Humidity can sometimes be the forgotten piece in a well-crafted cannabis growing formula. “A lot of people want to pay attention to lights, airflow and nutrients for their plant, but humidity is critical. It’s what optimizes plant growth and hopefully gets rid of any potential pest problems,” says John Pratt, a factory representative at Quest Dehumidifiers. “Humidity is what can optimize your business.”
Though the percent of relative humidity is the most common humidity measurement, there’s another piece of the humidity control puzzle that cultivators can utilize to improve plant yields, called vapor pressure deficit, or VPD, for short.
“Vapor pressure deficit is technically the differential between the amount of vapor in the air, and the maximum amount of vapor that the air can hold at a particular temperature,” Pratt says. “The Vapor Pressure Deficit scale is a method of growing to make sure the optimal level of pressure is exerted on the plant consistently throughout the grow, at different temperatures and different humidity levels.”
Cultivators need to be aware that there’s a “sweet spot” for VPD levels, which, if exceeded, can dry out cannabis plants. On the other hand, if the VPD level is too low, powdery mildew, bud rot and pest infestations can take hold.
To achieve your optimum VPD, first you need to thoroughly understand your humidity levels.
According to Cannabis Business Times’ and Quest Dehumidifier’s ‘Smart Humidity Report,’ in which 104 cultivators were surveyed for their humidity-regulation techniques, the majority of respondents indicated that vegetative room optimal humidity falls at a range between 46 percent and 65 percent, and flowering room optimal humidity levels ranges from 40 percent to 55 percent. Click here for more ‘Smart Humidity Report’ statistics.
Fortunately, there are companies out there that manufacture easy-to-use monitoring tools, Pratt notes.
Hygrometers (humidity-measuring instruments) and data loggers are two such devices you can use to help achieve your ideal levels.
“You can put them at any exhaust system of an AC; put them by a dehumidifier; put them near the plants, the ceiling, and what it’ll do is take measurements during any time period we dictate to it,” he says, though he recommends recording the data logger every 5 minutes. “You can do that over days, and see graphs and [using recorded data, see] charts about what they’re trending at certain hours of the day, and it can help you diagnose potential problems.”
Then, use a special calculator that can help you discover those desired VPD levels, Pratt says. Or, you can calculate the old-fashioned way, with standard formulas that are listed on websites of many universities with horticulture departments, though these can get a bit complicated to do by hand.
Once optimal humidity and VPD levels are determined, you can adjust for your desired humidity using dehumidifiers, increased air flow, AC and other methods. The result: optimal plant growth for higher yields.
Photo at top: Cannabis crop in Sydney, Maine. Quest dehumidifiers are another tool growers can use to hone in on desired VPD levels.
