How to Grow Cannabis with RO Water?

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Added 22 April 2022

Life exists on Earth primarily due to water — it is the source of life. So, no doubt, your cannabis plant also requires water. 

And cannabis is a thirsty plant. It requires a lot of water for everything from germination to growing big buds, photosynthesis, metabolism, etc.

But not all water is the same. For instance, most plants can't grow in stagnant water and saltwater. 

However, cannabis has one more requirement — it cannot grow healthy in water that's too hard or soft. 

In a quest to get the best yields and keep their plants happy, growers have started experimenting and finding the best water for their cannabis plants in recent years. One of the solutions they have discovered is RO water.

But, how does it work? Is it worth it? And, most importantly, how you can use it to grow cannabis? Too many questions...

In this article, we will cover all these questions and more. 

What is Reverse Osmosis Water?

Reverse osmosis

Reverse osmosis, or RO water, is a near-pure form of water, achieved by using RO purifiers.

These RO purifiers push water through a high-pressure filtration process to remove all the contaminants like dissolved salts, microbes, and other impurities. 

Generally, most RO systems contain three layers: a polyester web, a polyamide barrier, and a microporous polysulfone layer, but this can vary from manufacturer to manufacturer.

When the water passes through the semi-permeable membrane filters, the contaminants and impurities get trapped in the filters.

In this process, high pressure is used to force water through filters. The more the salt concentration in the water, the higher the pressure and vice versa. As a result, the water that passes through the filters is deionized or called demineralized water. On the other hand, water that couldn't make it through the membrane is considered waste. 

This process can remove up to 95% to 99% of contaminants from regular water, and the filters within the system can filter contaminants down to 0.0009 microns.

It is the purest form of water you can get your hands on easily because RO water is devoid of any contaminants that may affect your plant's growth.

On the other hand, tap water contains many minerals, dissolved salts, and other impurities. Plus, the PPM levels of the water differ from region to region, which can make the water too hard or soft for the cannabis plant.

However, that does not mean you can simply spray RO water on your plant. Unfortunately, the RO filter removes all the essential micronutrients, like calcium and magnesium, along with the impurities. Therefore, you need to fine-tune the water later to rebalance vital micronutrients before feeding it to your plant.

All growers, including commercial and home growers, can use RO water. In fact, many people have an RO purifier at home, especially if they are situated in a region with hard water.

Why Should I Use RO Water to Grow Cannabis?

Advantages of RO

Since RO water is free of almost all impurities, it offers you and your plant many benefits, such as the following.

Makes the Water Suitable for Your Plant

In many regions, tap water is not suitable for the plants since it can contain high levels of impurities and microbes or be too hard for the plant to absorb properly. In addition, hard water can lead to calcium and magnesium buildup in the root zone, preventing the roots from absorbing other nutrients.

In such cases, using RO water can make the water suitable for the plant, ensuring proper growth and nutrient absorption.

Allows You to Dial in the Nutrients

Similarly, tap water does not contain a consistent set of nutrients. For example, the water may have too much calcium in some regions, whereas it may not contain enough nutrients in others. Plus, tap water often contains chlorine and fluoride, which are not good for your plant.

RO waters can give you a fresh start with near-pure water in such cases. Once you get RO water with no contaminants and minerals, you can add your own minerals and fine-tune them to match your plant's requirements. This can significantly boost your plant's growth and health.

Can be Used by Home Growers

Home growers, who grow cannabis for fun, often have to compromise their setup due to financial or space restraints. However, using RO water allows them to circumvent this problem concerning water.

You can buy RO systems for cheap, and even a novice grower can use them to grow cannabis in regions where tap water is too hard or impure.

Plus, you can easily integrate RO systems into your home's plumbing system. Needless to say, RO is great for humans too. 

Requires Light Maintenance

An RO system is a simple mechanism, so it does not require much maintenance. Most RO systems only require servicing once every 3 to 5 years, depending on usage and tap water quality. And servicing the systems only requires the filters to be cleaned or replaced from time to time, nothing else.

Does RO Water Have Any Disadvantages?

Disadvantages of RO

Yes, RO water does have some downsides, such as the following.

Some Systems Require Pretreatment

RO water purifier systems come with two types of membranes — cellulose triacetate membrane and thin-film composite (TFC).

Here, the cellulose triacetate membrane has a lower rejection rate than TFC, but TFC is prone to damage from chlorine, which can cause premature failure of the system.

Choosing between the two is balancing between rejection rate and maintenance costs.

However, you can overcome the chlorine problem while using TFC by adding a pretreatment solution to the water to remove chlorine, which can add cost and labor to the entire process.

Wastes a Lot of Water

Traditional RO systems use a lot of water — typically, 3 to 4 liters of water is used to produce 1 liter of RO water. 

This is a significant waste of water, especially if you live in a water-restricted region, considering how much water cannabis requires to grow properly. You can still reuse wastewater, though, but it doesn't help you save all the water. 

