Your Guide to Growing Cannabis with RO Water

Imagine trying to cook the perfect meal, but every time you reach for ingredients, they’re already mixed with random spices. Some might make the dish better, while others completely throw off the flavor. Frustrating as hell, right?

That’s exactly what happens when you water your cannabis plants with unfiltered tap water. You never really know what’s in it, and sometimes, those extra “ingredients” do more harm than good.

That’s where reverse osmosis (RO) water comes in. Think of it as the purest, blank-slate water you can get — free from unwanted minerals, chemicals, and contaminants. By filtering out nearly everything except H2O, it gives you complete control over your plant’s hydration and nutrient intake.

But before you ditch tap water entirely, there’s a catch. RO water is so pure that it lacks the essential minerals your cannabis needs to thrive. That means you’ll have to tweak it before use. If done right, though, it can lead to stronger, healthier plants with optimal nutrient absorption.

So, is reverse osmosis water the secret to better cannabis growth, or is it more trouble than it’s worth? Learn all about RO water and growing cannabis in this guide below. 

What Even is RO Water?

RO or reverse osmosis water is one of the purest forms of water you can get your hands on — thanks to a filtration process that uses high pressure to remove almost all contaminants.

RO purifiers work by forcing water through specialized filters that strip away microbes, dissolved salts, and other unwanted particles. Most reverse osmosis systems have 3 filtration layers: a web of polyester, a polyamide filter, and a layer of microporous polysulfane. However, the exact setup can vary depending on the manufacturer.

The magic happens when water is pushed through a membrane that is semi-permeable. This traps impurities while allowing only purified water to pass through. As the concentration of salt in your water increases, a greater amount of pressure is necessary for the filtration process. 

This filtration process can eliminate up to 98% - 99% of the stuff in the water, filtering particles as tiny as 0.0009 µm. That’s about as pure as water can get without being straight-up distilled. Because it’s stripped of impurities, RO water is an excellent option for cannabis cultivation, ensuring nothing unwanted messes with your plant’s growth.

But before you start showering your cannabis with RO water, there’s a catch. The filtration process doesn’t just remove harmful contaminants — it also wipes out essential nutrients like magnesium and calcium, which your plants actually need. That means you’ll have to adjust and remineralize the water before using it.

The good news? RO water isn’t just for large-scale growers. Many home growers use it too, especially in areas where tap water is less than ideal. In fact, if you already possess an RO filter at home for drinking water, you’re halfway there.

Benefits of Using RO Water to Grow Cannabis

Since reverse osmosis water is devoid of nearly every impurity, it brings plenty of advantages for both the grower and the plants.

• Makes Water Perfect for Your Plants

Not all regular water from the tap is plant-friendly. In some regions, it’s packed with impurities, microbes, or excessive minerals that make it too harsh for cannabis. Hard water, in particular, can cause magnesium and calcium deposits within the root area, blocking the absorption of other crucial nutrients.

With RO water, you eliminate these issues and give your plants a clean slate, ensuring they can take in nutrients without any interference.

• Gives You Full Control Over Nutrients

One of the biggest issues with tap water is its inconsistency. In some areas, it’s overloaded with calcium, while in others, it barely contains any essential minerals. In addition, it often contains fluoride and chlorine — chemicals that aren’t exactly plant-friendly.

RO water gives you a new start. Since it’s nearly pure, you can customize the nutrient profile to match exactly what your plants need. This level of control can lead to healthier growth and better overall yields.

• Ideal for Home Growers

If you’re growing cannabis at home, space and budget constraints can make things tricky. But when it comes to water, RO systems offer a practical solution.

These purifiers are relatively affordable, easy to set up, and can help even beginner growers cultivate high-quality plants, especially in areas where regular water is very full of contaminants or too hard. Plus, they’re not just good for plants — RO water is excellent for drinking, too!

Drawbacks of Using RO Water to Grow Cannabis

While RO water has plenty of perks, it also has downsides. Before committing to an RO system, here are a few things you should keep in mind.

