Ideal grow room temperatures are more than just what shows up on a single thermometer in your grow room. While it’s easy to give you the ideal indoor grow temperature (between 65°F and the low to mid 80°F’s) we know that it’s not simply the room temperature alone that helps you get the yield you want. We want to go over the affect temperatures play in your grow room, from the temp outside your growing space to the lights inside of it, various temperatures play a big role in your garden.

External Conditions on Internal Temperatures
One of our biggest questions from growers is how outdoor temperatures have on their indoor grow. To answer that, it helps to know whether you want to grow in a tent or you want to use a room to grow your plants in.

If you’re converting a room into a growing space, then outdoor conditions can play a huge role in your grow. Summer heat can raise the temperature of your grow exponentially (especially when you’re using HID’s). With good insulation your dwelling can keep optimal temperatures where they need to be. In lesser conditions, the heat outside will raise temperatures and make it hard to maintain a good environment.

On the opposite end, using a basement to grow in the winter can make temperatures hard to maintain during lights off. Outdoor night temps can drop your grow’s temperature from the 70’s down to the 40’s within hours, which can create excess moisture around your lights and plants, and can even cause mold.

Now, if you’re growing in a tent you may have a little more of a buffer, provided your dwelling’s temps are okay. If you can maintain a good temperature in your home or apartment, you’ll be able to control a tent’s temperatures easier, too. But no matter what the application, outdoor temperature can (and usually will) have an effect on your grow. However, it’s up to you how/if you regulate how much of an effect it’ll have on your grow.

Ambient Temperatures
Temperatures inside of your garden play a big role in how well your plants breathe and absorb nutrients. Excessive heat will make your plants breathe and lose water quickly, while temperatures that are too low can slow down absorption of nutrients, stunt growth, and can lead to moisture build up (and all the problems that come with it).

Ambient temperature is the temperature your growing space sits at. It’s affected by external factors like the temperature outside, as well as internal factors like the heat (or lack of) coming from your grow light. This is important to know because this plays a huge factor in to the humidity of your grow room, which will affect how much your plants will sweat, absorb nutrients, and ultimately grow.

Your plants are constantly trying to grow, and when temperatures are off it makes it difficult for the plant to do so, which means you’re not getting the yield you want. Photosynthesis in plants can only be achieved at certain temperatures, so even if the humidity of your growing space allows your plants to breathe properly, a bad temperature may not allow for your plants to convert nutrients in to usable energy for your plant to grow.

No matter if your plants like it tropical or cool and dry, knowing the ambient temperature of your growing space is important in making sure your plants are getting what they need to convert nutrients in to usable food. That includes making sure the temp in your room isn’t affecting the temperature of your water or medium temperatures.

Canopy Temperature
Lots of growers will tell you that ambient temperature is what you need to focus on, and the temperature of your medium will fall in line. That’s fair. But if you’re not careful you can unknowingly overheat your plant’s canopy.

Canopy temperatures are the temperatures at the canopy level of your plants- simple enough, right? Right. So it makes sense, then, that the temp should be around the same as the ambient temp. If you set a thermometer at the canopy level of your grow, you should see a reading within the normal range your room sits at. If it’s outside of that range you’ll more than likely need to adjust the height of your light.

For example, if your plants want temperatures of around 65-80°F the canopy temperature should be within that range. Plants are pretty resilient, so a few degrees outside of that range won’t hurt. However, if your plants don’t get enough heat- or too much of it- check the height of your lights. The closer your lights are to your plants, the higher the canopy temperature will be so if temps are too high at the canopy level (say, close to 100°F) raise your lights. When lights are too far away, you might get too cold at the canopy, which will contribute to stunted growth, so bring the lights closer to the canopy.

Of course, there are variables that can skew these solutions. LED’s, for example, don’t generate much heat, so canopy temps can be relatively lower than most. In that case you’ll want to add a heater in to your grow. What we want you to take away, though, is that canopy temps are a good sign of how your plant is feeling; and even if your ambient temperature is stable, it’s important to make sure your canopy temps are correct. If your ambient and canopy temperatures are conflicting, you can bet your water and/or medium temperatures are off, too, and if that’s the case your plants are going to have a hard time eating.

Soil and Water Temperature
The temperature of your soil and/or water is important to know because if it’s not on point your plants are in for some trouble.

Ambient temperatures will affect the temperature of the nutrient-rich water you’re feeding your plants. Water temperatures should be around where your room is at, if not a little cooler. For example, if your room is at 77°F your water should be within 72-77°F. If the water’s too cold (in the 70°F or less range) nutrients may not be able to break down well before they’re taken up by the plant- and if/when they are, you’ll likely see slowed and stunted growth. Hotter water (in the 78°F+ range) can lead to root rot and mold at the root zone in hydro gardens (usually due to swampy conditions near the root zone).

If you’re growing in soil, make sure your soil temperatures are also within your ambient temperature range. This can increase depending on how close your lights are to your plant, and if it gets too hot at the soil level you can be frying your roots. Adversely, if your soil’s too cold your roots won’t be able to absorb nutrients effectively. Feedings won’t be processed if it’s too cold, and you’ll stunt your plant’s growth. With enough unprocessed food and bad temps, you can start to see water logging and root rot.

Temperatures all connect to get you the yield you want, and if there are conflicting temps in your grow room your plants can suffer. If canopy’s too high, then it’s likely soil’s too high so your plants can burn. If the water’s too hot but your canopy is right, you could be messing up your roots or exhausting your plant- the bad combos are endless.

There’s always a little wiggle room when it comes to correct temps, so the key is listening to your plants and making sure you give them they temperatures they like, not the temps they can withstand. Remember: just because a plant can withstand high or low temps doesn’t mean you should subject them to unhealthy conditions.