The Effects of Temperature
On Plant Growth

high temperature leaf burns

The ideal temperature range for plant growth without CO2 supplementation is 70-75 degrees. Temperature is probably the factor that causes the most problems in an indoor garden. When the temperature in a garden climbs higher than 85 degrees, plant growth damage can happen quickly. As far as temperature is concerned, you have the following factors working against you when you begin to improve your garden area...

  • the more watts you add to an area

  • the more enclosed you make the area

  • the less adequate your exhaust system is

  • the smaller the area is

  • The worse your heat problems get!

The general idea for controlling heat is to exhaust the hottest air out of the area and, at the same time, introduce cool, fresh air in to replace the exhausted air. This source of cool air is critical. Plant growth slows in hot, humid conditions, and your ability to control the temperature in the garden area is only as good as your source of cool, fresh air.

Indoor/Outdoor Garden Thermometer The most important thing is that you know the temperature in the hot spot of your garden area. This is usually directly under the center of your grow light, at the tops of your plants. The plant growth in this area is the most vulnerable to damage from high temperatures in the garden. For this, every indoor gardener should have an indoor/outdoor thermometer (the kind with a cord and remote probe).

Here are four strategies you can use to help reduce the temperatures in the plant growth zone of your indoor garden area...

  • 1. Open the grow area to allow air circulation with a larger volume of air

  • 2. Keep the grow light exhaust and the garden area exhaust as two separate systems

  • 3. Increase the size and number of exhauste fans for the area

  • 4. Add air conditioning

Opening the Grow Area

Larger volumes of air act as a buffer against temperature increases. A garden grown in a small closet with a 400 watt light, an oscillating fan for circulation, and no exhaust fan will have a problem with high temperatures.

On the other hand, the same 400 watt light placed in the corner of a large bedroom, with the same oscillating fan, may operate without any problems of high temperature (especially in winter months).

Running Two Separate Exhausts

Let's say you had a 600 watt light in a high quality reflector that had nice big 6 inch vent holes thru it and glass in the bottom. Let's say you placed this light in a completely enclosed 4 foot square box. You could duct air in from a window, through the enclosed light, and exhaust it back out the same window.

With the light efficiently cooled, far less heat ends up in the grow box and the temperature inside rises much more slowly. This means you can run an exhaust fan that kicks on and off as it is needed. Now you can run an exhaust out from the grow box and run it on a thermostat.

Once you no longer have to exhaust continuously to control the heat, you will be in a much better position to use carbon dioxide to try to maximize your plant growth.

The exhaust from a grow box should always be somewhere near the top of the box (because heat rises). This makes it only natural to make the fresh air intakes lower to the ground, around the bottom of the grow box or grow chamber.

Again, your ability to control the temperature will only as good as your source of cool, fresh replacement air.

Upgrading Your Garden Exhaust Fans

If the air temp outside is 70 degrees, than you should be able to get the temperature in your garden area down to 75 degrees....if your exhaust fans are good enough, that is. Box fans and oscillating fans are needed in the garden to circulate the air and to help all the plant growth breathe properly, but as exhaust fans they suck! They do not move nearly enough air.

There are two types of fans used (efficiently) to exhaust indoor garden areas. They are...

squirrel cage and centrifugal fans

  • squirrel cage fans

  • centrifugal fans

The main difference is that centrifugal fans are quieter, more powerful, and more efficient than squirrel cage fans. They are also more expensive. However, if you plan on using a carbon filter, have to get a centrifugal fan....squirrel cage fans just don't have what it takes when it comes to pulling air in volume through a carbon filter.

As a general rule, the CFM rating of your exhaust fan should be able to exhaust the square foot volume of your garden area in 5 minutes or less to be ok. Practical experience has shown me it usually takes even more than that to control temperatures in a garden, especially when no air conditioning is used.

Get more information on garden exhaust fans here.

Advanced Climate Control

Air conditioning is the final solution when it comes to controlling indoor garden temperatures. With AC, you are always guaranteed a quality source of fresh, adjustable cold air. Gardeners that use air conditioning do not exhaust their gardens continuously.

The strategy for using air conditioning is usually to run the lights on a separate exhaust system from the garden area. Whenever the temperature gets too warm, the garden exhaust fan kicks on at one end of the area while the air conditioner operating at the other end acts as the fresh air intake.

Gardens using carbon dioxide to maximize plant growth would then release CO2 once the exhaust fan(s) shut off.

An Important Sidebar on CO2 and Temperature

It is generally accepted that gardens can be ran at higher temperatures when using CO2 supplementation to maximize plant growth. While this is true, it is important to understand that this is only the case when all other growth influencing factors are kept in their ideal ranges. The ideal temperature range for the indoor garden is 70-75 degrees. With the addition of CO2 you can run your garden up to 10 degrees warmer without seeing any negative effects.

Temperature and Your Nutrient Reservoir

The other place where temperature is a concern is in your nutrient reservoir. In the plant growth and oxygen page, I describe how the uptake of nutrients (in the root zone) only occurs in the presence of oxygen. The fact is, water looses it's ability to hold dissolved oxygen as it warms up.

If the nutrient solution gets too warm, your plants will not be able to take up any nutrients (and therefor they will not grow). Worse yet, the nutrient solution will begin to favor a number of pathogenic micro-organisms at a warmer temperature....nasty things like fusarium and pythium. The ideal temperature for your nutrient solution is 65 to 70*F. If your garden is in the basement, keeping the nutrient reservoir on the concrete floor will usually keep it at an ideal temperature.

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