Control Humidity to Best Fight Powdery Mildew

April 23, 2015

Confusion Over Pesticide Rules Presents Conundrum for Colorado Growers

“John Andrle, the owner of Denver rec shop L’Eagle Services, estimated that 95% of cultivators across the country use some form of pesticide, and that Eagle 20 is one of the most commonly used, because it’s fantastic for killing mildew…”

‘Nothing else gets rid of powdery mildew…it’s a cure-all,’ Andrle said.”

The real answer is to not get powdery mildew in the first place.

The key to mildew control is moisture control

Mildew growth requires fungal spores, organic material, and sufficient moisture. In actual practice, you can only control your moisture level. Experts agree that 50% relative humidity or lower is a good target for preventing mildew.

If you have a moisture problem, solve it before it becomes a mold or mildew problem.

Humidity equalizes rapidly

Air movement is needed to equalize temperature in the grow room but humidity diffuses rapidly throughout a room all by itself.  This means dehumidifiers require no ductwork in most grow rooms.

Ventilation sometimes helps

Ventilation can work well for hobbyists and small grow rooms.  Ventilation works best in dry climates with moderate temperatures year round, but comes up short during humid seasons or in humid climates.  The incoming air must be filtered to keep out pests, mold spores, and other contaminants.

Ventilation does not work well for growers using supplemental CO₂ and it adds cost when odors need to be filtered out of the exhaust air.

Air Conditioning sometimes helps

Air conditioning can remove some moisture when lights are on, temperatures are near 80F, and there is a need for cooling.  Using an air conditioner to dehumidify a room when the lights are off is not a good idea.  Running an air conditioner to dry a room when the lights are off will often freeze-up the A/C system and will certainly consume excessive energy.

It isn’t wise to force your cooling system to play at being a moisture management system.

Dehumidifiers give positive control

Dehumidifiers are a sure way of controlling humidity at all times.  Most commonly available dehumidifiers are designed for light use in residential settings. For hobbyist grow rooms, light-duty dehumidifiers can be used as-is or modified by removing the condensate bucket and running a garden hose to a drain.  A more professional operation should consider the extended reliability, much greater energy efficiency, and higher moisture removal capacity of commercial grade dehumidifiers.

How much moisture must be removed?

Plants transpire over 97% of the water they receive, so, for most grow rooms—the amount of water you give your plants is approximately the amount you must remove each day. For example 30 plants each receiving 1/2-gallon per day comes to 15 gallons water removal per day.

Dehumidifiers are rated by their water removal capacity in pints per day, operating in an 80F/60%RH environment.

Dehumidifier sizing

The best starting point is the concept of, “What goes in, must come out.” Almost all the water you give your plants must be removed from the air after the plants use it.

For the example above: (15 gallons) x (8 pints / gallon) is 120 pints per day of water. You would use a 120 ppd dehumidifier.

The air conditioner argument

Some growers may argue that their A/C system removes some water…so the dehumidifier wouldn’t need to have all that capacity. The fact is, A/C systems are optimized to reduce the temperature in a room (they remove sensible heat.) That’s what you want to use them for.

A/C systems can sometimes remove some water but while they are removing water, they cannot do much cooling. And, normally, if you’re A/C system is running, it’s because your room needs cooling.

Environmental control strategy

The best strategy for precise, efficient environmental control in a grow room: Use your A/C system to control room temperature and use your dehumidification system to control room humidity. Anything less is a gamble.