What is relative humidity (RH)

What is Relative Humidity?

Relative humidity (RH) is the measure of the amount of moisture (water vapor) present in the air compared to the maximum amount of moisture the air can hold at a given temperature. It’s expressed as a percentage.

• 100% RH means the air is fully saturated with water vapor and cannot hold any more moisture, potentially leading to condensation.
• 50% RH means the air holds half the amount of moisture it could at that temperature.

Effect of Relative Humidity in a Grow Room

Relative humidity plays a critical role in plant health and growth. Here’s how it impacts your grow room:

1. Transpiration and Water Uptake

• Transpiration is the process through which plants lose water vapor through their leaves. The rate of transpiration is influenced by RH.
• In low humidity environments (e.g., below 40%), transpiration rates increase as the plant tries to pull more water from the soil. This can cause plants to dry out quickly and may stress the plants, especially in high-light environments.
• In high humidity environments (e.g., above 70%), transpiration decreases because the air is already saturated with water. This can lead to waterlogging of plants or a lack of nutrient uptake, as water and nutrients are not drawn up as efficiently.

2. Plant Growth and Development

• Vegetative Stage: During the vegetative growth stage, plants prefer a higher relative humidity (around 50-70%). This encourages optimal transpiration and ensures that the plants receive sufficient moisture, which helps with leaf and root development.
• Flowering Stage: During flowering, a lower RH (around 40-50%) is ideal. Lower humidity prevents the growth of mould and mildew on buds, which are more susceptible during this phase.

3. Risk of Mould and Fungi

• High humidity (over 60%) can create a perfect environment for the growth of mould, mildew, and other fungi, especially in densely populated grow rooms with poor air circulation.
• Mould, particularly powdery mildew, and bud rot are common in high-humidity conditions and can ruin a crop, making it essential to control RH during flowering and fruiting stages.

4. Stress and Plant Health

• Low RH can stress plants, causing them to lose too much water through transpiration and leading to dried-out leaves, wilting, and nutrient deficiencies.
• High RH can cause leaf curling, yellowing, or leaf drop as the plant struggles to manage excessive moisture, and it can also promote pests like spider mites.

5. Pest Management

• Low humidity can be beneficial in controlling certain pests, like fungal gnats or aphids, which thrive in high humidity.
• High humidity can encourage the growth of pests like spider mites and fungal infestations, which are more difficult to manage under moist conditions.

How to Control Humidity in a Grow Room

• Humidifiers can increase RH when it’s too low.
• Dehumidifiers can reduce RH when it’s too high.
• Ventilation is key—ensure proper airflow with exhaust fans, intake fans, and oscillating fans to help control moisture levels.
• Monitoring RH with a hygrometer will allow you to track and adjust as needed.

Ideal RH for Different Stages:

• Seedling Stage: 65-70% RH
• Vegetative Stage: 50-70% RH
• Flowering Stage: 40-50% RH
• Late Flowering/Harvesting: 40% RH (to prevent mould on buds)

Maintaining optimal relative humidity is crucial to ensuring healthy plants, preventing disease, and optimizing plant growth. Adjusting humidity based on the growth stage and using proper environmental controls can lead to better yields and healthier crops.