Water Demands for Cannabis

Recently doing some legwork on a design request I found myself making incorrect assumptions about the water demands of Cannabis. The overall demand of the plant is a little misleading. While Cannabis does in fact require quite a bit of water it prefers the application be across a long period of time and evenly distributed above the root base. Fortunately for growers, application options are plentiful and automation has made the waiting game not quite as painful as it used to be. For the project at hand I decided to dig deeper…and put the findings on paper.

Starting from the top; Cannabis like any plant is constantly using valuable energy trying to grow a root base. To be sure there are no interruptions in growth we must provide the plant with a combination of nutrients and water. As the plant uses water to grow we are left with a depletion level that must be replenished. In simple terms the water used or burned up by the plant is the plant’s Crop Coefficient. Knowing the Crop Coefficient of many comparable species to Cannabis we’re given a value of 1.0. The Crop Coefficient combined with the environments, soon to be explained ET value, is going to help guide a grower down the path of efficiency.

The ET value of our growing environment is going to help determine how much water we should apply to refill our plants depletion. Figuring out the ET value of an indoor environment can be a bit tricky. Fortunately the same variables that apply to our outdoor environments also apply to our indoor environments. Along with a plants growth stage and maturity level; humidity, temperature and solar radiation are all things to consider when determining an ET value for your room. For reference it’s currently 72 degrees in San Diego with a nice cool breeze and…ok sorry, and the Daily ET is .12. Not too far removed from coastal SoCal in Phoenix it is a very exciting 116 degrees, if I had to guess it’s a dry air. Current ET in Phoenix? .33 This pattern tells us that with dryer, warmer air our plants need more water.

Understandably growers do their best to keep rooms cool and well ventilated but with most conventional lighting setups creating room temperatures between 70 and 80 degrees we will use .18 as our value. With the majority of humidity coming from irrigation and what the plants emit our ET value should be relatively low. In other words our Cannabis needs .18 inches of water per day to thrive.

“No way!”

– Naysayers

They are actually correct! Keep in mind, not all the water leaving your emission device is making it into the plant. Stay with me here; on average, drip irrigation has what we call a DU or Distribution Uniformity rate of .9, meaning 90% of the water leaving the device makes it where we need it to go. With environmental conditions eating away at 10% of our water and multiple lighting cycles expediting photosynthesis we’ll now need to apply 2 X our previously calculated .18 demand.

Here is our formula:

1.0 x .36 = .36 inches of water per day

1.0 is the Crop Coefficient of Cannabis

.18 x 2 = .36 ET Value (based on two 10 hour lighting cycles)

.36 is the plants daily watering demand

We’ve now determined that Cannabis needs approximately .36 inches of water per day to replenish what the plant has used for energy. We can now move on to the fun part, the application of water!

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