• Some Systems Need Pretreatment

RO purifiers use one of two membrane types: cellulose triacetate (CTA) or thin film composite. CTA membranes have a reduced rejection rate, meaning they don’t filter out as many contaminants, while TFC membranes are more efficient but highly sensitive to chlorine. If chlorine isn’t removed beforehand, it can damage the membrane and shorten the system’s lifespan.

So, if you opt for a TFC membrane, you’ll need to use a solution to eliminate chlorine. While this keeps your system in top shape, it also adds to the cost and maintenance effort.

• RO Systems Waste Water

One of the biggest drawbacks of RO systems is water waste. On average, a traditional RO system uses a few liters just to produce a liter of purified water. That’s a lot of wasted water — more so if you are in a drought-prone area or somewhere with water restrictions.

Sure, you can reuse the wastewater for non-plant-related purposes, but you still won’t be able to recover all of it. Plus, if you’re running an RO system frequently, expect your water bill to climb.

There are more efficient reverse osmosis systems on the market, but they tend to be expensive since the technology is still evolving.

• Can Corrode Your Plumbing

Here’s something you might not expect — RO water can actually be too pure. Because it lacks minerals, it becomes corrosive when it comes into contact with certain metals. That means if your plumbing has galvanized or copper pipes, running RO water through them can lead to damage over time.

To avoid this, you’ll need to use RO-compatible materials like PEX or stainless steel for your pipes, tubing, and misters. This is an extra precaution, but it’s important if you don’t want your system to cause more harm than good.

Despite these drawbacks, RO water is still a popular choice for growers who want full control over their plants’ hydration. It just comes down to weighing the pros and cons and deciding if the benefits outweigh the challenges for your setup.

How is Tap Water Different From RO Water for Cannabis?

There’s nothing inherently wrong with regular tap water — plenty of growers use it without any issues. But if you’re looking to maximize your plant’s potential, switching to RO water can make a noticeable difference.

That said, tap water does come with some potential problems that might interfere with your cannabis plant’s growth.

• High PPM in Tap Water

Parts per million refers to the number of dissolved particles in water. If the PPM of your tap water is too high, it can cause a nutrient lockout, preventing your plant from absorbing essential minerals.

Cannabis thrives when the water’s PPM ranges between 100 and 200, with plenty of magnesium and calcium. Unfortunately, tap water doesn’t always meet these requirements — it might have excessive minerals or, in some cases, too little.

The best way to check is by using a PPM checking pen. If your regular tap water has a high PPM, an RO system can help tone it down, giving you better control over your plant’s nutrient intake.

• Risk of Nutrient Lockouts 

Tap water often contains large magnesium and calcium particles, such as magnesium carbonate and calcium carbonate. While these minerals are essential, their size can make them difficult for plant roots to take in. Instead of nourishing the plant, these particles can accumulate near the roots, blocking nutrient uptake and leading to deficiencies.

By filtering your water through an RO system, you strip away these oversized minerals and can later reintroduce magnesium and calcium in a form that’s more efficient for your plants to take in.

• Unwanted Microbes in the Tap Water

Just because tap water is safe for drinking doesn’t mean it’s ideal for plants. Municipal water supplies can carry microbes, bacteria, or other contaminants that may not harm you but could negatively impact your plants. In some cases, these microbes can cause moldy roots, nutrient lockout, or other health issues.

Of course, not every grower will face these problems. If your local water supply is relatively clean and balanced, you may not need an RO system. But if you’re dealing with contaminated or hard water with high PPM, poor water quality, or unexplained plant issues, switching to RO water might be the best move.

What About Distilled Water?

Distilled water, like RO water, is about as pure as it gets. The process involves boiling regular water until it vaporizes, then condensing the steam back into liquid form, leaving behind almost all contaminants and minerals.

On the surface, both RO and distilled water achieve the same goal — providing your plants with ultra-clean water. But there are some key differences that might make one a better fit for your setup.

For starters, buying small quantities of distilled water is cheap — you can even find it at an auto shop. But when you need a steady supply, setting up a distillation system can be pricey. 

These systems are also bulky, require more maintenance, and can’t be hooked up to your home’s plumbing like an RO system. That means getting large amounts of distilled water requires extra effort.