Plus, since they waste so much water, your utility bills may also rise significantly.

Some manufacturers produce more efficient RO systems, but they are generally more expensive to procure since it's a relatively new technology.

Can Damage Your Plumbing

Since RO water does not contain any minerals, it can be very corrosive to metal. So, you must never use RO water through copper or galvanized pipes. Also, other components like misters, tubings, pipes, etc., must be rated to withstand RO water.

So, What's Wrong With Tap water?

Disadvantages of Tap water

There is nothing wrong with tap water — most growers use tap water to grow their plants. However, if you want to take your cultivation to the next level, RO water is the right way to grow. 

Yet, tap water can sometimes be problematic for the following reasons.

Tap Water May Have Too High PPM

PPM means particles per molecule, so if the tap water has too high PPM, it can lead to a nutrient lockout. Ideally, water must have high calcium and magnesium PPM, but that's not always the case. So, you can use RO water to bring down the PPM and fine-tune it as per your plant's requirements. 

For a cannabis plant, the best water PPM should be between 100 to 200 ppm, with high calcium and magnesium content. 

We recommend investing in a PPM meter to check your tap water. If it has too high PPM, you must use an RO filter to bring it down.

It Can Cause Nutrient Lockout

Additionally, tap water may contain large particles of calcium and magnesium, like calcium carbonate and magnesium carbonate, which are too large for the roots to absorb. As a result, they can cluster around the roots, causing nutrient lockout. 

Instead, you can purify water with an RO filter and add calcium and magnesium that is easier for your plant to absorb.

It May Contain Unwanted Microbes

Even if it is fit for human consumption, tap water may not be good for your plant. It can contain various microbes that can damage your plant's health, leading to root rot, nutrient lockout, etc.

Remember, these problems only occur for some growers. If you live in a region with hard water (high PPM), unclean water, or experience water-based plant problems, consider using RO water.

Buying or Building Your RO System: Which is Better?

RO water purifier

RO water purifiers come in many forms, but you only need to focus on small-scale ones. For example, you don't need an industrial or commercial purifier for growing cannabis unless you are growing hundreds of plants.

Instead, you can either buy a RO system online or make one yourself. Let's take a look at them.

Purchasing an RO System

RO systems are commonplace in the modern world — you can buy them at your local electronics store or order them online. 

The prices vary drastically with prebuild RO systems depending on the output and filters used. While basic RO systems are affordable, they are still not worth the investment if you only want to experiment with one or two plants.

Here are a few factors you must consider when purchasing an RO system.

Water Output

RO filters come in all shapes and sizes, so you need to figure out the size of your system based on your cannabis cultivation. If you have a few plants, you can choose residential RO systems that can produce 15 to 50 gallons per day (GPD), which cost around $200 to $600, with a decent rejection rate.

If you need more water, you can choose RO filters with pressure pumps that can pump out 75 to 100 GPD with excellent rejection rates. These systems cost around $800 to $1000.

Avoid commercial RO systems as they are too expensive and produce a lot of water that may require costly massive water tanks. Also, read reviews on systems' water wastage — choose one with minimal water waste.

Filtration Stages

Most RO purifiers come with three stages of filtration, which is good enough. But if you manage to get your hands on a system with 4 to 5 filtration layers, choose that. They are much better at filtering water.


We recommend choosing RO filters with thin-film composite (TFC) since they have a higher rejection rate than cellulose triacetate membranes. However, it is prone to damage from chlorine. 

So, if your local water supply has too much chlorine, you must invest in a pretreatment system to remove the chlorine before it is fed to the RO system. Otherwise, you may have to shell out more money on maintenance to protect the TFC layer from premature failure.

DIY RO System

If you don't want to invest in an expensive RO system, you can simply build your own using products that you can buy from your local hardware store. Plus, it is easy to create and a fun science experiment!

To make your own RO system, you need a few major components like a pre-filter, reverse osmosis module with RO membranes, an activated carbon filter, a storage tank, and corresponding pipes and valves. 

BONUS: Buying RO Water

If you don't want the hassle of a DIY filter and the cost of a prebuilt filter, you can simply buy RO-purified water! Many cities' local water stations offer RO-purified water for a very cheap price. All you need is a refillable jug.

The only downside of this method is that you may have to drive far, carry the heavy jug, and you may also run out of RO water.

How to Use RO Water to Grow Cannabis?

Using RO water to grow cannabis is not so different than using regular water — the only difference is that you must treat RO water before feeding it to your plant. 

RO water is devoid of minerals and micronutrients essential for your plant's health and growth. So, you need to treat the RO water to make it suitable for your plant. Here are things you must do to prepare your near-pure water.

pH Balance

Check pH

RO filters bring down the pH level of tap water, which is great if the tap water has high pH. 

