Grow Tent Humidity: How to Lower or Raise It

Relative humidity is how much water vapour the air is holding, as a percentage of the most it could hold at that temperature. Too high and mould becomes the risk; too low and plants can lose water faster than they take it up. This guide gives the usual grow tent humidity targets for each stage, explains why the reading jumps at lights-off, and shows how to lower or raise it with the humidity control equipment we sell.

What humidity to aim for

The humidity ranges are from our airflow, temperature and humidity guide, the VPD ranges from our VPD chart. Young plants with few roots want humid air; plants in flower want it drier, because dense growth holds damp. The same humidity means something different at another temperature, which is what VPD takes into account, so the two columns will not always agree. At 24°C, about 50-55% in flowering sits inside both.

Measure first. Hang a hygrometer at plant height. Our advice: keep it out of the lamp's direct beam and add another for a propagator or second tent. For mould, watch the highest reading; it usually comes in the dark.

Why humidity climbs when the lights go off

Warm air can hold more water than cool air. When the lights go off the tent cools, so the same water becomes a bigger share of what the air can hold: relative humidity climbs with nothing added. Plants and wet pots still give off some moisture, and if the extractor goes off with the lights, or is slowed by a fan controller that only reads temperature, the damp air stays put.

How to lower humidity

1. Run the extractor through the dark period. An extractor swaps tent air for room air, so it can only bring the tent down towards the room's humidity. Sizing is in our fan and carbon filter guide.
2. Add a little heat in the dark period. The reading falls as the air warms. A tube heater gives gentle background heat: Gorilla Tube Heaters run from 60 to 240 W, Hylite Tube Heaters from 28 to 190 W (see grow room running costs).
3. Water early in the light period. Growers commonly do, so pots are not freshly soaked at lights-off. Empty the run-off trays.
4. Thin the leaves. As a rule of thumb, plants give back to the air most of what you water, and crowded leaves trap it. Take off shaded lower leaves with clean scissors, and keep a clip fan moving air through the plants.
5. Add a dehumidifier when extraction cannot keep up.

The dehumidifiers we sell

Which size? Go by the rule of thumb above: a sealed room needs a unit that removes about as many litres a day as you water, with some to spare; a room with an extractor needs less. For a tent, we would start with the RAM 12L. Growers commonly stand one in the room the tent draws from, so the tent takes in drier air. Both RAMs shut down when the tank is full, so use the continuous drain where you can.

Which controller? The RAMs, Quest 70 and 155 hold a target set on the unit. The G.A.S Day & Night Controllers come in a dehumidifier version with lights-on and lights-off targets. A unit switched at the plug must restart itself; only the Quest 70 and 155 pages say they do. The DEH 3000R controller and hanging kits are under Quest parts and controller.

How to raise humidity

• Young plants: use a propagator. A tray is far easier to keep humid than a whole tent. The Garland XL High Dome Heated Propagator has twin vents to control airflow and moisture; open them a little at a time as roots form. See propagators and cuttings.
• Slow the extractor. Less air leaving means the moisture the plants give off stays longer. The Variispeed Fan Controller turns down a single AC fan of up to 300 W; its page says most fans should be run at 35% or more. Watch the temperature does not climb.
• Add a humidifier if that is not enough.

The humidifiers we sell

Which one? For a tent, look at the Ram first: its page says both sizes suit small to medium growing areas. An extractor carries mist out with the air, so slow the fan before you size up the humidifier. Point the mist away from plugs, timers and the light.

Which water? The Ram and SonicAir Pro pages both say reverse osmosis (RO) water only, and the SonicAir Pro page adds that anything else voids the warranty. The Tornado page says clean water; the other two do not say, so follow the instructions in the box. See our reverse osmosis guide for the RO filters we sell.

Controllers that hold a set humidity

The three fan controllers act on humidity as well as temperature, so the extractor need not be left slowed at lights-off. If a humidifier and a dehumidifier share a space, leave a gap between their set points so they do not fight each other.

Humidifier or dehumidifier: what to do

Log the highest and lowest readings for a few days, light and dark.

• Too high only in the dark: run the extractor through the dark period, then add a tube heater, then a dehumidifier.
• Too high all day, late in flowering: check the extraction, thin the leaves, then add a dehumidifier.
• Too low: a propagator for seedlings and cuttings; in vegetative growth, slow the fan, then add a humidifier on a controller.

Quick answers

How do I lower humidity in a grow tent?

Run the extractor and add a little heat through the dark period, water early in the light period and thin crowded leaves. Still too high? Add a dehumidifier.

How do I raise humidity in a grow tent?

Slow the extractor with a fan speed controller, then add a humidifier. For seedlings and cuttings, use a propagator.

What should the humidity be in a grow tent?

As a rule of thumb: 65-75% for seedlings and cuttings, 55-70% in vegetative growth and 40-55% in flowering.

Which hygrometer for a grow tent?

One with a memory, so the tent stays shut in the dark: the ThermoPro TP358 or AirComfort; on a budget, the Digital Hygrometer.

More Growers' Guides

• VPD chart
• Powdery mildew
• How to set up a grow tent
• Grow tent intake and ventilation
• Growers' glossary

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.