How Cold Weather Affects Cannabis Plants, and How to Use It in Late Flower

Cold affects cannabis differently depending on the stage: a cool, dry finish on a mature plant can boost resin and keep terpenes intact, but temperatures under about 8-9 °C (46-48 °F) slow root activity, and frost can freeze the plant's sap and do serious damage. Temperature is one of the biggest factors in any grow, and in certain seasons and regions, cold can be a real obstacle to getting your plants to harvest. Still, if you know your plant well, you can turn it to your advantage.

Like any plant, cannabis doesn't do well with too much cold or too much heat. And at each growth stage, you need to know what the plant wants and what temperature works best. 

Cold at every stage: how much cold can a cannabis plant handle?

During the vegetative stage, a plant pours all its energy into producing cellulose, in other words, building solid plant mass both in its roots and above ground. 

If you start your grow in early spring, your plants get about four months of growth to build a strong enough structure, with big, deep roots and thousands of root hairs, those fine white rootlets that do most of the nutrient uptake.

By the time flowering kicks in, around mid-July in the Northern Hemisphere (mid-January south of the equator), the plant will have a solid structure that can handle many kinds of stress, including heat and cold stress, which is what we're talking about today. 

As harvest approaches, you may worry that the cold is a constant enemy for your plant. Don't panic: it's the classic signal that the plant's life cycle is winding down. The usual explanation is that, in response, and to catch pollen from a male plant, it ramps up resin production. That makes these weeks key, because cannabis is trying to fulfill its biological purpose: making seeds to carry on the species.

The more resin the plant produces, the better its chances of trapping male pollen. Since it doesn't have much else left to do at the end of its cycle, it throws everything into this stage: you'll see the buds firm up almost by the hour, and the leaves, still in decent shape, start to crinkle like those of deciduous trees in fall. The plant also gets coated in a thick, heavy layer of resin your eyes are going to love.

Keep in mind this only happens if the plant is fully mature and ready to enter its final stage. If the cold hits earlier for whatever reason, you get the opposite: buds that are still forming never firm up, and the plant starts to decline, leaving all that growth potential on the table.

On top of that, heat tends to evaporate terpenes, while cold doesn't. Plants grown in cooler conditions often have purer, more intense flavor and aroma.

Use the cold to your advantage: know your climate

As we said, cold can be good for your plants under the conditions above. But heads up: we're talking about cool weather, not extreme lows like anything under 8-9 °C (46-48 °F). Below that, root activity slows down and nutrient uptake drops. And if it dips below 0 °C (32 °F), brace yourself: the plant's sap freezes, its vital functions shut down, and the time it takes to get going again can be brutal for the plant.

Relax, that doesn't mean the plant is going to die, at least not a well-developed one. But there's no denying that exposing it to extreme cold does damage. 

Nature can surprise you, though: growers who followed the process to the letter, with a big dose of luck, have seen near full-size sativas covered in snow in early December (Northern Hemisphere) and harvested two weeks later with no problems at all. That's in the ground, mind you; in containers, the cold hits much harder.

That's why it's crucial to know your own climate before you commit to certain genetics. 

Rather than risk losing your plants, if your climate gets very cold in early fall, go for early-finishing mountain indicas: these genetics are built to handle the cold nights of late summer and early fall. They're also a good call where days are scorching but nights drop so far that the temperature swing is more than most strains can take.

A few cold-weather tips

If you've grown into the cold at the end of the season, or you figure you've got about a week left until harvest, it pays to strip the fan leaves and most of the small leaves. Basically, it's a trim on the plant a week before you chop. With the leaves it relies on for photosynthesis gone, the plant can focus on resin and trichome production instead of carrying on with its normal routine. 

Once the cold days and nights set in, stop watering. The reason is simple: the same frost that settles on the leaves can  freeze the water in the growing medium, which can be fatal for your plant. A long dry spell plus a mild frost, the theory goes, gives you a top-quality harvest.

As we've seen, the plant handles cold above ground pretty well, but not so much in its root zone. One of the best ways to protect the roots is a mulch of fresh grass clippings, fallen leaves, or even a sheet of plastic or a burlap sack. Grass and leaves heat up as they start composting. Composting is slow, though, so it won't harm a plant you're harvesting in a few days. 

Plastic, on the other hand, is a big help: it lets the soil warm up more during the day than it would bare, and at night it takes longer to cool off, holding the temperature almost until the next morning.

Drying and curing in the cold

Drying your harvest in cool, dry conditions is the way to go. And if you can cure it under similar conditions afterward, even better. 

Heat speeds up drying, but it also degrades THC and takes the edge off its potency. Cold doesn't, as long as it's dry and not damp, although you'll find drying takes a bit longer. 

So when you see a sharp temperature drop coming in the first days of fall, you should already know the basics about your plants: whether they're sativas or indicas, whether they're about ready to harvest or still have weeks to go, and even whether the genetics come from a cold climate or not. Once you know the answers, you can choose your genetics accordingly next year.

As we said earlier, too much cold or heat isn't good for anyone, least of all your plant. But if you know what you're up against, you can use each to your advantage, help your plant grow strong and bring in a killer harvest. 

What's the lowest temperature cannabis can survive?

Cannabis plants have some tolerance for cold, but how much they can take depends on the strain and the conditions. In general, temperatures below 10 °C (50 °F) can start to hurt growth. Some strains, though, are genetically better adapted to the cold, especially those that come from colder or mountainous regions. These tend to be hardier and can handle temperatures close to freezing. If you're expecting a long cold spell, you can protect your plants with greenhouses, plastic covers or frost cloth to create a barrier against the cold, or even heaters to keep the temperature where it needs to be. Hard frosts are especially dangerous, though, since they can damage the plant's cells and tissues for good. So protect your plants from frost and avoid long exposure to low temperatures to keep them growing healthy.

What's the best climate for growing weed?

The ideal climate for growing cannabis is generally warm and temperate. During the vegetative stage, aim for 20 to 30 °C (68 to 86 °F). That range gives plants ideal conditions for healthy, vigorous growth. During flowering, it helps to bring the temperature down a bit, to around 18 to 26 °C (64 to 79 °F), to encourage good bud development.

Keep in mind that some strains handle heat better than others. These genetically adapted strains can take higher temperatures, even above 30 °C (86 °F), without significant damage. If you're growing somewhere hot, these heat-tolerant strains are a smart pick for a better yield.

On the flip side, while cannabis is more sensitive to cold than heat, some strains are cold-hardy and can handle lower temperatures. If you live somewhere with cold winters, look for strains bred for cold climates that can take temperatures close to freezing.

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