Temperature is one of the most important variables for cannabis growers to control. As a general rule, cannabis plants aren’t big fans of high temperatures or the cold, and are happiest when the mercury reaches 20 to 30 degrees Celsius (70 to 85 Fahrenheit) during the daytime.
When things get a little too toasty, plants can start to suffer from heat stress, with potentially disastrous results for your cannabis grow. Likewise, allowing the temperature to drop too low can ruin a crop. Whether cultivating outdoors or in a grow room, it’s essential to ensure your cannabis plants don’t get overly warm or too chilly.
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Watch For Signs Of Heat Stress
Cannabis growers should constantly be on the lookout for signs of ill-health in their plants. Fortunately, heat stress is one of the easier problems to spot and diagnose. Excessive heat usually causes plants to droop and wilt, while fan leaves normally curl up due to overheating. If the problem occurs during the flowering stage, you may even notice foxtails appear, as small buds start bursting out of larger ones to produce a bushy tail-like structure.
Heat stress can occur when growing outdoors during a heatwave or if the lights and other equipment in your grow room produce too much warmth. Often, the problem is caused by hot grow lights that are too close to the tops of the cannabis plants.
Indoor Growing – How To Prevent Heat Stress
As mentioned, lighting is a classic cause of heat stress for cannabis plants. Sometimes, raising the height of your lights so that they stop burning the tops of your cannabis plants can solve the issue. However, changing your entire lighting set-up may also be necessary if your gear is putting out too much heat.
These days, many growers opt for LED grow lights, partly because they generate far less heat than HPS bulbs. The latter can cause a grow room to become quite toasty, so switching to a more energy-efficient alternative is often the easiest way to prevent overheating.
Installing fans keeps the air circulating and prevents the atmosphere from becoming stagnant while also keeping humidity levels under control. In hotter climates, an air conditioning unit may be required. Though this is more expensive than a simple fan, your plants will definitely thank you for keeping the temperature within an acceptable range. Fresh air is usually best.
Many growers also employ exhaust systems to remove hot air from the interior of the grow space, creating a convection current that keeps things nice and cool. Many people also choose to fit their exhaust with a carbon filter to remove any tell-tale odours that may otherwise be broadcast to nosy neighbours.
Outdoor Growing – Avoiding Heat Stress
For obvious reasons, growers have less control over the environment when growing cannabis plants outdoors. Keeping plants in pots rather than rooted to the ground can therefore be beneficial, as this provides the flexibility to move them around when a heatwave strikes.
Placing plants in a shady, breezy spot during the hottest part of the day can prevent heat stress. However, you’ll want to ensure they continue to receive enough direct sunlight for photosynthesis to occur; otherwise, you’ll get very little growth and no THC or other valuable cannabinoids.
Keeping plants watered is obviously essential when the weather is hot, although it’s important not to over-water as this can cause roots to become starved of oxygen. When the sun is intense, watering in the early morning will allow the roots to be cooled by evaporation as the ground heats up. Watering in the day’s heat is a terrible idea as water droplets can magnify the sun’s rays and burn your plants’ leaves.
Best Cannabis Strains For Hot Weather
Most Sativa strains obtain their genetics from landraces that developed in hot countries, meaning they tend to do better in warm weather than Indicas. Haze cultivars, for instance, blend Hawaiian, South Indian, Mexican, Thai and South American genetics.
Mama Thai, Amnesia Haze and Sugar Haze are all excellent choices when growing in a hot climate due to their heat-resistant genetics. Adding supplements like kelp extract or silica can boost plants’ heat tolerance by strengthening cell walls and improving nutrient uptake.
The Problem With Cold Temperatures
Cold cannabis plants can be pretty miserable. Once the temperature around the roots drops below 15 degrees Celsius, metabolism comes to a standstill and plant growth grinds to a halt. Cold temperatures and moisture or high humidity levels are even worse, as this mix encourages bud rot and mould. For these reasons, snow, frost, and rain are all bad news for cannabis growers.
Some cannabis strains also tend to become hermaphrodites when they get too chilly, which is another reason to protect your plants against the cold.
Growing Cannabis in Cold Weather
The simplest way to protect cannabis plants from cold weather is to move them inside when the temperature drops. This requires growers to keep an eye on the weather forecast and is obviously only possible when plants are grown in pots rather than the ground.
When moving photoperiod plants inside during the flowering stage, be sure to provide them with the same number of light and dark hours as they received outside. Giving plants too much light can cause them to revegetate, which is the last thing you want when you’re in the latter stages of a cannabis grow.
Alternatively, greenhouses and polytunnels can be erected to protect plants from the worst of the weather. Temperature and lighting can be controlled inside a greenhouse to a certain extent. However, if you find yourself in a race against time to finish your grow before winter kicks in, you may want to try force-flowering.
This can be achieved using blackout blinds to darken a greenhouse and trick cannabis plants into thinking it’s nighttime. Using this technique, growers can initiate a 12-hour photoperiod and trigger the flowering stage.
If none of the above is possible and plants have to be left outdoors and exposed during cold weather, propane-powered heaters can help to keep plants warm. Covering your crop when frost or rain is forecast can also help keep mould at bay.
The Best Cannabis Strains For Cold Weather
Indica strains originate in the cold Himalayan climate and therefore tend to be hardier than heat-loving Sativas. Autoflowering cultivars are descended from a lineage called Cannabis Ruderalis, which is native to Eastern Europe and Siberia and well adapted to the cold.
The fact that autos can complete an entire grow cycle in as little as eight weeks also makes them an excellent choice for growers with a short season. Perhaps more significantly, though, autoflowering cannabis strains can be grown with the lights switched on 24 hours a day, which means your grow room can be kept nice and toasty round the clock.
Cannabis cultivation is all about balance, and getting your temperatures right is so critical to healthy marijuana plants.