Best Temp and Humidity for Autoflowers (by Stage)
Quick answer: As a guide, autoflowers like about 20-25°C (68-77°F) and 65-70% humidity as seedlings, around 22-28°C with 50-70% in vegetative growth, and roughly 20-26°C with 40-50% humidity in flowering, dropping humidity to 40% or lower in late flower to avoid mould. Lower humidity as the plant matures, keep temperatures steady (avoid big day/night swings), and use the temperature-humidity balance (VPD) to guide fine-tuning. The chart below sums it up by stage.
Temperature and humidity are two of the biggest levers on an autoflower grow, affecting growth rate, health, yield and the risk of mould. Because autoflowers run on a fast, fixed clock with little time to recover from stress, keeping conditions in the right range at each stage matters even more than with photoperiod plants. The targets shift as the plant grows: warmer and more humid for seedlings, cooler and drier as it flowers.
Recommended Strains
Jack Herer x White Widow Auto
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THC | 20% - 24% (High) |
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Type | Autoflowering |
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Yield | High |
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Phenotype | 40% Indica / 60% Sativa |
Gorilla x White Widow
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THC | 20% - 25% (High) |
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Type | Feminized |
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Yield | High |
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Phenotype | 80% Indica / 20% Sativa |
This guide gives the ideal temperature and humidity ranges stage by stage, with a chart, plus why they matter and how to adjust them. A key concept is that temperature and humidity work together (through VPD, vapour pressure deficit), so it is the balance of the two, not either alone, that really counts. Keep conditions stable and in range, and your autoflowers will reward you.
| Stage | Temp | Humidity (RH) |
|---|---|---|
| Seedling | 20-25°C (68-77°F) | 65-70% |
| Vegetative | 22-28°C (72-82°F) | 50-70% |
| Flowering | 20-26°C (68-79°F) | 40-50% |
| Late flower | 18-24°C (64-75°F) | 40% or lower (avoid mould) |
Ideal Temp and Humidity by Stage
Seedlings want warmth and higher humidity, around 20-25°C and 65-70% RH, because their small roots take up little water and they rely partly on their leaves for moisture. Through vegetative growth you can run a bit warmer (22-28°C) and lower humidity gradually to 50-70% as roots establish. In flowering, aim for about 20-26°C and drop humidity to 40-50%, then 40% or lower in the final weeks to protect dense buds from mould.
Across all stages, avoid large temperature swings between day (lights-on) and night (lights-off), and keep good airflow. The reason humidity drops as the plant matures is both plant preference and mould prevention, flowering buds are vulnerable to grey mould (botrytis) in humid air. Temperature and humidity together determine VPD, the sweet spot for transpiration; for that, see our guide on the autoflower VPD chart.
Promos & Deals
Why Temp and Humidity Matter So Much
Humidity controls how fast plants transpire (lose water through their leaves) and take up nutrients. Too humid, and transpiration slows, growth can stall, and mould risk soars, especially on flowering buds; too dry, and plants transpire too fast, stressing them, curling leaves and wasting energy. The right humidity for each stage keeps this process efficient and healthy.
Temperature works hand in hand with it: warmth drives growth within limits, but too hot (above ~30°C) stresses plants, slows growth and can cause airy, loose buds, while too cold slows everything down. Because the two interact, growers increasingly use VPD to find the ideal combination rather than tweaking each alone. Stable, in-range conditions reduce stress, which is critical for fast, unforgiving autoflowers.
How to Hit These Targets
Hitting the targets is mostly about the right kit and monitoring. Use a thermo-hygrometer (ideally one that logs min/max) at canopy height to actually see your conditions, then adjust: a humidifier raises low humidity (useful for seedlings/veg), a dehumidifier lowers it (essential in flower, especially late), and good ventilation and an exhaust fan help control both heat and moisture. For heat, improve airflow, raise or dim lights, and cool incoming air.
Aim for stability as much as the exact numbers: steady conditions beat perfect-but-fluctuating ones. Watch day/night swings, lights-off periods often get cooler and more humid, which can spike mould risk in flower, so plan for that. Small, consistent adjustments keep autoflowers in their comfort zone right through to harvest.
Common Temp and Humidity Mistakes
A few mistakes trip up autoflower growers. The biggest is keeping humidity too high in flowering, dense autoflower buds plus humid air invite grey mould (botrytis), which can ruin a harvest fast, so humidity must come down to 40-50% (or lower late) in bloom. The opposite error, bone-dry air in veg or for seedlings, stresses young plants and slows growth, so seedlings in particular need higher humidity.
Other common issues are big day/night temperature swings (aim for stability), letting the grow room overheat above ~28-30°C (which stresses plants and loosens buds), and ignoring the lights-off period, which often gets cooler and more humid and spikes mould risk. Monitor with a thermo-hygrometer and adjust with a humidifier, dehumidifier and ventilation rather than guessing. Stability in the right range is the goal throughout.
FAQs of Best Temp And Humidity For Autoflowers
What is the best temp and humidity for autoflowers?
Roughly 20-25°C and 65-70% humidity for seedlings, 22-28°C and 50-70% in veg, and 20-26°C and 40-50% in flowering, dropping to 40% or lower in late flower to avoid mould. Lower humidity as the plant matures, keep temperatures steady, and use VPD to balance the two. Stability matters as much as the exact figures.
Is 30 degrees too hot for autoflowers?
Around 30°C (86°F) is at the upper limit and generally too hot for sustained growth, it stresses plants, slows growth and can cause loose, airy buds in flower. Aim to keep temperatures below about 28°C, improving airflow, raising or dimming lights, and cooling incoming air. Brief spikes are survivable, but prolonged heat hurts yield.
What humidity should autoflowers be in flowering?
Around 40-50% in flowering, dropping to 40% or lower in the final weeks. Lower humidity in bloom protects dense buds from grey mould (botrytis), which thrives in humid air. Keep temperatures steady and airflow good, and watch the lights-off period, cooler, more humid nights raise mould risk, so a dehumidifier is often essential late in flower.
What happens if humidity is too high for autoflowers?
Too-high humidity, especially in flowering, invites grey mould (botrytis) on dense buds and can ruin a harvest, and it slows transpiration and growth. Drop humidity to 40-50% in bloom (lower late), improve airflow, and use a dehumidifier if needed. Watch the cooler, more humid lights-off period, which raises mould risk most.
What temperature is too cold for autoflowers?
Sustained temperatures below about 15-16°C (60°F) slow growth and can stress autoflowers, though a modest night-time dip in late flower is fine (and can bring out colour in some strains). Aim to keep them in roughly the 20-28°C range by day, avoiding both cold and heat extremes, and minimise large day/night swings.
Do autoflowers need a humidifier or dehumidifier?
Often both, at different stages. A humidifier helps keep seedlings and veg in the higher 50-70% range, while a dehumidifier is important in flowering to bring humidity down to 40-50% (or lower late) and prevent bud mould. Monitor with a thermo-hygrometer and adjust with ventilation too. Stability in range matters as much as the exact numbers.
