Every cannabis grower knows the perils of improper watering. Most can recall a time or two when they damaged the health of one of their plants through an overzealous watering schedule or simple forgetfulness. But few recognize the epidemic of ongoing, low-level over-watering.
Mild over-watering is hard to detect. It doesn’t necessarily cause leaf droop and often presents no outward signs in the canopy, putting it into a category of “If it ain’t broke, don’t fix it.” High-quality cannabis and excellent yields are certainly possible despite imperfect hydration, but in the case of the world’s most lucrative cash crop, optimization is paramount. Even a 4% bump in yield can create enormous additional revenue. Prudent watering can also lower overhead by decreasing the runoff of pricey nutrients while minimizing the electricity used for dehumidification.
The Cause of Over-watering
As grow operations evolve, marijuana plants lead increasingly privileged lives. In the quest to increase quality and maximize yield, greenhouse conditions now approach environmental perfection. Climate-controlled temperatures, abundant CO2, boutique nutrients and the 100% chance of sunshine have pushed growth far beyond what nature anticipated.
But in the case of watering, these ideal grow conditions create some ironic liabilities. For example, greenhouses maintain high humidity levels to ensure the stomata of the plants stay open to maximize CO2 intake. Cool, humid mornings are the ideal opportunity for photosynthesis and greenhouse grows strive to replicate this time of day. This practice speeds growth, but plants are locked into a metabolic moment that only occurs in nature for a relatively brief period in the earlier hours of the daylight.
Throughout the rest of the day, the stomata open or close either to allow evapotranspiration to cool the plant in the hot sun or to conserve moisture if it’s in short supply. The afternoon is a critical time for moisture regulation, typically in the form of moisture conservation. This period also presents an opportunity to eliminate extra water in the soil. Transpiration through the leaves releases water, but a humid greenhouse stymies the process and growers must carefully regulate relative humidity and temperature — in addition to their watering schedule.
Vapor Pressure Deficit
Relative humidity (RH) is just that — relative. The amount of water vapor that air can carry is relative to its temperature: warm air can carry more moisture than cold air. Warm air at 70% RH contains more moisture than cooler air at the same RH. Cultivators carefully monitor the RH and temperature of their grow rooms, but those conditions are just indicators of a more important phenomenon.
The pressure differential between the inside of the leaves and the outside of the air affects the canopy’s ability to regulate and release moisture. More specifically, it’s a lack of pressure in the air — a relative vacuum — that “pulls” moisture out of the leaves, and, in turn, out of the soil and through the plant. This vacuum is known as the vapor pressure deficit (VPD).
VPD matters more in the “minds” of cannabis plants and it’s directly tied to the plant’s transpiration — which is directly linked to watering. Transpiration — evaporation from the leaves — increases the moisture in the air, which lowers the VPD and limits the plant’s ability to transpire. This compromises the plant’s ability to cool itself under hot grow lights — even though ample moisture is available in the soil. Irrigation (the water introduced to the grow room) is the key to regulating VPD.
Even if RH and temperature are maintained within acceptable ranges, a variance in either condition can create a significant difference in VPD, which changes the watering needs. In the vegetative growth phase, a wide range of humidity levels is acceptable and the temperature should be maintained between 72 and 82 degrees Fahrenheit. But the air in the grow room exerts significantly more pressure at 82 degrees and 70% RH than at 72 degrees and 50% RH. Such a difference requires an adjustment to the irrigation schedule to match the change in transpiration behavior.
So in the “mind” of the plant, changing VPD levels can make a set irrigation schedule seem irregular. It won’t really damage the plants, but it can disrupt the near-perfect environment that maximizes growth.
A positive pressure, or “push,” takes place in the soil. Hydrostatic pressure is a complementary force to VPD, meaning that water is present in the air pockets of the soil and pushes on the roots. When these forces are in harmony, the plant can move water and nutrients up the xylem of the stalk at the rate it prefers.
When surplus water in the soil (hydrostatic pressure) pairs with a low VPD, the plant begins to “drown” because the process of transpiration can’t keep up. Water displaces the oxygen needed by the roots. Because the ionic processes of photosynthesis require oxygen as a medium to release electrons, photosynthesis stops. Too much water can also drown beneficial soil microbes and spur the growth of other microbes that excrete alcohols and damage the roots. Once over-watering compromises the population of beneficial microbes, it may not recover, and nutrients that would have been provided by microbes are no longer available.
Hygrometers, thermometers and soil moisture meters help manage VPD, but the time-intensive process is best handled by an integrated automation system. Increasingly, computerized automation can adjust watering on the fly, yet simpler methods can eliminate the irrigation adjustments that are necessary to control VPD.
One such system, Tropf Blumats, uses the principle of hydrostatic pressure to deliver water slowly and at the plant’s preferred rate, allowing each plant to determine its own irrigation schedule according to changes in VPD, which minimizes wasted water and the need for dehumidification.
The ongoing assessment of VPD in relation to soil moisture can be a time-intensive endeavor. When a plant-responsive irrigation system is employed, plants have a variable they can control to regulate their own health, so they can adapt better to changes in grow room conditions that may be mildly unfavorable.
In fact, the plant’s response to irrigation might be the only way they can provide growers with definitive feedback. Cannabis ultimately asks for more water, speaking in the language of dry soil, or refuses the water with a soggy signal it’s had too much. When VPD is taken into consideration, irrigation can be planned from a better vantage point.
Letting the plants manage water intake can be a solution to the vicious irrigation/transpiration/dehumidification cycle.
Jack Bohannan is a Denver-based freelance marketing writer. He specializes in technical, cannabis-related subject matter. He can be reached at firstname.lastname@example.org.