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Grow Room Climate Control:

Five Steps for Better Yields

Marijuana Bud


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Many growers who are just starting to make the transition from hobbyist to a bigger setup don’t realize how quickly an entire crop can go bad. Or, while not quite as catastrophic, some growers simply don’t know how much better their yield could be if they optimized a few more variables. Because most growers take pride in what they do (and the crop is quite valuable), mastering grow room climate control should be up-top on the to-do list.

Our number one piece of advice to growers is to talk with your more experienced peers about perfecting grow room climate control. You’re not the only one who’s ever had the problems you’re facing. But we’ve spent a good deal of time working with expert growers.

Here are what we consider the key five steps to perfecting your grow room climate control.

1. Dial your light setup

Light isn’t something growers overlook when they think about plant health and indoor grows, but there is one element that can get missed: Most lights add a lot of heat to the room, so you really have to include that consideration as part of your climate control. Lighting dovetails with temperature control, and you’ll have to think all of that through.

While some growers are moving to LED lights, many are sticking with tried-and-true metal-halide and HPS (or high-pressure sodium) lighting systems, which can add a lot of heat and elevate your room beyond the 20-degree or so window that most growers aim to stay within.


Generally, that 20-degree temperature window—warm enough to not freeze your plants, cool enough not to fry them—is somewhere in the range of 65 to 85 degrees F. Because of lighting, most indoor growers usually only have to worry about cooling.

For hobbyists or growers who are just raising a few plants in their home, a simple window AC unit might be sufficient. Unfortunately, though, they’re not very efficient.

That’s one of the big things growers find out when they start to scale up; When they go bigger, some of those simple solutions fail. Window AC units don’t scale too well. When it comes time to scale up your operation, you’ll want to look at a system that functions more like what you’d call central air in a home environment.


Keeping air moving in your grow room serves two purposes: It helps with mold prevention and it helps build stronger stems and branches on your plants.

If you’re not familiar with transpiration, think of it this way: Your plants soak up water at the roots, and the water moves up through the plants. That water doesn’t disappear, it’s evaporated through small pores in the leaves. Circulatory movement of air within the grow room breaks up the boundary layer of air that, otherwise, would simply hang around your leaves, keeping them too moist and vulnerable to mold and fungus.

Oscillating fans are usually enough to keep air circulating, but if for some reason you want to do air exchange and bring outside air into your grow room – for instance, if you live in a dry environment and want to use exchange to keep humidity down – your solutions will be more complex.


Humidity control is really the primary climate control aspect of high-performance growing. Most people think of it as mold control, but another good reason to have a handle on your humidity is to influence your plants’ transpiration rates.

For humidity measurement, a handheld sensor may be enough to provide accuracy within about five percent. Larger grow operations are moving toward electronic monitoring systems that include sensors and will automatically record humidity and even take actions with fans and dehumidification equipment.

The problem with some of these solutions is that humidity is a deceptively complex topic. It sounds easy—it’s just water in the air. But when you start talking about relative humidity related to temperature and other variables, it’s not so simple.

Humidity control, overall, is one thing people miss particularly when they’re scaling up their grow operation, they don’t realize how much moisture each plant processes through itself. It gets to be quite a lot of moisture. In a small system, a small, residential portable dehumidifier that you buy at the hardware store will be fine, but once you get dozens or hundreds of plants, the way to handle that quantity of water is much different. Residential dehumidifiers won’t work as well and they won’t be as energy efficient as commercial dehumidifiers.

That’s why it’s a good idea to at least initially bring someone in or contact someone with some experience in that area to get you started.


CO2, or carbon dioxide, is the one nutrient that could be included under the umbrella topic of climate control.

Essentially, if a grower is using high-powered lights (generally higher powered than CFLs or fluorescents) and the plants are getting sufficient water and nutrients, they can use more CO2 than the environment can naturally provide. So, in the process of optimizing your growth environment, the CO2 level can actually be your limiting value.

Many high-performance growers shoot for 1,000 or 1,200 parts per million, whereas the average in the environment is around 400 ppm—though it’s on an upward trend of course. Like all the other nutrients, you can have too much or too little. The proper level varies from day to night and also depends on growth phase.

Many growers use compressed CO2; it’s an odorless gas. Generally, they have a valve and set a timer that turns the gas on for a fixed amount of time every so often. Or there are fully automatic methods with electronic sensors.

This is why it’s important to think through your whole grow process. If CO2 is going to be a limiting factor, you may want to build it into your climate control system, as well.


At the end of the day, it depends on your knowledge level at the outset whether you’ll have everything you need to plan for your grow room climate control yourself, or if you need to bring in some pros.

Those who come into growing with a construction or building background will often have a good deal of knowledge that’s useful for climate control and facilities issues. They know how to seal off and insulate buildings, and they’re often also the first to realize when they need a consult with a professional. It’s much the same as how those experienced in agriculture have a knack for plant needs and nutrients.

It’s your call whether or not to bring in a pro to consult on your grow room climate control system. But thinking ahead, making a plan and, of course, conferring with other more experienced growers is always a good idea.