Relative Humidity

Relative Humidity (RH) is the amount of water vapour in the air compared to how much moisture the air could possibly hold at that temperature. RH influences plant growth because it directly affects the amount of water that your plants move through their stems and leaves. This keeps your plants hydrated and transports vital nutrients.

What should my RH be?

For healthy plant growth, you should aim for a relative humidity of 50-70%. Humidity can be measured using a hygrometer.

Consequences of high RH

If the RH in your grow room is too high, you have the perfect environment for fungal spores to germinate and allow pathogens to infect your plants. The most common type of fungal pathogen you will encounter is Botrytis (Bud Rot). Others include leaf moulds, such as Powdery Mildew and Leaf Spot Fungi. There's also a risk of stem and root infections, although these are less common.

High humidity can be a sign that there's not enough ventilation in your growing area. If ventilation is adequate, a quick and easy way to lower humidity is to use an air conditioning unit or a dehumidifier.

Consequences of low RH

If the RH is too low, it can manifest itself in a few ways, the most common of which are:

• Leaf roll, which is a curling upwards at the margins making them look like a tube. This decreases the amount of transpiration as less of the upper leaf surface is exposed to direct light, which lowers the amount of water your plants will lose.
• Wilting, this in extreme cases will occur to the whole plant.

Generally, low humidity tends to be more common than high humidity. In fact, when growing indoors under hot lights, most hydroponic growers tend to start off with very low humidity. This is problematic because RH needs to be relatively high, around 60-70% during the vegetative growth stages. If the relative humidity in your growroom decreases to below 40% during early growth stages, you'll be likely to run into problems, one of these being over-fertilisation. High temperatures and low humidity will cause your plants to take up and release more water. Your plants will take up more water than nutrient, causing the nutrient to continually increase in strength. It is this increase in nutrient strength, coupled with an increase in transpiration that causes over-fertilisation and other nutrient related problems. Although the cause of many problems during the hotter summer months, the link between humidity and nutrients is often overlooked.

RH during the Dark Cycle

Some indoor growers find that when the lights go out the relative humidity shoots up. This can inevitably lead to mould problems like Botrytis on fruits or flowers. This can be corrected by making sure you have adequate night time ventilation or using a dehumidifier. Be sure that the dehumidifier does not remove too much water during this time as it can over-dry the air, causing the plant to loose water through its leaf tips. This water can collect on the leaves, creating the perfect micro environment for spore germination. Ideally, you want your growroom to have a lower humidity during the day compared to the night. The ideal figures to aim at during fruit/flower formation would be 50-60% in the light cycle and 55-65% during the dark cycle.

RH and Propagation

During the propagation stages when you are rooting cuttings or germinating seedlings, RH is controlled and kept high using a propagator, such as the Stewart Heat and Grow Electric Propagator. We recommend that you spray the lid of your propagator daily to keep the RH above 85%. This will minimise transpiration and concentrate the plants' energy on producing new roots.