# Temperature by stage: day, night and root-zone targets

> Run roughly 24–28 °C (75–82 °F) by day in veg, 22–26 °C (72–79 °F) in flower, cooling to 20–24 °C (68–75 °F) in late flower, with nights 3–6 °C (5–10 °F) cooler. Above about 30 °C (86 °F) photosynthesis in cannabis falls unless light and CO₂ are high. Keep the root zone and hydro reservoirs at 18–22 °C (64–72 °F).

- Canonical: https://genetixbay.com/knowledge/environment/temperature-by-stage.html
- Category: Environment · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review
- Stage: Seedling, Veg, Transition, Flower, Ripening · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What temperature should my grow room be in veg and flower?** A cannabis grow room should run about 24–28 °C (75–82 °F) by day in veg, 22–26 °C (72–79 °F) in mid flower and 20–24 °C (68–75 °F) in late flower, read at canopy height with a shaded probe. Add 2–3 °C when running 800–1,200 ppm CO₂. These setpoints are grower practice informed by one cannabis gas-exchange study and greenhouse horticulture.

**How much cooler should the night be?** Cannabis grow-room nights are usually run 3–6 °C (5–10 °F) cooler than the day. A smaller day-night difference during the transition weeks helps limit stretch, because a large positive difference lengthens [internodes](https://genetixbay.com/knowledge/glossary/internode.md), an effect well documented in ornamentals and grower practice in cannabis. Avoid sharp swings at lights-on and lights-off, which cause condensation and invite bud rot.

**Is 30 °C [too hot](https://genetixbay.com/knowledge/glossary/heat-stress.md) for cannabis?** 30 °C (86 °F) is at the upper edge for cannabis: research shows net [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) and water-use efficiency fall above about 30 °C [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) unless light and CO₂ are high. Sustained air above 30–32 °C is a hard limit in an un-enriched room. CO₂-enriched commercial rooms run 28–30 °C, but copying that setpoint into a tent without CO₂ is a common mistake.

**What reservoir temperature should I run?** A hydroponic cannabis reservoir should run at 18–22 °C (64–72 °F), which is established extension guidance for hydroponic [root zones](https://genetixbay.com/knowledge/glossary/root-zone.md). Above 24 °C (75 °F) in DWC or [RDWC](https://genetixbay.com/knowledge/glossary/rdwc.md), root-rot risk rises sharply. Keep a reservoir cool with a chiller, insulation, a larger volume, and by lifting it off a warm floor into shade; healthy roots stay white.

**Does a cold night really bring out [purple colour](https://genetixbay.com/knowledge/glossary/anthocyanin.md)?** Yes, cold nights can really bring out purple colour in cannabis, but only in cultivars that carry the genetics for anthocyanin pigments; colour is mostly genetic. Big cold nights also slow ripening and invite condensation and bud rot, so they are a trade-off. Late-flower nights of about 16–20 °C (61–68 °F) are the usual range.

**Do cold nights increase terpenes?** No, cold nights do not increase terpenes; that is a myth and the GenetixBay house position unless peer-reviewed data shows otherwise. No published evidence supports more terpenes, flavour or resin from cold. Cooler late-flower days of 20–24 °C may reduce heat losses of volatile terpenes, which is grower practice, but deliberately chilling nights adds colour at most.

**Short answer.** Cannabis photosynthesises best somewhere around **25–30 °C (77–86 °F) leaf temperature** when light is strong; above ~30 °C (86 °F) net photosynthesis and water-use efficiency fall in the one cannabis gas-exchange study we have [1]. In practice: **veg 24–28 °C (75–82 °F) day, flower 22–26 °C (72–79 °F) day, late flower 20–24 °C (68–75 °F)**, nights **3–6 °C (5–10 °F) cooler**, root zone **18–22 °C (64–72 °F)**.

## What's happening

Temperature sets the speed of every enzyme in the plant. Photosynthesis climbs with temperature until respiration and photorespiration catch up; at high light and high CO₂ the optimum shifts warmer, which is why CO₂-enriched rooms run 2–4 °C hotter [1][2]. Night temperature mostly controls respiration (sugar burnt) and stretch: a warm night relative to day (negative DIF) shortens internodes, a big positive DIF lengthens them (well documented in ornamentals; *Grower practice* in cannabis).

Leaf temperature is what matters, and it is not air temperature. Under [LEDs](https://genetixbay.com/knowledge/glossary/led.md) well-watered leaves usually run slightly cooler than air; under HID they run warmer [3]. See [VPD explained](https://genetixbay.com/knowledge/environment/vpd-explained.md).

