How to Keep Soil Warm Indoors

If you’re growing cannabis plants indoors during the winter months, or if you just live in a colder climate, it can sometimes be a challenge to keep your plants’ soil warm. If you don’t keep your soil warm enough, it can hinder your plant’s ability to absorb nutrients and grow successfully. 

Luckily, there are a number of fairly easy ways to keep your soil warm despite the old air outside. Here are the best ways to keep your soil warm so you can grow all season long.

How to Keep Soil Warm Indoors: 

Heated Soil Mats 

Heated soil mats are probably the best option for keeping your soil at an optimal temperature. This is because you can put your pot of soil directly on the heat mats so that the heat moves up through the soil. This is useful when you don’t have the capability to heat up the entire room you’re growing your cannabis in. 

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Many growers use these mats to assist with the germination process. But they’re also extremely useful when you’re trying to keep your soil warm indoors. When using these mats, you should try to set them at a temperature that keeps your soil between 75-85°F.

Pros: 

  • Can heat up soil without heating up the entire room 
  • Are inexpensive 
  • Exact heat can be adjusted 
  • Does not contribute significantly to heat costs

Cons: 

  • Will only heat up your plants’ soil and not the entire room. 
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Insulate your grow room with fiberglass 

This is another smart idea if you’re looking to grow cannabis indoors in a colder climate long term. While this won’t generate heat, it will help your room retain heat. This will be useful when paired with any of the other heating methods discussed in this article. For example, if you’re running a heater but have no room insulation, the room will go cold much faster. This can also be a major suck on your heating bill. 

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If you’re growing in a greenhouse, this method probably isn’t for you. But if you’re growing in your attic or basement, or even a solarium, insulation is a smart thing to install. If you’re doing a DIY installation of fibreglass insulation, you can expect to spend roughly $150 to $200 for a 500-square-foot room.

Pros: 

  • Makes other heating methods more effective 
  • Good for longterm grows 
  • Can save money on heating in the longterm 

Cons:

  • Insulation can store heat, but it won’t generate new heat. 
  • May not be sufficient if you’re dealing with extremely cold rooms.
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Electrical room heater

Electric room heaters can be a great option as they heat up an entire room, and usually come at a great value. If you’re looking to warm up a greenhouse, you can buy heaters that are designed specifically for that purpose. Otherwise, you’ll probably be able to find an adequate electrical heater at your local Walmart. 

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If you go the electrical heater route, you may also need to invest in some extension cords. When using these heaters, you should also keep a thermometer in your grow room so you can keep checking to make sure the room’s temperature is kept at an optimal 70-85 °F (20-30 °C). 

Pros: 

  • Effective at heating up entire room 
  • Great value 
  • Easy to find at a local store. 

Cons: 

  • Heat is directed at the entire room which, if you’re just trying to heat the soil, may not be necessary. May not work as well at heating soil if you’re growing in very large rooms.
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Insulate your grow room by making a DIY heat sink (with water) 

A heat sink is a medium, like water, that captures excess heat and then release that heat when the air around it gets cooler. Water is particularly interesting because it can absorb heat without heating up its own internal temperature too much.

You can use water as a form of insulation by storing large quantities of water open in the room where you’re growing your cannabis. This is particularly useful in a greenhouse or room with a lot of sunlight because the sunlight can heat up the water, and later, when the sun goes down and the room grows colder, the water can release this heat back into the room. 

A 55-gallon barrel or larger is a good size to store your water in. However, any types of containers will work. A good technique is to spray paint your containers black, or wrap them in black tape, so that they’re heated up quicker by the sun. Just remember, the more water you store in the room, the better this method will work.

Pros:

  • Renewable
  • Inexpensive
  • Works well in a greenhouse 
  • Allows you to have water on-hand inside your growing space, if you don’t already

Cons: 

  • Requires a room with lots of sunlight 
  • Takes up a lot of space
  • Works best in a greenhouse, but not practical for other indoor growing situations like basements.
  • Will not generate enough heat in extremely cold circumstances.
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Insulate your grow room by making a DIY heat sink (with compost) 

Have you ever worked a landscaping job where you had to shovel a big pile of cut grass, and notice that as you’re shovelling, steam starts pouring out of the grass as if the bottom of the pile is on fire? This is because when organisms break down organic matter like grass or compost, it generates heat. The more nitrogen-rich materials you have in this compost pile, the better it will generate heat. 

Creating a heat sink out of compost is an excellent way to heat an indoor growing room like a greenhouse because not only does it capture and generate heat, but the compost can be used for gardening later. 

The way to create a heat sink in your greenhouse is to simply dig a trench and fill it with compost. In the day when it’s warmer outside and the sunlight is streaming in, your compost pile will heat up. Then, when the temperature cools off at night, this compost will release heat back into the room. 

Pros: 

  • Inexpensive 
  • Great if you’re growing in a greenhouse 
  • Serves the double purpose of giving you a fantastic natural fertilizer on hand at all times to use on your soil. 

Cons: 

  • Only practical to use in greenhouses, or similar growing setups 
  • May not generate sufficient heat if you’re dealing with extreme cold.
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Heat lamps 

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Heat lamps are an effective way to heat up a room, but you have to be careful that they’re not drying out your plant too much. If you use heat lamps improperly, you could even burn your plant. Therefore, if you choose to use heat lamps, it’s recommended that you don’t point them directly at your plant. It’s also a good idea to keep a sprits bottle of water so you can mist your plants and help introduce moisture and humidity back into the room. 

Pros: 

  • Effective at generating heat. 

Cons: 

  • Can dry out the air and your plants 
  • Can burn your plant. 

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