Invest in a pH meter to measure your RO water's pH, and balance it accordingly. Ideally, cannabis grows best with a pH of 5.5 to 6.5 in hydroponic systems and 6.0 to 7.0 in soil.

If the pH is too high or low, you can use pH balancing kits to fix it quickly. These kits are affordable and readily available at most local gardening supply stores. 

CalMag Supplements

Add Cal-Mag

Water is the primary source of calcium and magnesium supplements for your cannabis plant, but RO water does not contain these compounds. So, you need to supplement calcium and magnesium in the water.

If you don't feed your plant adequate calcium and magnesium, your plant can suffer from malformation, brown foliage, or stunted growth. A lack of them can also affect photosynthesis. 

RO water would have a negligible PPM — too soft water — so you must add calcium and magnesium until the PPM of water is between 100 to 150 ppm.

As a general rule of thumb, adding a milliliter of water to a liter of water should be enough. But every CalMag supplement is different, so remember to read the manufacturer's instructions before adding it to your water.

Other Nutrients

Add other nutrients

Once you have balanced the RO water's pH and calcium-magnesium PPM, you must add other nutrients and supplements to the water. Here, you can prepare a nutrient solution like you would with tap water. 

Mix nutrients in the water as per your plant's requirements and tune it over time to ensure optimal growth and health of the plant.

Some growers also recommend preparing a mix of 80% RO water with 20% tap water. Yes, this may seem counterproductive at first, but it can deliver all the necessary micronutrients of tap water while maintaining RO water's purity. 

If you are using this technique, start slow and measure your tap water's PPM. If your tap water is too hard, consider using less tap water.

Watering Frequency

Watering frequency

Since RO water is pure and you have mixed specific nutrients, you need to rethink your watering routine.

As a general rule of thumb, you must water your plant every 2 to 3 days — only after the growing medium is dry. Doing so would give your plant enough water to absorb while keeping the root zone aerated enough for oxygenation.

The watering frequency depends on what kind of growing medium you're using. For example, a Rockwool-based grow medium has higher water retaining capacity, so you need to water the plant less frequently.

Your plant has different water requirements during each growth stage, so you must understand the requirements accordingly.

Your plant would not need a lot of water during its seedling stage since seedlings absorb most of their water from the air, so you must maintain a relative humidity of 65% to 70% during this stage.

And during the vegetative stage, your plant's water and nutrient requirements will grow. Even the roots will occupy most of the grow medium container. And during the flowering phase, your plant will grow twice its size and require even more water.

You can also alternate between using tap water and RO water for watering your plant to avoid overfeeding your plant and nutrient lockout — although it is hard to overfeed calcium and magnesium to your plant.

Can I Use Distilled Water Instead?

Distilled water, like RO water, is a near-pure form of water. In a distillation process, regular water is first boiled and vaporized. Later, the vapors are condensed and collected into a container.

Like RO filtration, distillation can also remove almost all contaminants and minerals from water. So, both methods can help you feed your plant near-pure water without any impurities, microbes, or unnecessary minerals.

But some differences set RO water apart from distilled water, helping you make your decision easier.

While distillation is cheaper — you can buy distilled water even at a car workshop — but if you want a lot of distilled water, you will have to install a distillation system, which can be expensive.

Distillation systems are also much larger and more complex, requiring higher maintenance. And perhaps the biggest downside of distillation systems is that they cannot be installed or connected to your home's existing water lines, unlike RO systems that simply connect to your water outlet.

There are other water filtration systems, too, like ion exchange, activated carbon, and activated alumina. However, we don't recommend using these as they leave the dissolved salts and minerals in the water. So, even if water from these systems is safe for human consumption, it can still be too hard for your cannabis plant.

Summary: How to Grow Cannabis with RO Water?

Growing cannabis with RO water is a terrific idea, and you can enjoy RO water even for yourself. 

Just remember these tips when using RO water for your cannabis cultivation:

  • Store RO water in a cool, dark place away from direct light to avoid fungus or algae growth.
  • Choose the right RO filtration system that works for you, whether it is a DIY filtration system, prebuilt system, or RO water purchased from your local water station.
  • Whichever system you choose, ensure it has at least three filtration levels with higher rejection rates.
  • And maintain the system regularly — if the system fails to filter the contaminants, you can end up feeding your plant hard water or overfeeding it with nutrients.
  • Ensure you have a pretreatment ready to remove chlorine from the water if you are using a filtration system that uses TFC.
  • Ensure you have checked and pH balanced the RO water.
  • Add CalMag supplements and other essential nutrients to the RO water as per your plant's requirements.
  • Water the plant only when the growing medium is dry.

You can even engage in your local grower forums to know your local water supply, especially its PPM, hardness, and local water wastage laws.

That's it. Follow these techniques, and you will successfully give your plant pure water that is free of any contaminants and microbes that would otherwise harm it. 



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