What’s Stomata With My Plants?

Cannabis, like all plants, requires gas exchange to live and grow. Carbon dioxide enters the plant, is fixed into sugar, and with the addition of oxygen gets metabolized to fuel the plant’s life functions. But with no lungs, how can a plant intake and exhaust gasses? 

The answer lies within tiny organs coating the plant’s surface called stomata. Stomata, along with lenticels (the little bumps on your plants’ stems), are responsible for a number of vital functions in a plant. From intake of carbon to increase your plants’ biomass, to uptake of water to allow nutrients to flow to your top colas, stomata are critical to growing a healthy plant; and understanding how they work will allow you to troubleshoot a number of problems in your crop. 

Levels of CO₂ 

If you’re a grower that supplements CO₂ you’re already aware of the relationship between biomass and carbon fixation. Every bit of tissue on your plant was formed from the atmospheric gas in your grow, so maximizing the available gas means maximizing your biomass, right?

In theory, yes, but in practice, not so much. Plants are extremely sensitive to their surroundings, and while an increase in available CO₂ typically means an increase in biomass, an excess of CO₂ can result in stomatal closure. If you are experimenting with CO₂ in your grow and notice that your plants suddenly aren’t drinking as much, this could be an indication that your CO₂ levels are too high. If you have a CO₂ meter, a target ppm of 1,000-1,500 will typically enhance your biomass without causing negative effects. Keep in mind that your cannabis plants are “C3” plants, meaning they will only fix carbon in the light. If you’d like to keep your gas bill low, consider only supplementing CO₂ while your lights are on. 

Water & Nutrient Content

Stomata do more than just intake gasses though, and without them, your plant would never be able to move critical water and nutrients from the soil to the plant’s apex. Through the influx and efflux of potassium ions in the cells surrounding the stomata, these apertures can open and close. With adequate control of your grow’s relative humidity, the flow of water and nutrients through your plants can be carefully manipulated. 

By sensing the humidity directly around the stomatal opening and the level of moisture in your root ball, your plant responds by either shutting their stomata to retain moisture, or opening them to allow water to flow from the root tips and out through the leaves. If you’re having trouble getting your growing media to dry back, or notice that your plants show certain nutrient deficiencies like low calcium, it’s possible your relative humidity is too high and your plants’ stomata can’t effectively exhaust water vapor. Water moves through your plant like a drink through a straw, and if the air surrounding your stomata is already saturated there will be no force pulling water into your leaves through your stems. Air flow is important! Adding a few fans to your grow will allow you to move saturated air off of your leaf surface, making room for more water to exhaust from your leaves and allowing more nutrients to move up from your root zone and into your leaves and buds. 

Your plants can’t hop out of their pots and walk somewhere else, so you are solely responsible for making sure their environment is just right for water and gasses to move through them. By understanding your plants’ stomata you can notice what your plants are trying to tell you and respond to maximize your plant health and yield. Happy growing! 

Michael Esposito is currently a Scientist at MCR Labs, focusing on microbiology. His interests lie in plant pathology and plant genetics. Previously, he has worked as a Cultivator & Plant Biologist for dispensaries and as a Researcher at the UMass College of Natural Sciences. He has a BS in Plant, Soil, and Insect Sciences from UMass Amherst.