Keeping cannabis stable in cold climates

## Introduction

Cold climates pose particular **climatic** challenges in cannabis cultivation: low intake temperatures, sharp day/night drops, cold surfaces with a risk of condensation and dry winter air that changes significantly once it warms up. The aim is not simply to make the room "warm", but to create **a stable, plant-compatible climate without harsh fluctuations**.

This guide focuses exclusively on temperature, humidity, air movement, supply air flow and CO2 aspects in cold environments.

## Typical climate problems in cold regions

In cold climates, several effects often occur simultaneously:

- **cold supply air** creates local cold zones

- Large night-time drops** slow down metabolism and growth

- cold walls, windows or metal parts** promote condensation

- warmed winter air often appears **very dry**, although the outside air may appear humid

- uneven air distribution causes **microclimates** within the stock

Particularly problematic are not only absolute values, but **rapid changes**. Cannabis often reacts more sensitively to abrupt drops in temperature than to slightly sub-optimal but constant conditions.

## Temperature control in cold climates

The most important rule is: **stable instead of extreme**.

### Sensible target ranges

**Young plants/cuttings**

- day: **22-25 °C**

- Night: **20-22 °C**

**Vegetative phase**

- Day: **22-27 °C**

- Night: **18-22 °C**

**Flowering phase**

- Day: **20-26 °C**

- Night: **17-21 °C**

**Drying after harvest**

- **18-22 °C** at **approx. 60 % rH**

### Important practical rule

The **day/night difference** should remain as moderate as possible in cold regions. A practical range is usually **about 3-6 °C**. Larger drops increase the risk of:

- slower transpiration

- condensation on cold surfaces

- damp boundary layers in dense stands

- Cold stress and growth stagnation

## Correctly classify humidity and VPD in cold conditions

Cold outside air often contains very little water. If it is heated in the grow room, its **relative humidity** drops significantly. As a result, the air inside can become too dry in winter despite humid weather conditions.

### Orientation about VPD

A practical orientation:

| Phase | Target VPD |

|---|---:|

| Young plants/cuttings | 0.4-0.8 kPa |

| Vegetative phase | 0.8-1.2 kPa |

| Flowering phase | 1.0-1.4 kPa |

|

Typical in cold climates:

- VPD** too high due to dry, heated air

- VPD** too low on cold nights with increasing RH

Therefore, temperature and humidity should **always be considered together**. A room can be too dry during the day and too humid at night, even though the average values look harmless.

## Defuse supply air in cold climates

Direct incoming outside air must not hit plants unbuffered. Cold air falls downwards and easily creates invisible stress zones.

### Proven measures

- Do not introduce supply air **directly at plant height**.

- Guide the inlet so that outside air **mixes with room air** beforehand

- Reduce cold air flows via **circulation or anteroom**

- Place sensors at several points to detect **cold air pockets**

The aim is **even temperature distribution** instead of a warm center and cold peripheral areas.

## Control condensation and dew point

In cold regions, it is not only the air temperature that is critical, but also the temperature of surfaces. If a wall, pane or pipe is close to the **dew point**, moisture can condense.

### Warning signs

- damp corners or cold tent walls

- Water film on windows or pipes

- musty odor despite acceptable average RH

### Countermeasures

- Improve air movement at cold edge zones

- Avoid sharp drops in temperature at night

- Keep humidity consistently lower during flowering

- Do not place plants in dead, unmoving cold air areas

Otherwise, the risk of botrytis and other moisture problems increases significantly, especially during flowering.

## Using air circulation correctly in cold weather

Even in cold climates, **gentle, continuous air movement** is needed. However, too much airflow directed at cold plants can cause additional stress.

### Aim of the air circulation

- equalize temperature differences

- Remove moist air from the canopy

- Prevent condensation zones

- Distribute CO2 more evenly

Fans should therefore **not blow continuously and sharply on individual plants**, but create an even, indirect exchange of air throughout the room.

## CO2 in cold, heavily ventilated rooms

Cold regions often lead to high exhaust air performance because humidity and temperature have to be constantly corrected. As a result, CO2 is exchanged quickly. In such setups, CO2 enrichment **only makes sense in well-controlled, relatively closed systems**. In very open or draughty rooms, the effect usually fizzles out.

## Conclusion

Cannabis in cold climates is successful if you actively manage **cold sources, airflow and dew point**. The decisive factors are not extreme values, but **stable day/night conditions, defused supply air, controlled humidity and uniform circulating air**. Cleanly integrating cold outside air prevents cold stress, condensation and unnecessary growth inhibitors.

## Pro Tips

- Always measure temperature and RH at the edges.

- Avoid direct cold air supply to the canopy.

- Keep night-time waste as moderate as possible.

- Pay particular attention to the dew point in winter.

- Never judge the climate by a single sensor alone.