Is Tap Water Okay for Hydroponics?

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Contaminated water collected in water bottles.

Some cannabis cultivators are lucky enough to enjoy clean tap water that’s always readily available for their hydroponic system. Unfortunately, that’s increasingly not the case across the country. Hormones, pharmaceuticals, and all kinds of toxic materials are making their way into the national water supply. In drought-stricken areas like Los Angeles and San Francisco, “toilet to tap” reclamation procedures are now in place, essentially straining solids, dosing with chlorine, then pushing the water back into the public drinking supply.

Sound gross? Well, the situation with clean water in America is rapidly changing. If you wouldn’t drink the tap water at your house or cultivation facility, why would you give that same water to your plants? One of the worst mistakes a cultivator can make is to invest money into a garden, but gloss over the water supply quality, perhaps the most important resource to having a genuinely healthy end-product.

A key indicator of water quality for plants is total hardness as expressed in the TDS of calcium and magnesium or in grains per gallon (gpg). With too much hardness, nutrient formulas can be thrown out of balance and plant deficiencies and lockouts quickly become a major problem. Any water source with over 50 parts per million (ppm) of TDS of hardness is considered a high ppm and should be purified through a reverse osmosis water filter or other purification method—reverse osmosis works by pushing feed water through semipermeable membranes filtering out contaminants. 50 ppm of hardness translates to 3 gpg and is considered soft water, which few facilities have straight from tap water.

Even if you believe your geographic area has a "good" water source, not investigating the water quality and giving your plants less than optimal water could choke their potential, and undermine an otherwise perfectly engineered operation. The TDS (total dissolved solids) in any type of untreated water varies for several reasons, and the relatively small investment into a reverse osmosis hydroponic water filter is a big step towards a consistent and reliable ecosystem.

Clean Water Report, First Things First

Municipalities provide free water reports, though water quality fluctuates greatly throughout an area, over the seasons, and can even vary from site to site.

Many cultivators use pure water—water from a source that has removed all impurities such as distilled water or RO water—to completely control the content of their nutrient formulas for each plant strain grown, making sure they properly and consistently dial in the amounts of each mineral vital to healthy plant growth. Getting a water test to determine source water quality helps cultivators to decide whether or not using a water filtration system would benefit their operation.

Growers using microorganisms such as beneficial bacteria, fungi, nematodes, mycorrhizae, and trichoderma must have chlorine and chloramine-free water in order for those helpful microbes to survive and flourish.

Water Contaminant FAQs

What is in tap water?

All city water contains chlorine and/or chloramines as they are both powerful biocides, meaning they are designed to kill all living organisms. Letting city water sit out and bubble overnight may get rid of chlorine, but it’s not effective to remove chloramines or other contaminants. Water from well or spring sources is often high in minerals such as calcium, magnesium, sulfur, and iron. Giving water with too high of levels of these minerals to heavy-feeding plants will contribute to nutrient lockout and lead to deficiencies.

Why is chlorine in tap water?

Chlorine is a biocide added to the water supply that kills beneficial bacteria, fungi, and microorganisms. However, any healthy organic or biohydro garden is chlorine-free. Hydroponic gardeners that use or brew compost teas or bioextract solutions typically remove chlorine from their source water.

What are chloramines?

Chloramines are a biocide that is a combination of chlorine and ammonia and is much more stable than chlorine. They are toxic to beneficial bacteria, fungi, micro-organisms, fish and amphibians.

How do you remove chloramines?

Chloramines will not dissipate by bubbling or even by boiling off. They can only be removed by proper filtration such as through an activated carbon KDF filter.

What is mineral hardness as it relates to water?

Mineral hardness is made up of the levels of calcium and magnesium dissolved in water and is the key cause of water problems in hydroponic facilities. High levels of hardness in untreated water lock out key nutrients to plants and also form scale on equipment and tubing. Calcium and magnesium are the most abundant minerals in tap water. The most typical form of these minerals in water is calcium carbonate and magnesium carbonate. The molecules of these compounds found in untreated water are far too large and immobile to be absorbed efficiently by the root systems and transported to where the plant needs them.

Is there fluoride in my water?

Fluoride is added to all municipal water supplies.

Is fluoride bad for plants?

Fluoride is a toxic substance to plants. Thirty-four enzymes in plants are affected by fluoride, as is seed germination. Enzyme additives will not do their job properly with fluoride in the water.

Do volatile organic compounds (VOCs) affect plants?

Some VOCs are known or suspected carcinogens. Trace amounts of these can end up in the plant’s tissues, flowers, and fruits.

How does Iron or Sulfur affect water and hydroponic equipment?

Water containing iron or sulfur may have a metallic taste and an offensive odor. Nutrient lockout, algae growth, and equipment staining can be results of too much iron in the water.

How does bacteria affect groundwater?

Groundwater sources may be affected by animal and human waste and other pathogens. These toxic substances can cause multiple issues in reservoirs and nutrient mixes and can be dangerous for human consumption and compromise the quality of your end-product.

How do nitrates affect plant growth?

Found in runoff from agriculture, animal yards, etc., these toxic substances contribute to over-nitrification and algae growth. High quantities of nitrates in untreated water can negatively affect the later stages of flowering, when cultivators typically try to limit nitrate levels in the growing medium.

How does pH affect plant growth?

Water that has either too high or too low pH will not allow nutrients to be absorbed properly and can be corrosive to equipment. Adjusting the pH of nutrient solutions may be difficult due to fluctuations in source water levels.
Using reverse osmosis to filter source water is the single most efficient, economical, and reliable way to ensure the removal of 98%+ of all contaminants mentioned above. As RO technology continues to advance, as well as new regulations in the United States go online, several simplified hydroponic water filtration solutions for commercial and hobby growers are now available. These RO systems ensure consistency and reliability of water input and are critical to the professional grower. Already have access to excellent source water? Consider yourself lucky. The rest of the country is having to deal with increasingly complex and sometimes dangerous water contamination issues as seen in areas like Flint, Michigan and Corpus Christi, Texas. Remember, if it’s not healthy for humans, it’s probably not healthy for plants.