From cuttings to the final product, cannabis producers make daily decisions about water. It is the transport medium for nutrients, serves as the plant’s coolant and interacts with CO2 capture.
But if water is not properly managed in the root zone and in the air, plants may see their performance compromised or disease persist, despite your best efforts. This is why water should be a top priority for your bottom line.
Water Analysis and Treatment
All water discussions should start with analysis and, if necessary, a treatment plan.
Municipal water seldom needs further treatment, as it typically contains calcium (Ca) and magnesium (Mg) at levels that do not require cultivators to modify nutrient formulations.
That is not the case, however, with well water. High calcium and magnesium content in limestone wells can be present in such large amounts that growers may not need to supply those nutrients in their fertilizer recipes. (Water from limestone wells can contain up to 50 parts per million (ppm) of Mg and 150 ppm of Ca, more than double the standard drinking-water levels.) An irrigation suitability test from a water and soil lab (your state’s university extension should have one), pinpoints nutrient content and alkalinity of the water. That data is used to assess what type of treatment the water requires.
Well water commonly has high levels of alkalinity from dissolved carbonates. These levels raise pH in the root zone. If lab-measured total alkalinity exceeds 150 ppm, acid treatment is recommended to control pH increases.
All water sources must be tested and monitored for biological and chemical contamination. Water’s amazing solvent properties also mean that it needs to be screened for heavy metals and chemicals. That is especially true of lake water (which is exposed to all manner of runoff). When using surface water sources, growers should be highly aware of potential contaminants in the catchment’s watershed. Depending on the analysis, required treatment may include filtering, chlorination, reverse osmosis, ozonation or ion-exchange columns. Water processing is no place to scrimp. Work with water-quality professionals to design these systems.
There is no Plan B for taps running dry. Storage is difficult to justify, but if your operation decides that it wants to be able to operate for three days without replenishment capability, you must plan for running your water through appropriately sized storage vessels. As for water delivery, most of the risk lies in mechanical and power failures, so look at redundancy in delivery systems and backup power.
Match Nutrients to Vapor Pressure Deficit (VPD)
Research has shown a direct correlation between the flow of water toward roots and nutrient uptake. So, when transpiration pulls water from the media, new water replaces the water absorbed from the media, bringing dissolved nutrients into contact with the roots for uptake.
To maximize nutrient uptake, you must ensure the environment and water-delivery scheme maintains transpiration throughout light periods. If plants dry down significantly, or if humidity gets too high, transpiration and nutrient delivery slow.
The degree to which a design can control VPD is tied to how well temperature and humidity can be controlled. Research growth chambers are able to achieve wide ranges of control, but are often no bigger than a walk-in freezer. Scaling that capability to production volume comes with significant expense. At scale, the use of these chambers is likely to be set by a cultivator’s access to capital and ability to deal with increased operating costs as much as by what the cultivation operation needs.
If one crop grows in a VPD environment of 10 kPa (kilopascals) and another at 20 kPa, more water will be drawn to the roots with the higher VPD. If the same nutrient solution is applied to these two sets of plants, the plants with the higher VPD will get more nutrients and will likely produce more yield. If the grower, however, gives the 10 kPa VPD plants double-strength nutrient solution, the two crops will now receive about the same total nutrients and will predictably perform quite close to one another.
While indoor growers are unlikely to experience that wide a range of VPDs, outdoor growers will. As a result, outdoor cultivators need to match supplemental nutrient concentrations to VPD to grow plants to their target harvest schedule and size, crop after crop.
Keep It Coming
The consequence of allowing the water supply to run low during light periods is that plant cells lose water pressure, causing the guard cells in the stomata to “deflate,” which, in turn, restricts the plant’s source of CO2 and delays photosynthesis. If plant stomata are closed for any significant time during the light period, growers lose precious growing time, nutrient uptake and yield.
The obvious way to detect low water is to look for changes in the environmental system’s run time or the amount of condensate being produced by the system. (Everyone measures their condensate, right? If not, you should.)
But there is no condensate to measure for field or greenhouse crops. Instead, those growers may find direct measurement of transpiration to be more helpful. (See the sidebar on p. 129 for more info on measuring transpiration and photosynthesis rates.)
Water as a Trouble Maker
While we discussed avoiding dry-down, the other problematic end of the environmental spectrum is when rooms drop below the dew-point temperature. This allows a film of water to condense onto leaf surfaces, which can activate mold and yeast spores.
Spraying pesticides (as a treatment or a preventative) also wets leaves. If the environment is not conducive to a quick dry-down, you are exposing your plants to powdery mildew outbreaks. If the room drops below the dew point temperature, trying to prevent or treat powdery mildew may actually encourage it. Eliminating spraying is the best solution for several reasons, but when it isn’t an option, we suggest avoiding low-pressure, high-volume spray equipment (i.e., hand pump) in favor of high-pressure, low-volume equipment (i.e., foggers or cold foggers) to limit moisture and humidity in a room.
Because humidity increases when spraying, VPD can drop to a point where transpiration is reduced significantly, so be sure to look at how long relative humidity (RH) stays high after spraying to understand how much of the plant’s overall growing time is spent with reduced transpiration to understand how to adjust nutrient delivery.
Put Water In, Take Water Out
As soon as harvest happens, all the water in the plants works against us. Water is essential to the growth of bacteria and fungus, which cannabis producers want to control. The relevant measurement of moisture relative to food safety and cannabis mold and yeast prevention is water activity, denoted as Aw.
For flower producers, the goal in dry and cure is to get product to an Aw of .65 or lower—commonly recognized as the point at which pathogens cannot thrive. An Aw of .65 is about the same as 15-percent moisture content, so dry-and-cure operations need to be designed around the goal of getting flower moisture levels to 15 percent as fast as possible without losing potency or terpenes. Once Aw is lowered, preserving product quality and longevity is achieved by keeping processed product in containers. This will prevent additional moisture reaching the product.
Stay on Top
While it may seem simple, proper water treatment and management is crucial to a grower’s success. To ensure water isn’t limiting performance, make time to evaluate water policies and practices on a regular basis.