Other filtration methods, like ion exchange, activated carbon, or activated alumina, exist too. However, they don’t fully remove dissolved salts and minerals. While the resulting water may be fine for drinking, it can still be too hard for cannabis plants. 

So, if you're serious about giving your plants the cleanest possible water, RO or distilled water remains the best choice.

How to Cultivate Cannabis with Reverse Osmosis Water?

The reverse osmosis (RO) water method for growing cannabis isn't complicated, but it does require some extra steps. Unlike regular tap water, reverse osmosis water is stripped of all minerals, which means you’ll need to reintroduce essential nutrients before giving it to your plants.

Here’s how to prep your purified water for the best possible growth.

• Getting the pH Right

One of the biggest benefits of RO water is that it naturally lowers the pH of your tap water, and this can be a good thing — especially if your tap water is too alkaline.

But here’s the catch: cannabis thrives within a specific pH range. For hydroponic setups, you’ll want to keep it between 5.5 and 6.5, while soil-grown plants prefer something closer, between 6.0 and 7.0.

To stay on top of things, invest in a pH meter and check the water before every feeding. If your pH is way too low or too high, use a pH up/down solution to adjust it. These pH adjustment kits are easy to find at most garden stores and make fine-tuning your water a breeze.

• Don’t Skip the Cal-Mag

Tap water naturally contains magnesium and calcium — two nutrients that cannabis absolutely needs for strong cell walls, proper photosynthesis, and overall plant health. Unfortunately, RO water removes these minerals completely, which means you’ll have to add them back in.

Without enough CalMag, your plants can develop stunted growth, brown leaves, or even twisted, malformed foliage.

A simple fix? Cal-Mag supplements. Most growers aim for a PPM (parts per million) between 100 and 150 before adding any other nutrients. A trick here is 1 ml of Cal-Mag per liter of your water, but you should always follow the dosage instructions on the bottle to avoid overloading your plants.

• Adding the Right Nutrients

Once you’ve dialed in the pH and CalMag levels, it’s time to mix in the rest of the nutrients. This process is the same as if you were using tap water — just make sure to use a well-balanced nutrient formula that provides all the macronutrients (nitrogen, phosphorus, and potassium) along with essential micronutrients.

Some growers swear by mixing 80% reverse osmosis water and 20% regular tap water to reintroduce trace minerals while keeping the water relatively clean. If you go this route, start small and measure the PPM of your regular water first. If it’s too hard (high in dissolved minerals), you may need to adjust the ratio.

• Adjusting Your Watering Routine

Since reverse osmosis water is super clean and only contains the nutrients you add, you’ll need to reevaluate your usual watering habits.

A good general rule is to water your plants every 2 - 3 days, but only if the medium has gone dry. This helps prevent overwatering while ensuring the roots are oxygenated.

The type of growing media you use also affects how often you’ll need to water. For example, Rockwool holds onto moisture longer than other substrates, meaning you’ll have to water less often.

Your cannabis plant’s growth stage also plays a role:

• Seedling stage: Young plants don’t need much water since they absorb a lot of the moisture right from the surrounding air. Keep humidity levels between 65% and 70% for best results.
• Vegetative stage: As your plant grows, so does its thirst. By now, the roots should have spread out, and the plant will require more frequent watering.
• Flowering stage: Expect an even bigger jump in water needs. Your plant can double in size during this phase, meaning its demand for water and nutrients will be at an all-time high.

Some growers also switch between RO water and tap water every other watering to prevent nutrient lockout and avoid overfeeding. Since it’s tough to overdo CalMag, you can use this method to help balance things out without harming your plants.

Should You Buy an RO System or Build One?

Reverse osmosis (RO) purifiers come in all shapes and sizes, but for growing cannabis, you only need a small-scale system. Unless you're running a massive grow-op with hundreds of plants, there's no need to invest in a commercial-grade purifier.

Instead, you have two options — buy an RO system or build one yourself. Let’s break them down.

1. Buying an RO System

RO systems are everywhere — you can find them at local electronics stores or just order one online.

Prices vary based on filtration stages, output capacity, and extra features. Basic models are relatively affordable, but when you want to just experiment with one or two plants, even the cheapest RO system might not be worth it.

