Every cannabis grower knows the perils of improper watering. And most can recall a time or two when they damaged the health of one of their girls through an overzealous watering schedule (or simple forgetfulness). But few recognize the epidemic of ongoing, low-level overwatering – both in at-home amateur grows and larger, automated operations.

Mild overwatering is hard to detect. It doesn't necessarily cause leaf droop, and it often presents no outward signs in the canopy. The whole issue can fall off the radar, or at least into the category of "if it ain't broken, 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. A 4% bump in yield creates enormous revenue. What's more, prudent watering can lower overhead by decreasing the runoff of pricey nutrients while minimizing the electricity used for dehumidification.

How Greenhouses Can Cause Overwatering

As growing operations evolve, marijuana plants lead increasingly privileged lives. In the quest to increase quality and maximize the quantities of crops, greenhouse conditions now approach environmental perfection. Never before have plants had it so good. Climate controlled temperatures, abundant CO2, boutique nutrients and the 100% chance of sunny days push growth far beyond what nature anticipated.

But in the case of watering, these ideal grow conditions create some ironic liabilities. Greenhouses maintain high humidity levels to ensure the stomata of the plants stay open to maximize CO2 intake. 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. Cool, humid mornings are the ideal opportunity for photosynthesis, and this time of the day is what greenhouse grows strive to replicate.

Throughout the rest of the day, the stomata choose to open or close for different reasons: either to allow evapotranspiration to cool the plant in the hot sun or else to conserve moisture if it's in short supply. The afternoon is a critical time for moisture regulation, and that typically means moisture conservation. But this period also presents an opportunity for moisture elimination should too much water be present in the soil. Transpiration through the leaves releases this water, but a humid greenhouse stymies the process, and growers must carefully regulate relative humidity (RH) and temperature – as well as their watering schedule. But relative humidity isn't the whole story; it offers only a limited understanding of how the ambient air affects the cannabis's moisture metabolism.

The "Pull" of Moisture Out of the Canopy: Vapor Pressure Deficit

Relative humidity 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 a 70% RH level. Cultivators carefully monitor the RH and temperature of their grow rooms, but those conditions are just indicators of a more important phenomenon.

What's actually affecting the canopy's ability to regulate and release moisture? It’s the pressure differential between the inside of the leaves and the outside air. More specifically, it's a lack of pressure in the air – a relative vacuum – that "pulls" moisture out of the leaves, and, in turn, through the plant and out of the soil. This vacuum is the vapor pressure deficit (VPD). A low VPD environment is similar to how a humid summer day feels to people. Perspiration stops working so well. Our sweat just doesn't have anywhere to go, so our "evaporative cooling" method becomes ineffective. In a drier climate, an even warmer day doesn't seem so bad because sweating works better.

VPD matters more in the "minds" of cannabis plants than RH, and VPD is directly tied to the plants' transpiration – which is directly linked to watering. Transpiration – evaporation from the leaves – increases the moisture in the air, which lowers the VPD, and it all comes full-circle to limit the plants' ability to transpire. This compromises the plants' ability to cool themselves 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 creates a change in watering needs. In the vegetative growth phase, a wide range of humidity levels is acceptable, and the temperature is maintained at 22-28 C°. Yet the air in the grow room exerts significantly more pressure at 28 C°/70% RH than at 22 C°/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. Will it damage plants? Not really, but it can disrupt the near-perfection that maximizes growth.

The "Push" of Moisture into the Roots: Hydrostatic Pressure

A positive pressure, or "push," takes place in the soil. The term hydrostatic pressure just means that water is present in the air pockets of the soil or growing medium, pushing on the roots. It's a complementary force to VPD. When these push/pull forces are in harmony with one another, the plant has the opportunity it needs to 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 vapor pressure deficit, the plant begins to "drown" because the process of transpiration can’t keep up. The 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, damaging the roots. Once overwatering compromises the population of beneficial microbes, it may not recover. The nutrients the dead microbes would have provided to the roots are no longer available.

Irrigation in Light of Vapor Pressure Deficit

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, Troph Blumats, uses the principle of hydrostatic pressure to deliver water slowly – and at the plants' preferred rate. A water-filled, ceramic cone is inserted into the soil when grow room conditions are close to ideal. As the soil dries, and hydrostatic pressure drops, the pressure inside the cone drops too, and the suction within the cone pulls open a valve to the irrigation line. With this system, each plant effectively determines its own irrigation schedule to match changes in VPD, minimizing wasted water and the need for dehumidification.

The ongoing assessment of VPD in relation to soil moisture can be a time-intensive endeavor – one best handled by automation or a system like Troph Blumats. When a plant-responsive irrigation system is employed, plants have a variable they can control, on their timeframe, to regulate their own health as only they know how. This allows them to better adapt to changes in grow room conditions which may be mildly unfavorable.

In fact, the plants’ 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, and the solution to the vicious irrigation/transpiration/dehumidification cycle becomes apparent – let the plants decide!

For more information on Tropf Blumats visit www.sustainablevillage.com or call (303) 998-1323.

Jack Bohannan is a Denver-based freelance marketing writer. He specializes in technical, cannabis-related subject matter. Contact: jack@jackbwriter.com