## Numbers that matter

| Stage | Day air | Night air | Root zone / reservoir | Notes |
|---|---|---|---|---|
| [Germination](https://genetixbay.com/knowledge/glossary/germination.md) | 22–28 °C (72–82 °F) | same | 22–26 °C (72–79 °F) | Hemp seed optimum ~19–30 °C [5] |
| [Seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) / clones | 23–27 °C (73–81 °F) | 20–24 °C (68–75 °F) | 21–24 °C (70–75 °F) for rooting | Warm root zone speeds rooting |
| Veg | 24–28 °C (75–82 °F) | 19–23 °C (66–73 °F) | 18–22 °C (64–72 °F) | Add 2–3 °C with CO₂ 800–1,200 ppm |
| Transition (flower wk 1–3) | 23–27 °C (73–81 °F) | 19–22 °C (66–72 °F) | 18–22 °C (64–72 °F) | Smaller DIF limits stretch |
| Mid flower (wk 4–6) | 22–26 °C (72–79 °F) | 18–22 °C (64–72 °F) | 18–22 °C (64–72 °F) | |
| Late flower / ripening | 20–24 °C (68–75 °F) | 16–20 °C (61–68 °F) | 18–21 °C (64–70 °F) | Cooler days avoid heat losses of volatile terpenes (*Grower practice*); cold nights don't add terpenes |
| Hard limits | Above 30–32 °C (86–90 °F) sustained, or below 15 °C (59 °F) | | Above 24 °C (75 °F) in DWC/RDWC raises root-rot risk [4] | |

## What to check first

1. Canopy-height reading, shaded probe, moving air. Ceiling and floor readings lie.
2. Leaf temperature with an [IR thermometer](https://genetixbay.com/knowledge/glossary/ir-thermometer.md); compare with air.
3. Day-night swing: plot 48 h. Sharp swings at lights-on/off cause condensation.
4. Reservoir or pot temperature, especially in hydro and [fabric pots](https://genetixbay.com/knowledge/glossary/fabric-pot.md) on warm floors.

## How to fix it

- **Too hot:** raise or dim lights 10–20 %, improve exhaust or add cooling, move ballasts/drivers outside the room, run lights during the cooler part of the day.
- **Too cold:** insulate, add a thermostatically controlled heater (never an open-element heater near water or plastic), shorten the dark-period swing.
- **Warm reservoir:** chiller, insulated reservoir, reservoir off the floor in shade, larger volume.

## How to confirm it worked

New growth is evenly spaced (no sudden stretch), leaves don't taco or droop at mid-day, water use is steady, and roots in hydro stay white.

## Common mistakes

- Copying a CO₂-enriched commercial setpoint (28–30 °C) into an un-enriched tent.
- Ignoring leaf temperature under HID.
- Big cold nights to "force purple" or "boost terpenes". Anthocyanin colour is mostly genetic; cold can bring it out in some cultivars (real), but it also slows ripening and invites condensation. More terpenes, flavour or resin from cold: *Myth*.
- Warm reservoirs in summer and wondering why roots go brown.

## Evidence

The cannabis gas-exchange response to temperature and light is *Supported* by Chandra et al. [1][2] (one cultivar, leaf level, short-term measurements). Stage setpoints and DIF effects are *Grower practice* informed by greenhouse horticulture. Root-zone 18–22 °C for hydroponics is *Established* extension guidance [4]. Germination cardinal temperatures for hemp are *Supported* [5]. Cold nights bring out anthocyanin colour in cultivars that carry it; that's real. Claims that cold nights add terpenes, flavour or resin are a *Myth* (house position) unless peer-reviewed data shows otherwise.

## Sources

1. Chandra S, Lata H, Khan IA, ElSohly MA, 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants 14(4):299–306. https://doi.org/10.1007/s12298-008-0027-x
2. Chandra S, Lata H, Khan IA, ElSohly MA, 2011. Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2. Physiology and Molecular Biology of Plants 17:291–295. https://doi.org/10.1007/s12298-011-0066-6
3. Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, HPS and LEDs. PLoS ONE 10(10): e0138930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598144/
4. e-GRO / Colorado State University Extension, Root disease problems and management in hydroponic systems (root-zone 68–72 °F). https://planttalk.colostate.edu/topics/miscellaneous/2043-root-disease-problems-management-hydroponic-systems/
5. Geneve RL et al., 2022. Temperature limits for seed germination in industrial hemp (Cannabis sativa L.). Crops 2(4):415–427. https://doi.org/10.3390/crops2040029