by Nebula Haze
Oh no! Why are my cannabis plants drooping this morning? And I see some brown spots on leaves out of no where. They looked wonderful last night. And brrrr it’s so cold in here. I remember the first time my grow room got cold I didn’t realize at first what was going on with my plants.
Cannabis plants don’t like the cold
Are my Cannabis Plants Too Cold?
Cold plants grow much slower than warm plants, and buds don’t fatten as much
Not only do cold cannabis plants grow slower in general because their metabolism is slower, but cool temperatures also cause the plant leaves to turn purple early in the flowering stage. This prevents some photosynthesizing, reducing yields and growth rates even further.
General droopiness and slow growth are the most common symptoms of plants getting too cold
Brown spots on leaves and brown curling leaf tips are common.
Cannabis plants often start getting brown spots on leaves in the cold. This is because water isn’t moving properly through the plant as opposed to a nutrient deficiency
Cold temperatures can cause plants to get odd brown spots because water isn’t moving properly through the plant
Other common symptoms include yellow or droopy leaves and ends that get brown and curl up, almost like nutrient burn only it happens even if you didn’t go overboard with nutrients.
Bleaching and plant death can happen. If there is a major cold snap where temperatures fall close to or below freezing, it can bleach plants or even kill them.
Freezing temperatures can cause leaves to get bleached or even kill the plant
It’s common for leaves and buds to turn purple in response to cold. This is fine at the end of the flowering stage, but if it happens too early it can interfere with photosynthesis and reduce your yields a little.
Cold plants tend to produce small buds with a lot of stem between buds.
Too Cold? Here’s What Cannabis Growers Can Do
Controlling cold is important because cold temperatures too often while growing greatly reduces plant growth speed, make them more likely to suffer from wilting, and greatly reduces the final yields.
1.) General principles for temperature control
- Set grow up properly to manage cold (more info below)
- Adjust time of day/night periods – Set your grow lights to turn on at night.
- Use auto-flowering plants so lights can be on 18+ hours/day in the flowering stage to help keep plants warm
- Cold-resistant strains – Some strains can handle the cold better than others. Try to pick a strain that is hardy to your personal challenges
2.) Use Light Schedules to Your Advantage
- Change your grow light schedule
- Make sure grow lights are on at night when it’s the coldest in the grow space.
- Make sure your “night” period happens during the warmest part of the day (at least above 70°F if possible)
- Keep your grow light at full power. If you need to reduce the light levels, move the whole lamp up instead of dimming it. This produces more heat overall.
Change your grow light schedule so lights are on the coldest parts of the day (use a timer)
3.) Which Grow Light for a Cold Grow Room?
Some cannabis grow lights are better than others if you’re struggling with cold. The #1 thing you can do is use “Big Bulb” grow lights like MH/HPS or LEC (aka CMH) grow lights.
- Avoid LED grow lights. LEDs are typically a great choice for cannabis growers, but aren’t ideal for a cold grow room. Not only do they put out less heat per watt than other lights, plants tend to be sensitive to cold. LEDs use tiny diodes to produce light, which is then concentrated by lenses and reflectors to put light down directly at the leaves. The temperature inside the leaf is barely affected by the light, and therefore if you use a thermometer gun on the leaves of a plant under LEDs, you’ll see the leaves are typically about the same temperature of the air. Plants like temperatures about the same as humans, and the temperature inside the leaf matters much more than the temperature of the air.
- Use HIDs (big bulb lights) like MH/HPS combo or LEC/CMH. Big bulbs tend to raise the inner temperature of the plant leaves compared to the air (think of the hot sun on your skin, or how your skin gets warm if you hold it next to a bulb for 10-20 seconds). Plants grow best in warm temperature, and this extra heat inside the leaves allows plants to do more photosynthesis and therefore grow faster than they would if they were the same temperature as the air.
- Keep grow light as close as possible to plants without seeing signs of light stress on the leaves closest to the light. Giving proper nutrients and water will allow you to keep the grow light closer without stress.
Cannabis plants tend to be much more cold-resistant overall if you use “bulb” grow lights like HPS or LEC, as opposed to LEDs.