Here’s what you need to consider before buying one:

• Water Output

Different RO systems produce different amounts of purified water. For small-scale cannabis cultivation, residential RO filtering systems that generate 15 to 50 gallons per day (GPD) are ideal. These typically cost between $200 and $600.

If you need more water, consider an RO system with a pressure pump, which can output 75 to 100 GPD. These higher-capacity systems cost around $800 to $1,000 and offer better water purification.

Avoid commercial RO systems — they’re expensive, overkill for small grows, and require massive storage tanks. Also, look for models that minimize water wastage to keep your utility bills in check.

• Filtration Stages

Most RO systems have at least three filtration stages, which are decent for cannabis growing. But if your budget allows you to get a system with four or five filtration layers, go for it — they offer better purification and remove more contaminants.

• Pretreatment Considerations

RO filters with thin-film composite membranes are better than cellulose triacetate (CTA) membranes, as they reject more contaminants. However, TFC membranes are vulnerable to chlorine, which can degrade them over time.

If your local water supply has high chlorine levels, you’ll need a pretreatment filter to get rid of chlorine before it reaches the RO membrane. Otherwise, you’ll be replacing filters more often — an expense you can avoid with the right setup.

2. DIY RO System

Not keen on dropping a few hundred bucks on an RO system? Build one yourself! It’s cheaper, relatively easy to assemble, and a cool DIY project.

To make your own RO system, you’ll need:

• A pre-filter (to catch large particles)
• A reverse osmosis module with RO membranes
• An activated carbon filter (to remove residual contaminants)
• A storage tank
• Pipes and valves to connect everything

Most of these parts are available at your local hardware store or online. With a little effort, you can have a fully functional RO system for a fraction of the cost of a prebuilt one.

3. Bonus: Buying RO Water

If setting up an RO system (whether prebuilt or DIY) sounds like too much work, there’s an even easier option — just buy RO-purified water. Many local water stations offer RO water at an affordable price. All you need is a refillable jug, and you’re good to go.

The only downside? You’ll have to haul heavy water containers back and forth. Plus, if you run out, a last-minute refill trip might be inconvenient.

Still, for small-scale growers who don’t need large quantities of RO water daily, this is a low-cost, hassle-free alternative.

Cost Considerations of an RO System

Initial Investment vs. Long-Term Savings

Setting up a reverse osmosis water filtration system is an investment but could save a lot of money in the long run, especially for cannabis growers. Here’s a breakdown of the typical costs.

Basic R.O. systems: $100–$200 (for small home grows).

Mid-Range RO Systems: $200–$400 (better efficiency and filtration) 

Top Grade Systems: $400-$1000+ (for high output and low waste) 

In addition to the system itself, ongoing costs include:

• Filter and membrane replacements cost $50-$150 yearly.
• Water may cost more due to RO systems' wastewater increasing the water bill.
• If you use a booster pump or storage system, it doesn’t take much electricity but it may matter with a large grow.

Is the Investment Worth It?

For small-scale growers, tap water that’s filtered properly may suffice. But if your water quality isn't good (lots of chlorine, chloramine, and dissolved solids), you can use RO water to keep from nutrient lockout, salt buildup, and unstable pH.

If you’re a commercial grower or just want the best performance from your garden, then the initial cost of RO is worth it. It gives complete control over water purity so that nutrients can be controlled, resulting in healthier plants and bigger, more potent buds.

How to Reduce RO Water Waste

Wastewater generated by RO systems is a major drawback. For every gallon of purified water, a conventional Reverse Osmosis (RO) filter wastes 3-4 gallons of water. This could be a considerable amount, especially in places with pricey water or scanty water.

Choose a High-Efficiency RO System

• Some more modern models come with a waste ratio of 1:1 or 2:1, which leads to a waste reduction of 50-75% when compared to the older water filtration systems.
• Choose systems that have internal pressure pumps for high efficiency.

