A dehumidifier beats any bottle on the shelf. In a 2024 study published in Plant Science, plants grown at 78 to 98% humidity produced 75% less total biomass and 71% less flower biomass than plants at 37 to 58%, and flowered 3 weeks later. No additive comes close to that effect. Most of the water you give your plants comes back out through the leaves, so a closed tent or room gets humid fast.
Read the litres per day properly
Dehumidifiers are sold on their maximum litres per day, which is measured in hot, wet air. Compare them at the same conditions. The Vivosun AeroDrain D12 shows why: 15.1 L/day at 35°C and 90% humidity, but 7 L/day at 27°C and 60%. The Ecor Pro EPD170LGR is rated up to 80 L/day, and 35 L/day at 27°C and 60%.
- Tents and small rooms: Vivosun D12 (7 L/day at 27°C and 60%), Trotec TTK 100 E (30 L/day max).
- Larger rooms: Trotec TTK 165 Eco (52 L/day max, rooms up to 90m²), Ora 60L, Ecor Pro EPD170LGR.
- Overhead units that hang out of the way: Trotec DH 25 S (40 L/day), Quest 100 (44 L/day), Quest 200 (90 L/day), Quest 375 (170 L/day).
- Humidifiers for seedlings, cuttings and dry winters: Havel 4L, Vivosun AeroStream H19, SonicAir 10L.
Pair it with a TrolMaster Beta-6 humidity controller or a simple thermo-hygrometer. High humidity also wears out carbon filters early, so see our fans and carbon filters too.
Humidity questions
What size dehumidifier do I need for a grow room?
Roughly as many litres a day as you water in, because most of that water comes back out through the leaves. Then check the rating at your room's real conditions, around 27°C and 60%, not the headline maximum.
What humidity should my grow tent be?
Around 75 to 85% for cuttings and seedlings, 65 to 70% in early growth, 60% in late growth, 55% in early flower and 45 to 50% in late flower. Temperature matters too, so read both together.
Do I need a humidifier or a dehumidifier?
Most grow spaces need a dehumidifier, especially once plants are big. A humidifier helps with cuttings and seedlings, and in dry winters or air-conditioned rooms where humidity drops too low.
Why does high humidity matter so much?
Humid air slows how fast plants move water and calcium, invites mould and mildew, and shortens carbon filter life. In the 2024 Plant Science study, high humidity cut total biomass by 75%.
