Often overlooked by most cannabis growers, irrigation strategy is a critical part of growing any plant. This is especially true when the goal is to maximize flower development, increase resin production, and refine flavor. Proper irrigation strategies need to be tailored to the plants’ genetic requirements and growing environment. Genetic variation, air temperature, humidity, CO2, air movement, irrigation system design, light type, and quality all play a role in how much water and nutrients a plant uses for optimum growth. Having the ability to monitor and steer water content, manage electrical conductivity (EC), and manipulate drybacks in the root zone can help ensure consistent and quality production.
Precision irrigation requires a system to apply the right amount of water at the right times. Ebb and flow irrigation is more commonly used in the propagation and early-growth stages, but can also be used in the final stages of growth. One caveat is that it does not allow for the same amount of plant steering and control that can be achieved with drip irrigation. Ebb and flow systems are good at keeping the root zone water content (WC) and EC the same in all the plants, but the right substrate volume and flood water height is needed to ensure full saturation of the growing media.
A well-designed pressure-compensated drip emitter is the best way to ensure every plant receives the same amount of solution. High-quality, commercial-grade drip emitters, pumps, filters, zone control, and timing systems with correctly- sized, light-blocking pipe are central to successful irrigation system operation. In addition, regular measuring of irrigation volumes from the emitters at different locations in the farm, cleaning of filters, and post-harvest maintenance of irrigation lines/emitters is critical to keeping the system applying water consistently throughout the entire farm.
With a proper irrigation system in place, timing and regular measurements of volumes applied to the plants compared to the total volume of runoff is required to dial in irrigation frequency. By changing irriga¬tion volumes, frequency, start times, and stop times, growers can control the WC and EC in the substrate, maintaining root health and plant quality. Adjust¬ing these factors allows growers to steer the plant between vegetative and generative growth.
IRRIGATION VOLUMES & FREQUENCY
The volume of the irrigation event should be equal to two- to six-percent volume of the substrate being used. For example, a three-percent irrigation event for one liter of substrate would be 30mL. Using a measuring cup, record the amount of time it takes dripper(s) to achieve the two- to six-percent irrigation volume. That time is your “pump on” time.
Many factors drive the ideal WC, EC, and total water consumption of a plant. This includes plant genetics, size of canopy, amount of light, air movement, temperature/humidity (VPD), etc. Measuring the volume of runoff is a simple way to help determine irrigation frequency which changes as the plants get older. The total amount of runoff should be about five to 20 percent of the total amount of water applied during the course of the day. This can be measured by placing one or three plants raised in a tray that allows you to capture the runoff. At the end of the day, measure total runoff volume collected, and divide by the number of plants being measured. You should have five- to 20-percent runoff of total volume applied to the plant over the course of that day. This also allows growers to reduce the amount of expensive nutrient that goes down the drain and into the environment while optimizing plant growth and product quality.
CROP STEERING
Crop steering through irrigation can help the plant grow leaves (vegetative growth) or flowers/fruits (generative growth). After the lights come on and the plants start transpiring, apply several irrigations that will slowly build up water content in the substrate. After a few irrigations, you should start to achieve runoff (drain) from the media in the afternoon. By achieving drain slowly, the substrate is allowed to fully re-saturate and EC to be stable at the point the plant is consuming the most and working it’s hardest. The drain also helps flush out old nutrients that are building up in the growing media. Stopping irrigation several hours before the lights go off allows for dryback to occur towards the end of day and overnight so WC is the lowest and EC is the highest when the plants is in a state of rest.
Doug Jacobs is a Technical Adviser with Grodan. He provides expert consulting on proven Precision Growing methods to optimize crop production with Grodan rockwool growing media, proper irrigation and other operational factors to produce the best quality plants using the least amount of inputs. Doug has extensive experience with indoor and greenhouse hydroponics, vertical farms, aquaculture and CEA system design. He has developed systems for large corporate institutions, non- profits, universities and urban farms throughout America. For more information about Precision Growing visit www.grodan-mj.com or email doug.jacobs@grodan.com