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Water Quality For Cannabis Production

April 24, 2020

  • Alkalinity & pH
  • Filtration & Hardness
  • Improve Biopesticide Performance

Water quality can be considered a part of the foundation for indoor cannabis production. Several factors can affect the quality of your irrigation water which include soluble salts, alkalinity, pH, and filtration. The quality of your water can affect the nutritional value of your irrigation water and affect the effectiveness of your biopesticide foliar sprays or chemigation (root drench) applications. If the water quality is poor, then you can expect slow or stunted growth, various nutrient deficiencies, and the possibility of burning leaves.

Alkalinity & pH

Alkalinity and pH are similar, but different and they are both equally important factors. pH (potential for hydrogen) is a measure of the concentration of hydrogen ions (H+) in liquids. For cannabis production, a pH between 5.5 - 7.0 for irrigation and foliar sprays is adequate. The exact pH needs of a cannabis plant changes slightly throughout its life cycle so bear that in mind. Alkalinity is the capacity of water to resist lowering pH changes, in other words it is the strength of a pH buffer solution. One of the most common pH buffer solutions in many hydroponic and soil mixes is limestone.

Filtration & Hardness

In case you didn’t know, it is not recommended to use tap water in a hydroponic system. The size of your filter will affect your flow rate so consider how long you are willing to wait to fill up water reservoirs. Filtering out chlorine, chloramines, heavy metals, and other solids from the water is necessary for hydroponic systems. Even though chlorine and chloramines are a part of water treatment and are necessary to kill pathogens that can affect humans, they are detrimental to the health of your cannabis plants by killing beneficial microbes in your grow medium. It is true that cannabis plants need trace amounts of chlorine, but the amount that is found in tap water exceeds this threshold. Many grows use carbon filters for their water filtration needs as reverse osmosis (RO) filtration systems can be expensive to maintain in the long-term. Many small and medium-sized grows use Hydro-logic carbon filters which are offered in various sizes and materials to suit your water filtration needs.

Water is considered “hard” when it has a high amount of calcium and magnesium. You can measure the hardness of your water by measuring the parts per million. Water with +150ppm without any additives is hard. The amounts of calcium and magnesium are too high and the form that they are in means that they are more harmful than beneficial to cannabis plants.

Improve Biopesticide Performance

It is important to check the quality of your water when using the water as a carrier for biopesticides. The quality of the water can directly affect the effectiveness of any biopesticide you use for control. The pH and temperature of the water are factors that can determine the quality of the water for biopesticide use. The temperature of the water should be warm for mixing, then the water temperature should be adjusted to room temperature when it is time to apply. Cold water will prevent pesticides from mixing properly and hot water can damage spraying equipment as well as plants. The pH of the water should be between 5.5 - 7.0 prior to adding pesticides and then should be closely monitored after mixing. Make minor adjustments to the pH depending on what the label’s recommendation states.

In addition to factors that can diminish the quality of the water for biopesticide use, there are other general rules to follow as good practice such as mixing in warm water, not using tank mixes that have been sitting overnight, and only mix biopesticides that are not prohibited by the label and have been proven safe for use.


  1. Voight, Delbert G. Jr. (2017). “pH and Water Modifications to Improve Pesticide Performance”. Penn State Extension.

  2. (n.d.). “Water Quality for Crop Protection”. University of Massachusetts Amherst. The Center for Agriculture, Food, and the Environment.

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