How cold is too cold for my grow room at night?

A cool night is normal and good for her: aim for 5 to 10°F below lights-on, and not below the mid-60s. Trouble starts below about 55°F at the canopy, and near 50°F or lower is an emergency. In winter, fix the room around the tent first.

Warm the room, not the tent.

Where the lines are

• Lights on: 75 to 82°F in veg, 72 to 80°F in flower.

• Lights off: 5 to 10°F cooler than lights-on, and not below the mid-60s.

• Below about 55°F at the canopy: trouble for most warm-season plants. Near 50°F or lower: treat it as an emergency.

• The roots: 68 to 75°F in the pot.

One cold night usually costs growth, not the plant. Several in a row do serious harm. Read the overnight low on a sensor at canopy height, shaded and out of the fan's path, before you decide anything.

What cold looks like on the plant

• Growth slows. New leaves come in smaller and slower.

• She looks hungry when she's just cold. Cold roots take up water and food slowly, so a cold plant can pale and fade like an underfed one. Warm her before you feed her.

• The pots stay wet longer. A cold pot drinks slowly, so the same watering rhythm that worked in summer can leave the mix wet for days.

• Purple can creep into the stems and leaf stems. Some plants do this after cold nights, and some do it no matter what, so purple on its own isn't a diagnosis. Read it alongside the overnight low.

Fix the room first, in this order

1. Slow the cold air. In a cold room the exhaust pulls cold air through the tent faster than the light can warm it. Turn the fan down one speed notch at a time, and if you can, pull the intake air from a warmer room. Don't switch it off: heat and humidity still need somewhere to go.

2. Heat the room, not the tent. A small heater on a thermostat in the room keeps things steadier than chasing the temperature inside the tent. A sealed oil-filled radiator, with no glowing element, is a common pick. Give it space on every side, and check its watts against whatever it plugs into; heaters pull a lot.

3. Get the pots off a cold floor. A pot on concrete runs as cold as the floor. Set it on a board or a stand.

4. Water at room temperature. Cold water straight from the tap chills roots that are already slow. Let it come up to room temperature first.

5. Let the light help. The light is your heater for half the day. If the nights are the cold part, you can run the light period through the coldest hours instead. Make that change in veg or between runs, not in the middle of flower.

Change one thing at a time, about 2°F at a time. Give it at least an hour to settle before the next change, and judge the result over a full day and night, overnight low included.

Warm air gets dry

Heating cold air drops its relative humidity: the same water, in warmer air, reads as a lower percentage. So get the heat steady first, then see where the humidity lands before you add any. A steadier night also takes the edge off the 15 to 20 points humidity often jumps when the lights go off, because most of that jump is the temperature dropping, not new water.

If the power goes out

The lights are already off, so the risk flips to cold and still air. Keep the tent closed so it holds the heat it has. When the power comes back, check the timer, the clock and the fan before you walk away, because timers lose their place. A sensor at canopy height that sends your phone a low-temperature and an "offline" alert tells you before morning, not after.

What not to do

Don't put a heater inside the tent. It makes a hot spot and puts the heater next to fabric and wet pots.

Don't make up for a cold night with a hot day. A big swing is its own stress. Bring her back into the lane in steps.

Don't feed a cold plant to fix the pale. Warm her first, then judge the new growth.

When it's something else

• Humidity jumps when the lights go off: see What humidity should I run, and what if I can't hit it?

• A droopy plant in a heavy pot: see How often should I water, and why is she drooping?

• The air's too dry now the heat is on: see My tent is too dry this winter. What do I do?

• Purple stems or leaves in a cold room: see My plant is turning purple. Is it the cold, hunger, or just her genes?

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

Keep going

Running the exhaust slower in a cold room: see our tent airflow shelf guide.

A steady room, day and night. 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.