How to Deal With a Cold Grow Room?

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Added 10 July 2023

When you feel cold, you can put on a hoodie and be at ease, but the same isn’t true for your cannabis plant. Unfortunately, they don’t have hands or money, so they can’t get their hands on a sweater as easily as you. So, what happens when your grow room gets too cold during the winter? 

A lot of things. Cold temperature is usually not healthy for your cannabis plant and can lead to various problems. Fortunately, it’s easy to fix with a few simple tricks and, if you are lucky, your plant can even benefit from the cold temperature. 

Like heat stress, your plants can also experience cold stress. Yes, cold stress. You’ve definitely heard of heat stress, but cold stress? Not so much. 

In this article, we look at why cold temperatures are detrimental to your cannabis plant, how they can affect your plant, and how you can spot cold stress. 

Then, we delve deeper into some of the best techniques for heating up your grow room and list down some excellent heating sources. So, let’s take a look at how to deal with a cold grow room while growing cannabis in this guide.

Is Cold Temperature Bad for Cannabis?

Is Cold Temperature Bad for Cannabis?

When you grow cannabis outdoors, your cannabis plant is at the mercy of the weather, but you don’t have to worry about that indoors. However, some indoor setups may experience problems during the winter months when either the grow room cools down too much or the intake pulls in cold air from outside. 

This is not an ideal situation. Cannabis grows best in a temperature between 70°F to 75°F or 21°C to 24°C when the lights or on (daytime, outdoors) and 60°F to 70°F (16°C to 21°C) during dark hours. 

If the temperature goes lower than this during any time of the day, your cannabis plant can suffer from cold stress. Let’s take a look at how cold temperature affects cannabis plants.

Effect of Cold on Cannabis Plants

Different cannabis strains react differently to cold, but most cannabis plants start experiencing cold stress under 60°F or 15.5°C, approx. In cold temperatures, the plant’s metabolism slows down drastically, which can lead to stunted growth or other growth-related problems. This is because the oxygen concentration is higher in water in cooler conditions. 

Also, if the temperatures are not maintained during the flowering phase, there could be a delay in flowering. You’ll also notice that the buds won’t develop as much as expected, leading to less yields and returns. If you live in very chilly areas, frost is a big problem. When the temperatures dip below 32°F or 0°C, the cells can get damaged as the water inside the plant cells can freeze up. Ice crystals created within the plant cells can force the cells to rupture, leading to discoloration and wilting. In serious cases, the plant may not recoup and could also die. 

At the same time, the plant’s biochemical and enzymatic processes are hindered, and in severe cases, your plant’s vigor may collapse entirely, i.e., your plant will stop growing entirely or grow really weak. 

Even the roots fail to absorb nutrients below a certain level, which further leads to nutrient deficiencies. One of the primary nutrients that fail to get absorbed in cold is magnesium, which is vital for your plant’s growth. And too much water during this time will only make matters worse. 

Cold weather can also affect seedlings drastically. Cannabis seeds germinate best in warm temperatures between 65°F to 75°F or 18°C to 24°C, but if the temperatures dip too low, the seeds may fail to germinate properly. 

Lastly, cold temperatures give rise to hot spots around the grow room, which are damp and susceptible to mold or fungal issues. Cold plus damp conditions can create the perfect atmosphere for the development of mold, mildew, and also rot which can be destructive to marijuana plants. This is because a cold grow room with a lot of moisture offers a great breeding place for mold and mildew spores to thrive. Thus, you need to prevent the room from getting too cold and damp before mold attacks your plants. 

The only good thing about cold temperatures is that they help with trichome production. However, make sure the temperatures don’t go below recommended levels or you’ll have no plants, let alone trichomes!

Another way cold temperatures help has to do with storage. Have you ever stored the buds in a container only to find mold on it after a few days? If, yes, you already know the importance of storing cannabis buds properly. In this regard, cold temps help because they can not only preserve the potency of your buds but also ensure that the buds stay fresh and smell good for a long time. 

Storing the buds in cold temperatures decreases the deterioration of cannabinoids and terpenes. Typically, buds stored in temps between 32-68°F (0-20°C) will smoke well and last longer too. 

Symptoms of Cold Temperatures

But how do you know if your grow room is too cold for your cannabis plant? Cannabis is quite communicative, and if you have a keen eye, you can easily spot the symptoms of your grow room being too cold, such as the following:

  • Stunted growth of the plant without any other apparent reason
  • Damp areas around the grow room or signs of mold 
  • The root zone is too cold, especially in a hydroponic setup
  • The plant smells weird 

Leaf discoloration

One obvious sign that your plant is struggling with the cold is that it may develop yellow, purple, or black colors. These color changes occur as a response to stress and can also be attributed to disrupted chlorophyll production or issues in nutrient uptake caused by roots that have been damaged due to the cold. 