Recycle Wastewater

• Garden and landscaping - RO wastewater isn’t great for cannabis but is fine for watering outdoor plants, trees, or lawns.
• This can be used to mop the floors, wash the car, or even flush the toilet.
• Hydroponics Pre-Treatment: Some growers mix RO wastewater with tap water for non-sensitive plants.

Maintain Your RO System

• Using filters and membranes on time increases efficiency and reduces wastage.
• If the system is cleaned regularly, it will not clog.

Use a Booster Pump (if needed)

• Low water pressure raises waste generation in RO systems cleaning. Installing a booster pump can minimize water waste and enhance filtering speed if your home’s water pressure is less than 60 PSI.

Collect and Store RO Water

• Collect filtered RO water in a storage tank, so the system doesn’t work constantly.
• Automate refills with a float valve to avoid overuse.

So, as you can understand, serious growers may consider investing in a RO system as it is worth it. Sure, the installation and maintenance will cost money but you will reap more yields at the end of the day. When a plant gets all the essential nutrients and minerals, it results in a healthier and higher yield and you’ll be a happy camper with clean water to boot!

A Note on Maintenance and Filter Replacement

To ensure that your reverse osmosis (RO) system continues to run well and provide clean water to your cannabis plants, it must be maintained properly. If you maintain it regularly, it will also last longer, and your plants will get the same water without contaminants and with no fluctuations in pH levels.

Why Regular Maintenance Matters

Over time, filters and membranes get clogged with dirt, minerals, and other impurities. If you don't replace them regularly, your system will underperform, which leads to: 

• Lower water output
• Higher wastewater production.
• Poor water quality.
• Potential damage to the RO membrane.

Regular maintenance guarantees:

Stable Water Purity (0–10 PPM)

RO systems can eliminate nearly all the impurities from tap water including chlorine, heavy metals, and dissolved solids. Fresh filters and membranes consistently provide water with ultra-low PPM, helping growers begin mixing nutrients on a blank slate. If you don’t service your system regularly, TDS levels start increasing, and with that come contaminants that interfere with nutrient absorption and stunt growth.

Efficient Water-to-Waste Ratio

As time passes, blocked filters make the system work harder and create more wastewater for every gallon of clean. If you change the membrane and pre-filters on time, then the system will run at its designed efficiency ratio so that water waste and operating costs don’t increase. With clean filters, high-efficiency systems can achieve 1 to 1 or even 2 to 1 water recovery, which can make them sustainable for cannabis growing over the long haul.

Longer Lifespan of the RO System.

When you change the filters regularly, the RO membrane — which is the heart of the system — does not wear out quickly. If sediment (sand) or chlorine access is allowed to reach the membrane, it will get damaged. However, with a proper pre-filter, the membrane life can be extended to 3-5 years saving growers hundreds of dollars on replacements over time.

By sticking to a regular maintenance schedule, growers can help the system perform at its best, save on water bills, and ensure their plants always get pure and dependable water — the cornerstone of a successful cannabis grow.

Schedule to Replace Filters

Most RO systems use a multi-stage filtration process, each with different replacement timelines.

Filter Type	Purpose	Replacement Frequency	Price Range
Sediment Filter	Removes dirt, sand, and rust	Every 6–12 months	$10–$30
Carbon Block Filter	Removes chlorine, chloramine, and chemicals	Removes chlorine, chloramine, and chemicals	$20–$50
RO Membrane	Removes dissolved salts, heavy metals, and contaminants	Every 2–3 years	$50–$150
Post-Carbon Filter (Optional)	Polishes water for taste	Every 6–12 months	$20–$40

 

How to Replace Filters

1. Turn Off the System: Turn off the water supply and the valve of the storage tank.
2. Release the pressure by opening the RO faucet.
3. Take the old filters out. Unscrew the filter housing to safely extract them.
4. Rinse with soap and warm water to remove any debris from the housings.
5. Put New Filters in Place: Insert new filters in the correct order (dirt > carbon > membrane > post-filter).
6. Run the system for a couple of hours to remove any residue before using it.

Signs You Need to Replace Filters

Watch out for these signs that your filters need replacing:

• Reduced water flow.
• Higher TDS (Total Dissolved Solids) readings.
• Unusual taste or smell in the water.
• Cloudy or discolored water.
• The system is making more noise than usual.

