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 here to understand is waterlogging. You often hear about overwatering and underwatering — that can be confusing because it can often be linked to the volume of irrigation you apply and the amount of times that you irrigate. Container capacity is the amount of water that your cube or your 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, then you can start seeing something called waterlogging. Overwatering is often a misnomer for waterlogging.

If your substrate is wet for a long period of time, that's waterlogging, and you start running into issues with low root-zone oxygen. You have a high incidence of root-zone pathogens like Pythium and Fusarium, and you start also running into issues with 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 your grow in terms of environment or fertilizer rates. You're going to have problems with your growth.

So the ultimate goal here 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. So we're looking for adequate irrigation and also adequate drybacks between irrigation.

So how do you make sure that you're having adequate drybacks each day? 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 the water in that pot very quickly and you're going to have to water it very frequently — which is operationally challenging. It's still acceptable and you can still grow a quality cannabis plant in a small pot, but if you miss an irrigation or several irrigations, you're at high risk of crop failure. If you have too big of a pot, then the plant will naturally not pull up enough moisture from the pot overnight and you'll have waterlogging conditions.

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

Phase 2 is used to extend the period of container capacity so that the last irrigation of the day is representative of that point that allows that proper dryback. In phase 2 we're applying pulses — it could be an irrigation 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 the subsequent day. Important to consider: when you're hitting container capacity, you're hitting it to runoff. To mitigate variability across plants, we try to irrigate 10 to 15% more than you would need to just hit container capacity — we call that the leaching fraction. You want to maintain that between 5 and 20%.

Phase 3 is the period of time from the last irrigation of P2 all the way to the first irrigation of P1. Typically we like to see a dryback of 30 to 50% over the rest of the day, overnight, and to the beginning of P1 — so from container capacity the total moisture content is decreasing 30 to 50% overnight during P3. That allows you to avoid waterlogging and maintain those dry conditions that cannabis likes so much. The 30 to 50% range is a wide one — if you're somewhere between that range, you're already doing quite well.

You might be thinking: why not always hit that bigger 50% dryback or even more? Well, the risk is that if some of your plants are having a 50% dryback, there's other pots in your crop that might be going even further. Some growing media — Rockwool, for example — once it hits those really low levels of moisture, it's hard to re-saturate, and you run into issues of not being able to hit container capacity at P1 and P2 the next day. So it's important to balance the drybacks with the capacities of your growing media. Coco is more forgiving — it allows you to get a little bit drier and still go back up to container capacity.

So how do you measure the moisture content in your growing media? The easiest way is the lift-and-feel method. If you irrigate to container capacity — you know you're at container capacity because you've reached runoff and waited 30 minutes — you lift that plant and you know what that feels like when your plants are fully saturated. You want to bring that weight down by 30 to 50% in the morning. If you have a scale, that's an even better way. You can weigh your plant at container capacity and bring it down to 30 to 50% of that weight first thing in the morning. Or use volumetric moisture content sensors, which you put into the growing media to get an accurate estimation of moisture content. Make sure those sensors are calibrated to your growing media, pot size, and located in the right position.

Remember: don't get too dry with certain growing media because you reduce the amount of water that you can actually maintain in that growing media. Maintaining field capacity is incredibly important. If you're irrigating with a water breaker, you're able to fully saturate the media from the top, ensuring every part gets saturated. If you're using drip stakes, you're able to administer irrigation more precisely — but depending on where the drippers are and how many you have, you might leave a portion of the top of the growing media completely dry. Use at least two stakes for pots of half a gallon or larger. If you're using emitters with too high a flow rate, you start running into channeling where water flows through the pot without actually saturating the media. A slow, controlled irrigation approach with drippers is a great way to fully saturate your growing media and hit container capacity without channeling.

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