RO Water for Cannabis

What RO water actually is

Reverse osmosis is a filtration process that forces water through a semipermeable membrane under pressure, removing 90–99% of dissolved solids, most heavy metals, chlorine/chloramine (with appropriate prefilters), and the bulk of dissolved minerals like calcium, magnesium, sodium, and bicarbonates ^[1]. The result is water with a very low TDS (total dissolved solids), typically under 20 ppm, sometimes near zero with a DI (deionization) post-filter.

For reference: municipal tap water in the US averages around 150–400 ppm depending on the region, and well water can run anywhere from 50 ppm to over 1000 ppm ^[2]. RO units waste water during production — older systems discharge 3–4 gallons of brine for every 1 gallon of permeate, though newer units improve on that ratio ^[1].

Why growers use it

There are three legitimate reasons:

1. High starting TDS. If your tap is 300+ ppm before you add nutrients, you have less 'room' to dose fertilizer before hitting EC ceilings that stress roots. Starting near 0 ppm gives full control over the nutrient profile. Strong evidence
2. Problem ions. Sodium, chloride, and bicarbonates in some tap and well water can accumulate in the root zone, lock out nutrients, or push pH up persistently ^[3]. RO removes them.
3. Chloramine. Unlike chlorine, chloramine doesn't off-gas if you let water sit out. Carbon prefilters on RO systems (or standalone catalytic carbon) are the practical fix ^[4].

Reasons that are not good on their own: 'cleaner is better,' 'pros use it,' or 'it makes terpenes pop.' There's no controlled evidence that RO water improves flavor or potency versus clean, properly amended tap water. No data

When to start (and when not to)

Test your tap first. Get a cheap TDS meter and, ideally, a water quality report from your municipality. Look at:

• TDS / EC
• Sodium and chloride
• Calcium and magnesium (Ca:Mg ratio around 2–4:1 is convenient)
• Bicarbonate / alkalinity
• Chloramine vs. chlorine

Consider RO if:

• TDS over ~250 ppm
• Sodium over ~50 ppm
• Persistent pH drift upward from high alkalinity
• Well water with iron, sulfur, or unknowns

Skip RO if your tap is under 200 ppm with reasonable mineral balance — just dechlorinate (carbon filter or 24-hour sit for chlorine; carbon block for chloramine) and dose normally. Don't switch mid-flower; the EC and ion shift can cause deficiencies within days.

How to do it: step-by-step

1. Size the system to your grow. A 4-plant tent might use 5–15 gallons a week in late flower. A 50 GPD (gallons per day) unit is plenty; a 75–100 GPD unit if you're filling reservoirs.

2. Install with pre-filters. Standard setup is sediment → carbon block → RO membrane. Add a second carbon stage if your water has chloramine ^[4]. Replace sediment/carbon every 6–12 months and the membrane every 2–3 years, sooner if input water is hard.

3. Collect into a clean reservoir. Food-grade plastic or a dedicated tote. Cover it to keep light out (prevents algae).

4. Re-mineralize before nutrients. This is the step people skip. RO water has essentially no calcium or magnesium, and most base nutrient lines assume some background Ca/Mg. Options:

• Use a base nutrient line designed for RO/soft water (it'll include adequate Ca/Mg).
• Add Cal-Mag supplement (typically 0.5–1 mL/L) before other nutrients. Strong evidence
• For organic/soil grows, this matters less because the soil itself provides minerals.

5. Mix nutrients in the standard order. Cal-Mag first, then Part A, then Part B (or per your manufacturer), then pH adjust last. Target EC and pH per your medium (typically pH 5.8–6.2 hydro, 6.2–6.8 soil).

6. Check final TDS. A fully mixed feed in veg might be 800–1400 ppm (500 scale); flower 1200–1800 ppm. These are ranges, not rules — read your plants.

Common mistakes

• Feeding plain RO water. Pure RO at 0 ppm is mildly aggressive on roots and provides nothing. Always re-mineralize, even for 'plain water' days — a pinch of Cal-Mag is enough.
• Forgetting Ca/Mg. Classic RO deficiency: interveinal yellowing on new growth (Mg) or rusty spots and tip burn (Ca). Fix by adding Cal-Mag, not by panicking and changing nutrient brands.
• Ignoring the waste stream. Old RO units waste 3–4 gallons per gallon produced. Capture the brine for lawns or non-edible plants if you care about water use.
• Letting the membrane go too long. Output TDS creeping up means the membrane is failing. Check with a TDS meter monthly.
• Switching mid-grow. Going from tap to RO (or vice versa) in flower changes the ionic background and often causes a deficiency cascade. Pick one and stick with it for the cycle.
• Assuming RO = better yields. It doesn't, on its own. It only helps if your tap was the bottleneck. Disputed

Related techniques

• Dechlorination only: Cheaper alternative if your tap is otherwise fine. Activated carbon inline filters or letting water sit (chlorine only, not chloramine).
• Rainwater collection: Naturally low TDS, but variable purity depending on roof and air quality. Check local regulations.
• DI (deionization) polishing: A post-RO stage that strips remaining ions to near zero. Useful for hydroponics where precision matters; overkill for soil.
• Cal-Mag supplementation: Essential companion to RO use.
• EC and PPM basics: Understand what you're actually measuring before you commit to an RO setup.

Sources

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