4.) Other Equipment Tips
- Use a grow tent to help contain heat vs growing plants in an open room.
- Insulate grow tent or grow space to help prevent heat from escaping out the sides
- Get a fan that controls fan power based on temperature, such as an AC Infinity fan. They’re a little pricey but can reduce or turn off the fan if the temperature starts dropping. That way if it gets really cold one night it will turn off the fan and help contain heat. (Note: Turning off the fan reduces the amount of oxygen available to the plants and makes the carbon filter ineffective since no air is flowing through it and stinking up your grow room.
An AC Infinity fan has the ability to turn off the fan if the temperature drops too low, to help keep warm air in the grow tent until the room warms up. As a bonus, they’re super quiet.
5.) Are there supplements to help cannabis plants deal with cold?
Unfortunately, not really. Though there are some things to remember with cold and nutrients/supplements for cannabis plants.
- Silica supplement helps plants have a more sturdy structure. This helps them resist temperature changes in general.
- Giving the right amount of water and nutrients makes plants more resistant to cold (and any other stress)
- Maintaining proper pH also makes plants more resistant in general)
This cannabis is stressed because of the cold night temperatures. A silicon supplement and good nutrient care can help it still produce well.
6.) Keep roots warm
Plants are much more resistant to cold when their roots are warm. So be careful not to put your plant pots directly on a cold cellar floor. If plants are sitting directly on a cold surface (like a basement or cellar floor), put a piece of plywood between the plants and the floor to help keep their roots from getting cold. It can also help to put your plant pot inside a bigger pot. This helps add an extra layer of insulation around the roots.
Protect plant roots from a cold floor. This grower put the plant pot inside another plant pot, and sit both on a saucer that raises the plant off the ground so roots never sit directly on the cold floor.
7.) No overwatering! Cold cannabis plants drink less water
Cold cannabis plants drink less water and are more prone to drooping and overwatering. Cold cannabis plants still tend to droop or wilt when they’re getting the right amount of water, so drooping may not be a sign of a watering problem.
Cannabis plants are simply more prone to drooping and overwatering. Cold cannabis plants still tend to droop or wilt even when they’re getting the right amount of water, so drooping may not be a sign of a watering problem.
Be extra careful not to overwater your plants in a cold growing environment. For small plants give just a little water in a circle around the plant.
This plant was overwatering and then got cold, giving it curling leaves.
8.) Strain Choice
Mainly you want to grow auto-flowering strains because they allow you to leave the grow lights on 18-24 hours a day, which reduces the time the lights are off (when plants tend to get cold). The other thing is to opt for a cold-resistant strain.
- Autoflowering strains – Allow you to keep your grow lights on for 24 hours/day, which can help prevent cold times when lights go off.
- Cold-resistant strains – Even if it does get cold, these plants can handle it better than some other strains.
- Indica strains – Indica-leaning strains tend to be more cold-resistant than Sativa or Haze, but not always
LSD-25 Auto is an example of a cold-resistant autoflowering strain that makes great bud quality. Check out this gorgeous LSD plant by StickyFingers.
9.) Get a heater
Get a heater if your grow space is just struggling to stay warm enough to keep plants happy and healthy. If you are struggling with high humidity, get a big dehumidifier which will not just lower the humidity but also greatly warms a room. If you’re struggling with low humidity (more common in a cold climate) consider getting a heater that also has a humidifier.
If your cannabis plants keep struggling with the cold and none of the other options work, you may need to get a heater in the grow space.
10.) Power Outage Prep
Cold climates often also struggle with problems like blackouts or power outages, especially during winter storms.
Cold Grow Rooms Do Have One Cool Advantage
Lets end on a positive note. Having a cold grow space sucks but you do have one big advantage. When buds are air-drying, they do best around 60°F. Most growers have to use an AC to achieve that temperature but for a grower dealing with cold, usually that’s not a problem.
For most growers, dealing with cold is usually easier than dealing with heat!
Hey, the one good thing is bud quality tends to be better when it’s cool!