In some cases, cold temperatures can trigger the accumulation of purple pigments called anthocyanins in the leaves. This response is a protective mechanism as the plant produces these pigments to shield against cold damage. However, excessive purple coloring may indicate prolonged exposure to cold stress.

Leaf curling

Primarily, leaf curling occurs when there are changes in turgor pressure within the plant cells. Needless to say, cold temperatures disrupt nutrient intake and water absorption as well. Marijuana leaves might crinkle, and curl upwards or downwards when exposed to chilly temperatures. Again, this is a defense mechanism by the plant to reduce the area subjected to a cold environment. In short, the leaves curl to minimize moisture loss. 

Stunted Growth

Cold temperature levels can dramatically influence the development of marijuana plants. The metabolic procedures of the plant decrease when the plants are subjected to too much cold, minimizing nutrient efficiency and absorption. This means that although you provide all the necessary nutrients, the plants may not be able to absorb them, leading to stunted development with much shorter internodal spacing.

Stunted growth occurs because cold stress affects the plant's ability to carry out essential physiological functions, such as photosynthesis, respiration, and nutrient uptake. The reduced metabolic activity limits the plant's energy and nutrient availability for growth and development.


Cannabis plants may also start wilting due to extremely cold temps. As you already know, cold stress prevents the plant from absorbing nutrients efficiently. Moreover, it also disrupts nutrient transportation, thereby rendering the plant weak. Due to lack of water, the leaves may start wilting, making you assume that you’re underwatering. 

Note that you can still save the plants even when they wilt because it’s just a temporary response from the plant to prevent more damage from the cold. It will recoup once the temps return back to normal. 

Delayed Flowering

Cold temperatures can postpone or prevent the start of blooming in marijuana plants. Blooming in marijuana is set off by changes in the light cycle as well as temperature. When the temps dip very low, the vegetative stage is extended, delaying the onset of the blooming phase. 

Although an extended vegetative phase may not seem like a big deal, it’s concerning if you have a small grow tent. Outdoors, cannabis plants have a limited time window to bloom before harsh weather begins, so timing is critical even indoors if you want to mimic the conditions it experiences outdoors. Plus, a delay can affect the yields ultimately as the plants don’t respond favorably in extremely cold temps. 

Bud development issues

Cold temperature levels can negatively impact the growth of marijuana buds as the plant’s ability to absorb nutrients and water is restricted. Instead of thick, dense buds, you may notice that the buds are small compared to buds growing in normal temperatures. 

How to Warm Up Your Grow Room?

How to Warm Up Your Grow Room?

So you’ve found your grow room to be cool and your cannabis plant is starting to show signs of cold stress. Don’t start worrying yet, it’s not the end of the world, but it can be the beginning of the end if you don’t take prompt action. Here are some of the hacks you can try to keep your grow room warm during the winter.

1. Set Up a Heater in Your Grow Room

One of the most effective ways — as you’d have guessed by now — is to use a heater in your room. Heaters are easy to install, affordable, and quite effective at keeping your room cozy. Remember the thumb rule is to purchase a heater that is powerful enough for your room.

There are also a lot of nuances to choosing the right heater since there are so many kinds in the market. We will get into more detail later in the article. 

2. Insulate Your Grow Room 

Sometimes, grow rooms can get cold due to the ambient temperature around your house — the outside environment cools down your house. A great way to prevent this is by insulating your growing space. This insulation will act as a barrier between the ambient environment and your plant, keeping the heat trapped while keeping the cold temperatures out. 

Some of the best ways of insulating your plant are as follows:

  • Wrap your pots in aluminum foil 
  • If your container sits on the floor, place a polystyrene sheet under it so the cold cannot transfer onto the container so easily 
  • Close off any holes or gaps in your grow tent or grow room to keep the cold air from making its way into the space 
  • You can even wrap your entire plant setup in blankets, water heater insulation, or spray foam, depending on your setup’s type and scale 
  • If nothing fixes this, you should consider renovating your house insulation and installing a more resilient insulator in the walls

3. Make Changes to the Airflow 

Air plays a major role in temperature regulation, so let’s take a look at some of the best ways to use air to keep your grow room warm and cozy for your cannabis plants. 

Improve Air Circulation

Wait, wouldn’t improving the air circulation make the room cooler? It would, but we want to improve the air circulation to get rid of any hot spots first. These hot spots may not seem like much of a problem but can lead to mold in our grow room. 

The goal of improving the air circulation is to make sure the warm air gets circulated around the entire grow room, including toward your plant, and you can facilitate this with the help of a simple oscillating fan. 

