Climate is one of the most misunderstood factors for beginning and intermediate growers. The key to good climate control is understanding the relationship between your plants and nature, and how you mimic and manage that relationship when you go indoors or into a greenhouse to grow. The three big aspects of climate control are temperature, air movement, and humidity. Other factors, such as CO2 concentration, can also be considered and managed.
Temperature is often one of the most difficult things to manage for new growers. It is so crucial to the development and lifecycle of plants that it can’t be overstated. While most new growers tend to air on the side of overheating, overcooling your garden is an equal concern. Either extreme will cause your plants too shut down and stop growing, and in the most severe cases, even die altogether.
First, in order to prevent guessing, get yourself a decent hygrometer. This is a handy little device that will tell you the temperature and humidity of your environment. The common ones available in garden stores today have a minimum/maximum feature, which will tell you at any given time the highest and lowest temperatures recently recorded, as well as the corresponding humidity levels. The magic numbers are 75 degrees during the day (when your lights are on) and 68-70 during the night (your plants expect a slight cool down when the sun isn’t out).
Anything above 80 degrees is getting towards the dangerous end of the scale for most indoor fruits and flowers. Transpiration (water leaving the leaves due to evaporation) will begin to exceed the rate of hydration (water entering the plants roots), therefore drying your plants out, no matter how much water you give them. As you approach 90 degrees, this will start to have a noticeable effect on the health of your plant. Dry, brittle, yellow fan leaves, and dead, shriveled old growth will begin to appear. If you notice that your high/low is getting up to 90 or more, and you are witnessing these symptoms, you should first address your temperature problem. You may in fact be under-watering as well, but you’ll never know until you get your transpiration/hydration balance in check. There are other problems that can come with high heat; certain pests and parasites, including mildews and mold, thrive in warmer temperatures, and unhealthy plants are at a distinct disadvantage in trying to naturally to fight them off. As well, your plants will manifest strange growth patterns in an attempt to cool and aerate it’s fruit. Stretching stems and flowers, airy or “fluffy” fruit development, and overall low yield will result. In addition, extreme temperatures can even trigger hermaphroditic behavior, which is a natural defensive reproductive reaction the plant has to shock or stress.
Extreme lows can be equally problematic. In colder climates and winter months, night-time temperatures indoors can easily reach 30 degrees or lower without the aid of a heater. When plants become too cold, the rate of transpiration slows so much that there is no need for hydration (there is literally no room in the plant for more water) which dramatically slows the uptake of nutrients and essentially brings the growth process grinding to a halt. This can shock the plant into thinking an early winter has arrived, and that it needs to pollenate itself, which will result in male flowers and eventually seeds. As well, cold temperatures tend to encourage high humidity and moisture in the plant and growing medium, which will invite mold, mildews, and pests.
Humidity is the other most crucial factor in protecting the health of your growing environment. Even after beginning growers learn to control their temperature, they often experience powder mildew, botrytis (brown/grey mold), and funguses and other yield-threatening problems. The culprit? High humidity, almost every time.
Humidity is a measure of how much moisture (water) is in the air. This is important for three reasons. One, the moisture level of the environment has an important influence on the moisture balance your plants. If the environment’s humidity is extremely low, plants will loose moisture faster, and certain pests like spider mites benefit. If the humidity is too high, the plants won’t loose moisture fast enough, and powder mildew and botrytis benefit from that environment. The trick is keeping it right down the middle, at around 45%. This is best achieved through the use of a commercial dehumidifier which you can get at any home improvement store. Lastly, the moisture in the air is vector (delivery system) for parasitic spores and microbes itself, so it’s important to keep it in check.
Keeping it in Check with Airflow
Heaters and dehumidifiers are nice, but without a good, comprehensive airflow system, they are just fighting an uphill battle. The biggest thing a grower can do to manage both temperature and humidity, as well as exposure to unwanted foreign particles and parasites is to carefully manage airflow. Ample intake and outtake CFM (cubic feet per minute) are necessary for managing heat, humidity, and cleanliness of your environment. For larger rooms, 10′x10′ or more, consider a 12″ or larger outtake fan that will provide at least 1000 CFM or more. Smaller rooms can use smaller fans, all the way down to the 4″ models that are carried at many garden stores.
The key with airflow is maintaining a good ratio of flow to temperature. Too much incoming outdoor air, when lights are off, will cool a room down too low, and may not be needed. Too little outgoing air, when lights are on, will let heat pile up. Some growers dial this in with timers, others carefully adjust using a manual method, and others use temperature-based controllers to turn the fans on and off. These are our favorite, as they allow the room to “breath” with the temperature and bring in cool air when needed, turn it off when it isn’t, and stop exhausting when the room isn’t hot. 4 or 5 oscillating fans keep the air in the room moving in between bursts of in and outtake air activity. If you do use a timer to turn on your fans, make sure you set the timer to run long enough to actually completely exchange the air in the room. This should take 1-5 minutes to happen.
For keeping your intake air clean, it is important to use HEPA certified intake filters. Ask your garden store to recommend a filter size for your room. Remember that an intake filter will slow down the airflow in, so it’s important to get a fan and filter with enough combined CFM to compensate. Your outtake filter is another very important part of keeping your air clean. That activated carbon filter you have is the same stuff they use to keep hospital air clean, so it will work for you. Make sure again that your output filter and fan are large enough, that with the combined CFM between them they are able to quickly (< 3min.) clean and replace the air in your room.
Lastly, make sure that your intake CFM and outtake CFM are roughly equivalent. This will keep air moving and prevent vacuums and pile-ups of dirty air.
Remember that every grow space is it’s own environment and will require monitoring and adjusting. By carefully measuring your room however, and consulting with your local garden store, you can make sure you have the recommended amount of CFM both in and out when you start. Add in a dehumidifier and a basic space heater and you are good to go!
Controlling it All
Now that you have the basic elements of your climate control system in place, it’s important to consider how you’ll run it all. There are dozens of products on the market that will help you do this, from single-device, thermostat-based controllers to large, elaborate “brains” that will run your lights, your heater, your A/C, your dehu, and more. Some of these new systems even come with an ethernet connection so you can access your information from the internet, your PC, and even your iPhone! For more information on this, it’s best to consult your local garden store. A knowledgable owner will know what item is appropriate for the budget, scale, and concerns of your garden.