How Reverse Osmosis Helps Home Cannabis Growers
Test your water first. Reverse osmosis (RO) helps when tap water is hard or high in dissolved solids, but many growers do not need it. If your water is already clean and soft, good nutrients and correct pH matter far more than an RO machine.
RO gives you very pure water, which means you control exactly what your plants get. That control comes with tradeoffs: wastewater, maintenance and the need to add back minerals. Below, you’ll see how RO works, how to decide if you need it, how to choose a home unit, and how to feed plants with RO water.
What RO is
Reverse osmosis pushes water through a very fine membrane under pressure. Water passes through, while most dissolved minerals, salts and many contaminants are held back. University of Nebraska Lincoln Extension explains what happens to the leftovers: “The rejected contaminants on the concentrated side of the membrane are washed away as wastewater.”
A home RO system usually has several stages. A sediment filter catches dirt and rust, a carbon filter removes chlorine and some chemicals, and the RO membrane does the fine filtering. Some units add a final carbon or remineralizing stage. The University of Nevada, Reno Experiment Station notes that “the largest component of a reverse osmosis system is the pressure tank that holds treated water and supplies a separate faucet.”
The result is water with very low dissolved solids. For a grower, that’s a clean starting point to build on. You add nutrients to it, and you know your readings come from your feed, not from your tap.
Do you need RO? Test your water first
Plenty of growers don’t need RO at all. Cannaseuse, a German grow site, points out that RO is not automatically essential for every grow, and that the EPA notes RO is not sensible or necessary for all applications.
Start with a water test. A cheap EC or PPM meter tells you how much is dissolved in your tap water, but that is only part of the picture. A full water report, from your water supplier or a lab, tells you about hardness, alkalinity and whether the water is treated with chlorine or chloramine.
RO tends to help in a few situations. If your water is very hard, it can leave mineral buildup and make pH hard to control. If your water has a high starting EC, it leaves less room for nutrients. If your report shows problem contaminants, RO can remove many of them. If your water is soft and clean, a simple carbon filter or letting water stand may be enough.
How to choose a home RO machine
If you decide you need RO, a few specs matter most.
Output per day. RO units are rated in gallons per day. Work out how much water your grow uses at peak, when plants are large and drinking most, and choose a unit with room to spare. A slow unit can leave you waiting for water.
Wastewater. Every RO unit sends some water to drain. The amount varies a lot by unit and by your water pressure, so check the maker’s specs. Plan where that water goes. Some growers use it for cleaning or for garden plants that tolerate it.
Space and setup. Units with a pressure tank take up room under a sink or in a utility area. Tankless units are smaller but may need more pressure. Make sure you have a water line, a drain and enough space.
Stages. More stages are not always better. For most growers, a sediment filter, a carbon filter suited to your water’s chlorine or chloramine, and a membrane cover the basics.
Filter costs. Prefilters and membranes need replacing. Check how often the maker recommends changes and what replacements cost before you buy.
How to set up RO for a grow, step by step
- Test your tap water’s EC or PPM and ask your supplier about chlorine or chloramine.
- Decide if you need RO from a full water test (hardness, alkalinity, chlorine or chloramine), not EC or PPM alone.
- Pick a unit sized for how much water you use per day.
- Follow the maker’s installation instructions for stage order, and plan where wastewater goes.
- Test the output water to confirm it is working.
- If your nutrients do not already supply calcium and magnesium, add them as the label directs, then set pH before feeding.
Keep a log of your output readings. A slow rise in the output EC over weeks is an early sign that filters or the membrane need attention.
Calcium, magnesium and pH with RO water
RO removes nearly everything, including minerals plants need. University of Nebraska Lincoln Extension lists calcium and magnesium among the minerals that household RO units remove. Libera Herba adds that RO removes calcium and magnesium that must then be added back.
Cal mag supplements are often added to RO water, but they are not always needed. Some nutrient lines already include enough calcium and magnesium for soft or RO water. Check your nutrient label and readings first, and only add a supplement if your feed falls short.
pH needs care too. RO water has very little buffering, which means its pH can swing easily. Measure pH after you mix in your nutrients, not before, because the nutrients change it. Adjust slowly and recheck.
Troubleshooting
| Sign | Likely cause | What to do |
|---|---|---|
| Output water reads high | Worn membrane or filters | Replace filters and retest |
| Very slow output | Low water pressure or clogged prefilter | Check pressure and change the prefilter |
| Leaf spots after switching water | Several possible causes | Check water quality, feed and pH before assuming a calcium or magnesium shortage |
| pH swings | RO water has little buffering | Measure after mixing nutrients |
When you switch to RO, try to change just one thing at a time. If you also change nutrients or pH targets on the same day, it is hard to tell what caused any new problem.
RO vs tap, filtered and distilled
Tap water is fine for many growers, especially where it is soft. Let chlorinated water stand or use a carbon filter if needed, and check your pH.
Carbon filtered water removes chlorine and some chemicals but leaves most minerals in place. It is a good middle option when your water is clean but treated.
RO water is very pure and gives you full control, at the cost of wastewater, maintenance and the need to add minerals back.
Distilled water is similar to RO in purity, but buying it by the jug gets expensive for a grow, and home distillers are slow. RO is usually more practical when you need a lot of pure water. Whatever you choose, feed and pH matter most. For another common additive question, see our guide on molasses at https://kindseed.com/why-use-molasses-for-cannabis/.
Sources: University of Nebraska Lincoln Extension G1490, Drinking Water Treatment: Reverse Osmosis (https://extensionpubs.unl.edu/publication/g1490/2014/html/view). University of Nevada, Reno Experiment Station, Selecting a Reverse Osmosis Unit (https://naes.unr.edu/publication.aspx?PubID=4758). Cannaseuse, RO for growing. Libera Herba, grow manual. Cannabis Industry Institute, Using a Reverse Osmosis Filtration System.
FAQ
Do I need an RO machine for cannabis?
Not always. Test your water first. RO helps when tap water is very hard, high in dissolved solids, or has problem contaminants. If your water is soft and clean, good nutrients and correct pH matter more, and a simple carbon filter may be enough.
How much wastewater does RO make?
It varies by unit and by your water pressure. Every RO system sends some water to drain to carry away the rejected contaminants. Check the maker's specs before you buy, and plan where the wastewater will go.
Should I add cal mag to RO water?
Often, but not always. RO removes calcium and magnesium, and some nutrient lines do not supply enough for pure water. Check your nutrient label and readings first, and only add a supplement as the label directs if your feed falls short.
RO vs distilled water?
Both are very pure. Distilled water is expensive to buy in the amounts a grow needs, and home distillers are slow. RO is usually more practical for regular use, though it produces wastewater and needs filter changes.
How often do RO filters need changing?
Follow the maker's schedule for your unit. Prefilters usually need changing more often than the membrane. Test your output water regularly, because rising readings are a sign that filters or the membrane need attention.
