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Fix the air the tent breathes. Heat first, then humidity.

My tent is too dry this winter. What do I do?

GROWER ANSWER

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Winter air holds very little water, and heating it drops the humidity reading even further. A typical exhaust then swaps the tent's air for that dry room air every 1 to 3 minutes. So the fix starts outside the tent: get the heat steady, slow the swap, and add moisture a little at a time. Outside of drying, don't run below about 40% in any stage.


Fix the air the tent breathes.


How dry is too dry

The humidity lane moves with the stage:


  • Seedlings and fresh cuttings: 65 to 75%

  • Veg: 55 to 70%

  • Early flower: 45 to 60%

  • Late flower: 40 to 50%


And the floor for all of them: don't run below about 40%. Very dry air pulls water through the plant fast, so the pot dries faster and the feed hits harder at the leaf tips.


Notice that late flower is meant to run dry. A dry winter tent is mostly a problem for seedlings, cuttings and veg plants, the ones that want it most humid.


What dry air looks like

  • The pots dry out faster and she drinks more than she did in the fall.

  • Leaf tips and edges crisp at a feed that was fine before, because she's pulling water and salts through faster.

  • Seedlings and cuttings stall or wilt even with a damp mix.


Check the reading first

A lot of humidity problems are a meter problem. Test your hygrometer with the salt test: table salt with just enough water to make a wet paste (not dissolved), sealed in a bag or container with the meter, not touching it, for 6 to 8 hours. A good meter reads about 75%. Then put the sensor at canopy height, shaded and out of the fan's path.


Raise it, in this order

  1. Get the heat steady first. Heating cold air is what dried it out, so settle the temperature before you chase the humidity, then see where the humidity lands.

  2. Slow the exhaust. Turn the fan down one speed notch at a time. The tent keeps more of its own moisture when it swaps its air less often. Don't switch it off: the heat from the light still needs to go somewhere.

  3. Add moisture to the air going in. A humidifier in the room the tent breathes from, or a small one in the tent, running on a controller instead of flat out. Fill it with low-mineral water, RO or distilled, so it doesn't leave white dust on the leaves.

  4. Use the temperature too. Temperature and humidity work as a pair. If the humidity won't come up, running the cooler half of the temperature lane takes some of the pull off the leaves.

  5. Move in small steps. About 3 to 5 points at a time, then wait at least an hour and judge it over a full day and night.


Watch the lights-off jump

When the lights go off, the humidity usually climbs about 15 to 20 points on its own, because the air cools. A humidifier still running hard at that moment can push the tent from too dry to wet in an hour. Let the controller ease it off at night, or turn it down before lights-off. Cold air plus a humidifier spikes fast.


What not to do

Don't mist the plants to raise the reading. It lifts the number for a few minutes and leaves water sitting on the leaves.


Don't run a humidifier and a dehumidifier against each other. They undo each other's work, and the room never settles.


Don't push late flower back up. That stage is meant to be the driest part of the run.


When it's something else


On our shelf in Worcester: Fan speed and climate controllers are in Climate, air & controls. Call (508) 459-9609 or get directions.


Keep going

Running the exhaust slower without cooking the tent: see our tent airflow shelf guide.


Steady heat, then steady humidity, one change at a time. The whole system for a steady grow is written down in the free Stability Playbook. Get the free Stability Playbook


More answers: all grow guides. Every shelf guide and maker feed chart: Grower Resources.

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