Route Intake from a Warmer Space

Where is your intake air being sourced from? If it’s from the outdoors, it will clearly pull cold air. So, it’s best to reroute your intake from a warmer space, like your kitchen or your garage which are usually warmer than other spaces in your house. Make sure you install some kind of mesh screen to keep the insects out and keep an eye on the humidity levels. 

Use a Timer on Your Exhaust Fan 

Chances are, your grow room is using negative air pressure with the exhaust fan throwing out more air than the inlet pulls in. This is great to keep the air fresh, but it’s time to use a bit of damp air to our advantage. 

Install a timer on your exhaust fan and allow it to shut down occasionally, which will lead to the internal air getting a bit warmer. If you want to go a step further, connect the timer to your thermostat so it turns on/off whenever the temperatures reach closer to the preset limits. 

4. Rework Your Grow Light Setup

Grow lights not only nourish your cannabis plants but can be a source of heat as well, especially if you use more conventional types of light panels. So, let’s take a look at some ways you can rework your grow light setup to warm up your grow room.

Switch to HIDs

LED grow lights are amazing and they offer benefits like energy efficiency, ultra-long life, customizability, and so much more. However, another benefit that works against you is its running temperature — LEDs don’t heat up so much since they use tiny diodes, whose light is concentrated with reflectors and lenses. 

So, if your room gets cold often, not just during a few weeks of the year, you should consider switching to HIDs like MH or HPS lights. These grow lights pass electricity through various compounds, which heat up and shine bright. The result is bright light along with warm temperatures. So, you can use these lights to increase the ambient temperature of your room. 

Remember, when placing HIDs or other types of conventional grow lights, always use the correct wattage of light, follow the manufacturer’s recommendations, and do not place it too close or far from the canopy. Also, carry out the palm test by placing your hand on your plant — if the grow light feels hot on your hand, it is too hot for your plant, too.

Change the Light Cycle Schedule 

You should also consider changing the light cycle, i.e., you can change the lights to be on at night when the temperatures outside are too cold and keep the room dark during the day when the outdoor temperatures are warm. 

For a vegetative plant, you can also increase the daylight hours from 18 to 24. While this is a debatable topic in the community, its benefits far outweigh the cons in this case where managing the temperature is the priority. 

If you are growing autoflowering cannabis plants, one of the best things you can do is leave the grow lights on. This won’t affect your plant’s health as much but it will save you a lot of money since running grow lights 24/7 will always be cheaper than running your heater for 10 hours. 

5. Check the Nutrient Solution

Nutrients can play a major role in your plant’s health during a cold snap, albeit indirectly. Let’s look at two ways you can use nutrients to keep your cannabis plant warm.

Install a Nutrient Heater 

If you are growing cannabis in a hydroponic setup, one of the major risks to your plant’s health is that chilled nutrient water bathes the roots. This can not only cause cold stress but can also shock the roots. 

So, you need to heat up your nutrient solution with a heater. Additionally, you can also increase the nutrient concentration in the solution by up to 20% so the plants can absorb the nutrients despite the cooler temperatures. 

Use Supplements 

There are a few supplements that you can try that can help your cannabis plant cope with the cold weather. For example, silica supplements can help your plant become more sturdy, thereby increasing its resistance to temperature changes. So, visit your local gardening store and check out silica supplements that can work well for your cannabis plant. 

Avoid Overwatering Your Plants

Overwatering is a common mistake that many cannabis growers make, and under normal circumstances, a little more water won’t harm anyone. But when the temperatures dip, overwatering can be risky for your cannabis plants. 

This is because, in cold weather, your plant consumes a lot less water. So, if you give it the same amount of water as you would during summer, your plant may start drooping or showing other signs of overwatering. Thus, when the temperatures decrease, you should cut down on the water you give your plant. 

What are the Best Heat Sources for a Cold Grow Room?

What are the Best Heat Sources for a Cold Grow Room?

You’ve tried all the remedies but there’s still some room for improvement, then the only thing left to do is get your hands on some effective heat sources for your cannabis grow room. Here are some of the best heat sources you can use to warm up your grow room.

1. Thermostat Controlled Electric Grow Heaters

The most effective and popular solution to warm up your grow room is to use electric grow heaters. Broadly, electric heaters can be classified into two types: those for grow rooms and those for grow tents. Let’s look at both of them below.

The first difference between the two types of grow heaters is the size of their heating capacity. Grow room heaters are larger and consume up to 2000W whereas tent heaters are much smaller and consume up to 1000W. 

