We all know what humidity is- it’s water vapor carried in the air. We all know what relative humidity is, too- at least, we think we do! Trouble is, humidity may be affecting your plants in ways you maynot be aware of, and worse, might well be stunting the growth of your little green friends. This is timely because traditionally fall and winter are the driest time of year in Colorado. So, grateful growers,when you implement these humidity building tips you’ll see noticeable acceleration in the growth of your crops!
 
First, a few definitions. Relative humidity (RH) has nothing to do with Uncle Ernie’s damp armpits- rather it’s a measure of the percentage of moisture currently in the air relative to its maximum carrying capacity at the current temperature. That last bit is the part that tripspeople up, since air’s carrying capacity for water changes drastically with temperature- 80% RH at 60 degrees might be only 25% RH at 80 degrees! Since your skin isn’t as sensitive to humidity as it is to temperature, it’s essential to put a thermometer/hygrometer in your grow room to keep tabs on it. Spend a little money to get a good one with a memory function since you don’t want to be working with inaccurate information. I have one with a remote sensor so I can check my growroom’s conditions even when I’m not in there- maybe not required but darn convenient, let me tell you!

 
Unless your address is the middle of Cherry Creek Reservoir, chances are you will have a nasty shock when your new unit tells you that your RH is 12%. Yep, that’s Colorado, all right- small wonder then, that you can get zapped just walking across the carpet. Now, for the science. Vapor Pressure Deficit (VPD) is the mechanism by which all land based plants transpire and breathe. If RH is too high, they won’t be able to transpire and won’t be able to move nutrients up the stem to your precious growth tips- where the
action is. If humidity is too low, then the plant becomes stressed because it must spend too much energy trying to move enough water to keep from wilting! For a more in-depth discussion of VPD, there is a great article on the subject in the July/August issue of Urban Garden magazine, but for now, suffice it to say that if your humidity is much below 65%, your plants are spending too much energy moving water and not enough actually growing.

 
Now, how to get it up there? The answer, my friends, is blowin’ in the wind- from a swamp cooler. Of course you have a ventilation fan pulling warm, stale air out of your growroom, but unless you have a swamp cooler chances are it’s pulling all the humidity out, too.Go out and get one, and not the kind that hooks up to your furnace- they just don’t put enough moisture in the air. When you get it going, you will discover that it will cool off your growroom substantially, since evaporating water cools the air. Good news, since now you can reduce the amount of air being pulled through your growroom without worrying about overheating! Playing with your swamp cooler and exhaust fan settings will enable you to balance temperature and humidity. Ideally, that means about 70% RH if you like to keep your temps in the 70s and if you want to run in the low to mid 80s for best CO2 absorption, go for RH settings in the 75-80% range. If anyone tells you that blooming plants need less humidity than in the vegetative stage, don’t believe them! The best way I now of to minimize essential oils in the end product is to let the humidity fall during mid and late blooming phase.

 
With higher RH comes the possibility of fungal problems like powdery mildew, but don’t be afraid- use a natural antifungal such as neem oil, and be sure the air doesn’t stagnate around your plants by keeping the lower parts of them trimmed up and by circulating air through the understory of your grow. And since most coins have two sides, you will enjoy the benefits of higher humidity on discouraging spidermites and aphids as well! Other benefits include better temperature stability, making for smaller swings in temps when conditions change. You’ll also notice your plants will be asking for a lot less water, which helps reduce the danger of nutrient burn.

 
Lastly, be aware of the type of humidifier you get. Again, skip the furnace type, don’t get a ‘hot air’ type as that will overheat things, and don’t use a spray mister because that will atom-ize the dissolved minerals in the water. When they dry they’ll precipitate out of the air and stick to everything! The type that sucks air through a damp matting is what you’re looking for. That’s it for this month, so keep up the good work and happy growing!

 
About the Author:
Tyler Davidson – Indoor Cultivation Consultants