How to Grow Cannabis in Winter

When growing cannabis in winter, even an indoor garden is at the mercy of the elements. Cold, dry air and dropping temperatures can create problems with relative humidity and light intensity. But that doesn’t mean that growing in winter is impossible. Actually, with the right conditions, an indoor garden can perform just as well in winter as it does in the summertime.

It is true that growing in the winter takes a little more attention and a little more TLC. With the right advice and the right frame of mind, you can successfully grow all year round. That is what we are here to help you do!

How to maintain grow room temperature in winter

Temperature is one of the most important concerns when growing in the winter. Optimal daytime temperature for cannabis is 24-30°C (75-86°F), and optimal night-time temperatures falling in the range of  18-22°C (64-72°F).

As a grower, you want to avoid temperatures outside of the prescribed range but you also want to avoid huge discrepancies between day and night temperatures. Growth rate is severely affected by inconsistent temperature changes as much as by an incorrect temperature bracket.

As well as this, if there is too great a discrepancy between daytime and night-time temperatures in the first 2-3 weeks of the flowering period (during which time plants ‘stretch’ noticeably), very widely-spaced internodes will result. Conversely, keeping the discrepancy as small as possible throughout this time reduces the space between nodes.

A temperature gap of 2-4°C (3.6-7.2°F) is ideal for the first 2-3 weeks of flowering. A gap of no more than 10°C (18°F) should be maintained for the rest of the flowering period.

If you are an old-school cannabis grower, then you probably love growing in the winter. That is because last decade’s HID (high-intensity discharge lightning) technology emits alot of heat. And if you are using them in the summer time, then you need to pay through the roof for air conditioning. But in winter, HID lights can keep your grow room at the optimal temperature.

With that being said, a temperature drop when the lights are turned off is something to think about. To mitigate this, winter growers use their lights during the night time and use daytime as the lights off period. This lets a grower take advantage of warmer temperatures during the day.

While daytime temperatures are higher, they are not always high enough to maintain the plants’ required ‘night-time’ temperatures of 18-22°C (64-72°F). If temperature is consistently dropping below this range when lights are off, it is advisable to use central heating or an electric heater to maintain adequate temperatures.

A digital thermostat will come in handy here to automatically control the heater according to the ambient temperature. If using cold lights such as LEDs, heaters may be required round-the-clock.

Controlling humidity of cannabis grown in winter

Mould is one of the biggest threats to your cannabis garden during the winter. In the winter, there is a tendency for low temperatures to increase relative humidity (RH) to a point of danger for your plants. Not only do cannabis plants detest high levels of relative humidity, but it also makes them a breeding ground for mould and fungi. Low temperatures can create issues in maintaining relative humidity.

Essentially, the volume of water in the air continues to condense as the temperature drops. And if you have ever grown cannabis before, you know that this can open up a proverbial can of worms (or better yet, mould) all of your plants.

The obvious way out of this problem is to keep temperatures at the optimum level, whether by using lights or by using central heating. If the problem is extremely hard to contain, then it may be necessary to use a dehumidifier.

It is also highly recommended that winter growers purchase a device called a hygrometer. This device can test the relative humidity of your soil as well as your grow room. It is essential to regularly measure the relative humidity in order to stay on top of it and avoid huge spikes or drops in the concentration of water in the air.

In many areas, winter is actually the driest time of the year, as well as being the coldest. If you are not facing issues of relative humidity, then cold dry air will present problems of its own in the grow room. If air is taken in from outside at a temperature of 10°C and an RH of 50%, it will contain water vapour at 4.7g/m³. If this air is heated to 25°C without the addition of extra moisture, its RH will drop to around 20%, which is far too low for healthy cannabis plants to grow.

In the growroom, a moist growing medium along with transpiration will generally raise levels of water vapour in the air. However, relative humidity should remain consistently between 40% and 60%.

Other things to consider when growing in winter

Temperature and humidity are the main issues that winter-time growers will have to deal with. Lighting is usually not an issue as plants are typically grown under HID lights.

Having said that, some hobby growers might still be eager to utilise as much natural sunlight as possible, such as growing on a windowsill. The problem with this is that light intensity or simply the amount of sunlight hours may be insufficient for growth. Each area will vary in its wintertime habits, and each grower should make choices accordingly.

Some clever growers will use extra lighting during winter only, to ensure that their plants have enough light to grow. While plants will usually not achieve the yields and qualities achieved in more favourable times of the year, there are plenty of smart hobby gardeners out there that will ensure their supply remains steady even in the harshest times of the year by following this principle.

Greenhouse growers are similarly affected by the reduction in daylight hours during winter, and unlike those growing on their windowsill at home, additional lighting may attract unwanted attention.

However, some adventurous greenhouse growers will add the supplementary lighting and then make sure that the greenhouse is covered so that light does not escape when it is dark outside. Thick, heavy blackout curtains or Mylar sheeting can do an excellent job here. Then, all that remains is to heat the greenhouse sufficiently and provide adequate airflow, and growing throughout the winter should become possible.

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Consider growing winter-appropriate strains

Every grower understands the temptation of growing their favourite strains throughout the winter, even if those strains aren’t really appropriate for winter growing. If you can let go of that temptation and choose winter-appropriate strains, you might be able to avoid some of the difficulties in growing over a snowy winter.

Let’s look at autoflowering strains as an example. In as little as 8 weeks from germination of the seed, you can have a ready-to-harvest cannabis plant. This means that a grower can avoid growing throughout the harshest part of the winter and maximize their growing opportunity.

Any strain that contains ruderalis genes is also more likely to make it through a winter. Ruderalis is tougher and hardier than more common strains. It also isn’t photoperiod dependent, meaning it will flower when it’s ready rather than as a response to a change in light hours. It can bloom within 30 days of planting.

Growing in winter might mean doing some additional planning in advance to prevent the demise of your plants. But that doesn’t make it impossible. After a couple of seasons growing weed in a snowy winter, it should start to feel like second nature.