cooling pads evaporate water.
• Humidifiers – Release vapor to increase humidity.
• Ventilation – Vents, fans, and open doors decrease humidity.
• Heating – May use dry heat to lower humidity.
• Dehumidifiers – Remove moisture from air to reduce RH.

Shade cloths and insulating materials also influence greenhouse humidity. An integrated pest management plan reduces the need for chemical sprays that change moisture levels as well. Monitoring with a hygrometer and making adjustments as needed keeps humidity in the optimal range.

Ideal greenhouse humidity levels by time of day

Humidity levels fluctuate naturally throughout the day in greenhouses. Evapotranspiration from plants causes relative humidity to peak in the early morning. As sunlight warms the air and plants transpire, humidity drops through the late morning and early afternoon. Humidity then rises again late afternoon as temperatures cool and transpiration slows. Daily humidity management involves monitoring these natural cycles and adjusting to maintain ideal RH levels. Typical daily humidity ranges are:

• Early morning: 60-80% RH
• Late morning: 40-60% RH
• Afternoon: 40-60% RH
• Evening: 50-70% RH

Adding morning humidity and increasing ventilation midday keeps the greenhouse within optimal ranges. Monitoring equipment helps track daily humidity patterns.

Signs that greenhouse humidity is too low

The following plant symptoms may indicate greenhouse humidity is too low:

• Wilting or drooping leaves
• Leaf scorching or browning at edges
• Stunted growth
• Dry soil even after watering
• Thirsty or stressed appearance
• Tip burn on leaves
• Poor germination rates

Slowed growth and transplant establishment can also indicate excessively dry air. Adding a humidifier, misting system, or evaporative cooling pads can raise humidity when needed.

Signs that greenhouse humidity is too high

Excessively wet greenhouse air creates the following problems:

• Algae or mold on floors, walls, benches
• Disease development on leaves and stems
• Flopping or weak stems in seedlings
• Nutrient deficiencies
• Slow growth
• Condensation dripping from ceiling and covers

Improving ventilation with vents, fans, and dehumidifiers can lower humidity to the optimal range.

Ideal greenhouse humidity for common crops

Here are the typical optimal humidity ranges for some popular greenhouse crops:

Tomatoes

45-60% RH

Peppers

45-55% RH

Cucumbers

40-50% RH

Lettuce

45-55% RH

Basil

40-50% RH

Chrysanthemums

60-70% RH

African violets

50-60% RH

Orchids

55-65% RH

Succulents and cacti

40-50% RH

Seedlings and propagation

65-70% RH

Monitoring individual crops and adjusting humidity as needed ensures optimal moisture levels.

Using a greenhouse hygrometer

A hygrometer is essential for monitoring and managing greenhouse humidity. Hygrometers measure and display relative humidity. Place hygrometers at plant level in multiple greenhouse zones. Check readings 2-3 times daily, ideally during cooler morning and warmer afternoon. Adjust humidity up or down to stay within optimal range for the crop. Calibrate hygrometers annually for accuracy.

Greenhouse humidification methods

If humidity needs to be increased, consider these greenhouse humidification techniques:

Evaporative coolers

Evaporative coolers, or swamp coolers, can double as humidifiers. As water evaporates from saturated pads, it humidifies the air. These are only effective in drier climates where cooling is needed.

Misting systems

Targeted misting of plant foliage or floors releases moisture into the air when humidity dips too low.

Humidifiers

Steam, misting, evaporative, and ultrasonic humidifiers raise humidity in greenhouses. They can target the entire greenhouse or smaller propagation areas.

Hand watering

Thoroughly watering plants in the morning helps saturate air with humidity early in the day.

Greenhouse dehumidification methods

If humidity needs to be lowered, consider these techniques:

Ventilation

Roof vents, side vents, roll-up curtains and fans exchange humid air. This is the most energy efficient dehumidification method.

Heating

Where possible, heating greenhouse air during the day can help lower humidity levels.

Dehumidifiers

Specialized greenhouse dehumidifiers, including desiccant and condensation models, actively remove moisture.

Air conditioners

Air conditioning units cool air and condense out excess moisture. This is energy intensive but lowers humidity.

Conclusion

Controlling humidity is critical for optimal plant growth in greenhouses. Most plants thrive between 40-60% relative humidity. However, crop needs vary and humidity levels fluctuate by season and time of day. Growers need to actively monitor humidity with hygrometers and use humidification, ventilation, heating and dehumidification to maintain ideal conditions. Keeping humidity within optimal ranges encourages healthy transpiration, photosynthesis and growth while preventing drought stress and diseases.