Temperature And Humidity

Temperature and humidity are the two numbers that quietly decide how your grow goes. They do not cause dramatic overnight failures the way overwatering does. They cause slow underperformance, and then one expensive disaster in late flower if you ignore them.

The good news is that both are cheap to measure and mostly manageable with equipment you already bought. A ten dollar combined thermometer and hygrometer at canopy height, an exhaust fan, and a small humidifier cover nearly every situation in a home tent.

Put the meter at the top of the plant, not on the tent floor. The temperature the leaves experience is what matters, and the canopy sits several degrees warmer than the floor because it is directly under the light.

The targets#

Stage	Temp, lights on	Temp, lights off	Humidity
Seedling	70 to 78F	65 to 70F	65 to 70%
Vegetative	72 to 82F	65 to 75F	45 to 60%
Early flower	70 to 80F	65 to 72F	40 to 50%
Late flower, last 3 weeks	65 to 78F	62 to 70F	40 to 45%

Two patterns to notice. Humidity comes down steadily as the plant grows. Temperature comes down slightly at the end.

Humidity starts high because a seedling has almost no roots and takes in moisture through its leaves. It comes down because a mature plant in flower has dense buds that trap moisture, and moisture trapped in a bud is how you get rot.

Temperature#

Cannabis grows well in a range most people find comfortable in a t-shirt. If your room sits at 72F, you are already close.

Too hot, above about 85F. Growth slows, the plant drinks far more water, leaves may cup upward or fold along the middle to reduce surface area, and in flower you get airy loose buds and a real loss of terpenes. Sustained heat above 90F in flower will noticeably cost you quality.

Too cold, below about 60F. Growth slows and nutrient uptake drops, particularly phosphorus, which shows up as purple stems and dull growth. A cold pot is also a slow drying pot, which loops back into overwatering.

The night drop. A 5 to 10F difference between lights on and lights off is normal and fine. Bigger than about 15F and you risk condensation on the leaves, because cooler air holds less moisture and the relative humidity spikes when the light shuts off. That is a real mold risk in flower.

Fixing heat, in rough order of cost:

• Turn the light dimmer down 10 or 20 percent, or raise the panel.
• Run the lights at night when the room is cooler.
• Turn the exhaust fan up. Most inline fans come with a speed controller.
• Leave the bottom tent vents open so cooler room air gets pulled in.
• Air condition the room. The tent is only ever going to be a few degrees above ambient.

That last one is the honest answer for anyone growing in a hot climate without AC. A tent cannot get colder than the room it is in.

Humidity#

This is the one people neglect, and it is the one that costs whole harvests.

Too high. In veg it slows transpiration, which slows nutrient uptake, and the plant grows lazily. In flower it invites powdery mildew on the leaves and bud rot, botrytis, inside the flowers. Bud rot is the nightmare scenario: you see a single browning leaf sticking out of a cola, you pull it apart, and the inside is gray mush. It spreads through a plant in days.

Too low, below about 30 percent. The plant transpires faster than the roots can supply, leaf edges get crispy, and growth slows. Less dangerous than too high, but not free.

Raising humidity is cheap. A twenty to thirty dollar ultrasonic humidifier feeding into the tent handles a 2x2 or 2x4 easily. A wet towel or an open pan of water works in a pinch. So does turning the exhaust fan down, since the fan constantly pulls humid air out.

Lowering humidity is where it costs money. Turn the exhaust fan up first, since that is free. Add air circulation inside the tent so no pocket of still, damp air sits in the canopy. If that is not enough, a small dehumidifier runs sixty to a hundred fifty dollars and is the only reliable fix in a damp basement or a humid summer.

Airflow is part of this#

Two different jobs, and you need both.

Circulation is the clip fan inside the tent. It moves air around the canopy so no pocket sits still and damp, it strengthens stems, and it evens out the temperature between the hot zone under the light and the cooler floor. Point it so leaves flutter. If they are thrashing, you get wind burn, which looks like leaf edges curling and drying.

Exchange is the inline fan pulling old air out and drawing fresh air in. This is what actually removes heat and humidity and replaces the carbon dioxide the plant used up. A 4 inch inline fan exchanges the air in a 2x4 tent many times an hour, which is what you want.

Run the exhaust continuously. Some people run it only during lights on, but humidity spikes at night when the temperature drops, which is exactly when you want the air moving.

Getting stable is worth more than getting perfect#

A tent that sits at a steady 76F and 55 percent all day is better than one that hits the ideal number at noon and swings 20 degrees overnight. Plants adapt to conditions. They do not adapt well to conditions that change every few hours.

So set your fan speeds, check the meter for a few days, adjust once, and then stop fiddling. Most cheap hygrometers also record a high and low, which tells you what happened while you were asleep. That reading is more useful than whatever the display says when you walk by.

When you cannot hit the numbers#

You will not nail every range, especially in a bedroom in August. Prioritize in this order:

1. Keep late flower humidity under 55 percent. This is the one that loses harvests.
2. Keep the canopy under 85F. Above that you are losing quality steadily.
3. Keep the night to day swing under 15F.
4. Everything else.

A grow that runs a bit warm and dry the whole way still produces good weed. A grow that hits perfect numbers for ten weeks and then sits at 70 percent humidity for the last two produces moldy flower.

With environment settled, the plant starts eating through what is in the soil, and you have to decide when to start feeding it.