Tips for Extending Filter Life

• If your tap water has high sediment levels, use a pre-filter.
• Install a pressure gauge to monitor performance.
• Use vinegar or a sanitizing solution to clean the filter housing.
• Don't forget the system for long periods as water becomes stagnant and attracts bacteria for growth. 

Cost of Filter Replacement

The annual cost of filter replacement is around $100–$200 depending on the system and water quality. A good system filter may cost more but can often last a long time and work better.

Troubleshooting Common Issues with RO Water Systems

Even if you install and maintain it properly, reverse osmosis (RO) systems can sometimes encounter problems that reduce water quality. However, if you diagnose and fix them quickly, your cannabis plants will continuously receive clean, pH-stable water. 

1. Low Water Pressure or Slow Water Flow

Symptoms:

• Water trickles out of the RO faucet.
• Storage tank takes longer than usual to fill.

Causes:

• Clogged sediment or carbon filters.
• Worn-out RO membrane.
• Low incoming water pressure (below 40 PSI).

Air pressure loss in the storage tank.

Solution:

• Replace clogged pre-filters (every 6–12 months).
• To ensure the system functions properly, check if the storage tank bladder has sufficient air pressure; it should not be lower than 7–8PSI.
• Put in a booster pump for low-pressure water pressure in the house.
• If your Reverse Osmosis system is 2 to 3 years old, change the RO membrane instantaneously.

2. High TDS (Total Dissolved Solids) in Water

Symptoms:

• PPM (Parts Per Million) readings are higher than expected.
• Nutrient lockout or pH fluctuations in plants.

Causes:

• Expired or damaged RO membrane.
• Clogged pre-filters are allowing contaminants through.
• Improper membrane installation.

Solutions:

• If TDS readings continue to be high after changing the filter, replace the RO membrane.
• Make sure the filters are put in properly. Membranes should fit into the housing properly.
• After you change the filters, let the RO run for 2-4 hours to wash away carbon dust and clear airlocks.

3. Leaking System

Symptoms:

• Water pooling around the system.
• Dripping fittings or filter housings.

Causes:

• Loose fittings or O-rings.
• Worn-out or cracked filter housings.
• Improper filter installation.

Solutions.

• Tighten fittings using Teflon tape on threaded connections.
• Change damaged O-rings and put on a thin film of food-safe silicone grease.
• Make sure filters are seated before closing housings.

4. Bad Taste or Odor in Water

Symptoms:

• Water tastes stale, metallic, or chemical-like.
• Unpleasant smell from the RO faucet.

Causes:

• Expired carbon filters.
• Bacteria buildup in the system.
• Stagnant water in the storage tank.

Solutions:

• Replace carbon filters every 6–12 months.
• Sanitize the entire RO system during filter replacements.
• Use the system regularly to prevent stagnant water buildup.

5. Excessive Wastewater Production

Symptoms:

• Higher water bills.
• RO system drains a large quantity of water compared to the filtered water output.

Causes:

• Clogged filters.
• Low incoming water pressure.
• Worn-out RO membrane.

Solutions:

• Replace filters on schedule.
• Install a booster pump to increase water pressure.
• Upgrade to an efficient RO system with a 1:1 or 2:1 ratio of waste to pure water.

Summary: Your Guide to Growing Cannabis with RO Water

RO water might not be a magic potion, but it sure gives your cannabis plants the VIP treatment. By stripping out contaminants and unwanted minerals, it offers a level of purity that tap water simply can’t match. 

With the right adjustments — like adding calcium and magnesium, and dialing in the pH — it can be a game-changer for you if you’re looking for precision and consistency.

Of course, it’s not without its downsides. Water waste, plumbing concerns, and the extra step of remineralization can make it less appealing for some. But when you want total control over the plant’s environment, the benefits often outweigh the drawbacks.

In the end, whether you go all-in on RO water or stick with a tap, the real key to thriving cultivation is balance. Your plants don’t need perfection — they just need the right care. And hey, at least they won’t complain about the water quality like a picky restaurant customer. So, go ahead and grow some amazing cannabis today!