Some of you may have been struggling with too much heat in your greenhouse this summer. Even in a relatively cold climate, it’s easy to get temperatures too high for plant growth within the greenhouse.
Ideal maximum temperatures for most greenhouses are in the range of 75 to 85 degrees Fahrenheit. When you get above these temperatures, most plants will suffer and sometimes may react by dropping flowers or fruit.
The great thing about growing in cold climates is that we can exclusively use outdoor air to provide our cooling. At least in our climate, it’s fairly rare that our outdoor air temperatures are higher than the maximum temperature we want in our greenhouse.
Certainly, the easiest of temperature control methods is to just open the greenhouse door each day. Or, if you have vents, to open these. But, automating this process by using fans and temperature controllers is a very worthwhile benefit to the grower.
There are some basic standards we can use to determine optimal air circulation. These guidelines are to replace the entire volume of air in the greenhouse once per minute. This can be figured out by roughly calculating the cubic footage of the greenhouse. Fans are rated in “cubic feet per minute” that they move.
It can take some surprisingly large fans to achieve these levels of circulation, even in a small greenhouse. For example, we have two 16 inch fans. This is roughly double our actual requirements, but we wanted two fans to provide some redundancy in case of failure.
When you pursue active cooling like this, you’ll also need to address air intake. Generally speaking, a good starting point is to have equal sized intakes to the size of fans you use. We like using actuated louvers here, so they open and close with the fan’s status. In our case, we have two 16 inch louvers as well.
In our case, we’ve hooked up our fans to a temperature controller. It turns the fans on when it’s 85F and off again when it’s 80F. This maintains relatively warm temperatures, which is ideal for many plants like tomatoes.
If you can’t achieve the “once per minute” rule, it’s good to try and at least get close. We were quite surprised how much air movement is required to maintain air temps. The closer the outdoor air temperatures are to the greenhouse temperatures, the more air volume is required to keep things cooler. It seems like overkill, but it’s definitely not.
We can definitely tell you, it’s nice to not worry about greenhouse temperatures. We’re far too busy (and honestly not disciplined enough) to remember every single time we should have the greenhouse door open. It’s definitely worth the cost to achieve automated cooling, in our opinion!