The second difference is their runtime. A small heater can easily run for half a day without any problem but since it can only heat up a tent, it’s not suitable for a grow room. Instead, you need to use a bigger heater for the grow room that can run constantly without fatiguing. 

Here, you can choose between an oil-filled or a ceramic heater, depending on your preferences and budget, but whichever one you use, ensure it offers enough heating for your grow room or tent.

Lastly, it’s a good idea to choose heaters that can be controlled by a smart thermostat, which will take the burden off your shoulder of regulating the temperature. A smart thermostat will automatically tune the heater to compensate for the temperature, maintaining the ideal environment in your growing space at all times. 

2. Grow Tent Tube Heaters

If you are growing a couple of plants in a small grow tent, you cannot go wrong with a tube heater, either. This is an easy yet effective solution that only consumes 90 to 200W of power and is more than enough for a small step. And despite their small price tag, these heaters come with various features like a safety cut-out, overheating prevention, adjustable legs, etc. 

If you want to use a tube heater for your cannabis plant, it’s recommended that you place them on the floor and not on the tray. Raise the tray a bit and place the tube under it and ensure there is enough airflow there to spread the heat under the plant, which will heat up the root zone and lead to a much better yield. 

3. Heating/Cooling Splits 

Many growers live in regions where the winters are too cold and the summers are too warm, and if you live in such a region, you can even invest in heating/cooling splits. These devices consist of an indoor and outdoor unit, offering a wide range of BTU in terms of heating and cooling.

Unfortunately, these units are expensive and consume a decent amount of energy, but if you want to grow cannabis for a long time, they are a worthy investment. 

4. Soil Mats

Whenever your grow room gets too cold, don’t just focus on the ambient temperature, focus on the growing medium too. So, one of the first things you should do is heat up the soil with a soil mat. 

These soil mats are cheap and can keep your soil warm and it is much cheaper than warming the ambient air around your plant. Just don’t confuse soil mats with germination mats, the latter are used for germinating seeds and soil mats can be used for plants in all growth stages.

5. Radiating Cables

A great yet cheap alternative to soil mats is radiating cables, which work well when you can’t install mats or tube heaters. These cables are usually used in propagators for germination of cuttings, but they can work well when wrapped around your plant’s containers. Like the soil mats, these radiating cables heat up the substrate of your plant and protect it from cold stress.

6. Aquarium Heater

As mentioned earlier, you must up the nutrients if you are growing cannabis hydroponically, and the best way to do that is with an aquarium heater. These are cheap and can be found in your local pet shop since they are usually used by people that have fish. 

7. Compost

Another terrific method that can warm up your grow room is compost since it releases a lot of warm air and is rich in carbon dioxide. It offers multi-pronged benefits to you as it not only heats up the air but also releases carbon dioxide that’s good for your plant’s health, and it is also a terrific fertilizer for your plants. 

Take Advantage of Cool Temperatures for Growing Cannabis

Take Advantage of Cool Temperatures for Growing Cannabis

Low temperatures are bad for your cannabis plant, especially if it’s a youngling, but if your plant is in the final weeks of bloom, don’t worry about the temperatures much. If your cannabis plant has all the other essentials it needs to grow healthy, the cold temperature can in fact improve your yield’s quality. 

Essentially, your cannabis plant has resin glands on the surface, whose job is to protect your plant from extreme conditions like cold, pests, and UV rays. So, if the grow room is cool, your plant will go into defense mode and start producing a lot more of these resinous glands to protect itself.

And what do you know — these resinous glands also contain the terpenes and cannabinoids that make us want to consume these flowers. So, the more the resin glands on the buds, the tastier and more potent it can feel, and they will smell a lot better too!

And, as mentioned already, another advantage of cold temperatures during this stage is that it can encourage your plant to grow colorful buds. So, depending on your strain’s genetics, the buds can develop various shades of red, blue, or purple if the temperatures are cool. And who doesn’t like purple buds?

Summary: How to Deal With a Cold Grow Room?

Cold temperatures and cannabis don’t usually go hand in hand. Cold can lead to various problems for your plant, primarily in terms of stunting or halting its growth, affecting its nutrient uptake, and making it prone to overwatering or mold. 

So, if you think your cannabis plant is suffering from cold stress or if your grow room is too cold, you need to start acting. You can use various methods like using space heaters, installing better insulation, changing the airflow or the row light setup, and reworking the nutrient solution.

And you can also invest in some excellent heat sources for your grow room like a thermostat-controlled electric heater, soil mats, radiating cables, heating/cooling splits, compost, and aquarium heaters. 

But if your cannabis plant is in its final weeks, don’t bother much. Let the cold weather do its magic and make the buds more juice with resins (terpenes and cannabinoids. You won’t regret it. 


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