Hi, I'm Darren Kaplan from the CannaCribs Horticultural Consulting team, and today we're here to share some concepts that we discuss and implement to help commercial cannabis growers operate as efficiently and successfully as possible. Making the right irrigation decision can make the difference between a poor-quality and a good-quality crop — if you've been growing for months or years, you know that to be true. Today we're going to talk about the fundamentals of irrigating cannabis.

There's a lot of information out there in the industry about precision irrigation, crop steering, drought stress — but I'm going to focus today just on the fundamentals. The first most important thing to consider is that cannabis, compared to other horticultural crops, likes a dry root zone. It likes to have its roots relatively dry between irrigation events.

An important concept to understand is waterlogging. You often hear about overwatering and underwatering — that can be confusing because it can be linked to the volume of irrigation you apply and the number of times you irrigate. Container capacity is the amount of water your cube or pot can hold after it's been completely irrigated to saturation and allowed to drain off completely. That's the most water you can have in your root zone after runoff. If you maintain that amount of water in your root zone for an extended period of time, you can start seeing waterlogging. Overwatering is often a misnomer for waterlogging.

If your substrate is wet for a long period of time, that's waterlogging. You start running into issues with low root-zone oxygen, high incidence of root-zone pathogens like Pythium and Fusarium, and reduced nutrient uptake. Cannabis is very sensitive to these things. If you maintain your substrate at a high moisture content or waterlogged for extended periods of time, you're going to run into issues with crop quality and yield — and it doesn't matter what else you're doing in terms of environment or fertilizer rates. You're going to have problems.

The ultimate goal is to irrigate your plants completely — reach that container capacity, ideally once a day (could be more) — and allow a sufficient dryback between irrigations so that the roots have time to dry out and the cannabis plants can perceive the benefits of the dry root zone. We're looking for adequate irrigation and adequate drybacks between irrigation.

The first step is to match your pot size with your plant size and the growth stage. If your plant is very big in a small pot, it's going to use water very quickly and you'll have to irrigate very frequently — which is operationally challenging. Still acceptable, but if you miss an irrigation you're at high risk of crop failure. If the pot is too large, the plant won't pull up enough moisture overnight and you'll have waterlogging conditions.

For simplicity, I'm going to break down the irrigation strategy into three phases — P1, P2, P3. Phase 1 is your first irrigation of the day — it can involve multiple pulses or shots to reach container capacity. You're looking to reach container capacity starting 30 minutes to an hour after lights are on and plants have begun to transpire. Remember: transpiration before irrigation.

Phase 2 is used to extend the period of container capacity. We're applying pulses — could be every half an hour or an hour. The idea is that the timing of the end of P2 allows for sufficient dryback until the next P1. When you're hitting container capacity, you're hitting it to runoff. To account for variability across plants, irrigate 10–15% more than needed to just hit container capacity — we call that the leaching fraction. Maintain between 5 and 20% depending on how much EC control you need.

Phase 3 is the period from the last P2 irrigation all the way to the first P1 irrigation. We like to see a dryback of 30 to 50% over the rest of the day, overnight, and to the beginning of P1. From container capacity, total moisture content decreases 30–50% during P3. That avoids waterlogging and maintains the dry conditions cannabis prefers. The range is wide — if you're somewhere between 30 and 50%, you're already doing quite well.

Why not always hit the bigger 50% dryback? The risk: if some plants are having a 50% dryback, others in your crop might be going even further. Some growing media — Rockwool, for example — once it hits really low moisture levels, it's hard to re-saturate. You run into issues of not being able to hit container capacity at P1 and P2 the next day, and that might last for the duration of the crop. Balance drybacks with the capacities of your growing media. Coco is more forgiving; it allows you to get drier and still return to container capacity.

After transplant, roots are not fully developed — there might be several days where you don't have to irrigate at all. When you start seeing that first dryback to 30–50%, that's when you have to start thinking about P2 timing. As plants grow, you extend P2 by another hour, then another, responding to the morning moisture content. You keep increasing P2 duration as plants grow and use more water — until they start to finish flowering and you'll see you don't need to increase P2 anymore, and may even need to bring it back.

How do you measure moisture content? The lift-and-feel method: irrigate to container capacity, wait 30 minutes, lift the plant — you know what fully saturated feels like. Aim to bring that weight down 30–50% by morning. If you have a scale, that's even better — weigh at container capacity, set a target morning weight. Or use volumetric moisture content sensors, which give accurate moisture readings when calibrated to your media, pot size, and placement. Even uncalibrated sensors are useful for tracking dryback trends over time.

Don't get too dry with media like Rockwool because you reduce the amount of water you can maintain in that media going forward. Maintaining field capacity is incredibly important. If you're irrigating with a water breaker, you fully saturate from the top. If you're using drip stakes, you can irrigate more precisely — but depending on where drippers are located and how many, you might leave the top of the growing media completely dry. Use at least two stakes for pots half a gallon or larger.

If you're using emitters with too high a flow rate, you start running into channeling — water flows through the pot and runs straight to drain without saturating the media. A slow, controlled irrigation approach with drippers is the best way to fully saturate growing media and hit container capacity without channeling. If using drip stakes, make sure they're not overly pushed in — leave 40–50% of the top of the stake sticking out so you don't get channeling starting halfway down the media.

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