What is the best water for marijuana plants?

Water nutrients cannabis homegrown
May 01, 2020

Plants need water, we all know that. But what kind of water do they need?

There is more than meets the eye when it comes to water, especially the water we give to our precious marijuana plants. Using the right kind of water is essential if you want the purest and cleanest buds.

Knowing what parts per million (ppm) is and why it matters is essential to understanding water quality. It’s a measurement of the total dissolved solids (TDS) – all the dissolved inorganic and organic substances – in the liquid. You can think of ppm as an indicator of the overall mineral content of the water.

These minerals, though, are considered impurities – especially at high concentrations. Hence, the higher the ppm value, the more the “contamination”. To keep cannabis plants healthy, you should only use water that has a sufficiently low ppm reading.

Common types of water for cannabis plants

Water can be categorized into two types – hard and soft. The concentration of calcium and magnesium, which is measured in ppm, reveals how “hard” the water is. Essentially, the higher the mineral content (or, the higher the ppm value), the harder the water.

When growing marijuana, your water should contain about 100 to 150 ppm of calcium. If it is “very soft” or about 50 ppm, you will have to treat the water with cal-mag supplements.

There are many scales measuring water quality, each with their respective ppm ranges for both softness and hardness. For some, the upper limit of soft water is 100 ppm, while for others, it can be a bit higher at 125 ppm. The table below will give you an idea of the values.

Hardness PPM
Very soft 0 to 70 ppm
Soft 70 to 125 ppm
Medium hard 125 to 200 ppm
Hard 200 to 300 ppm
Very hard 300 to 500 ppm
Extremely hard Over 500 ppm

Can I water cannabis with hard water?

Hard water has high amounts of calcium and magnesium. These minerals are captured as water flows over rocks and soil. Excess calcium deposits increase the alkalinity of water. When used to irrigate cannabis plants, this can interfere with nutrient absorption at the roots, resulting in nutrient deficiencies.

At about 200 to 300 ppm of calcium salts, water is considered hard. At 300 to 500 ppm, it is already very hard and it’s extremely hard once it goes beyond 500 ppm. Tap water generally sits at around 170 to 390 ppm – but it could be even less in some places. While this range is safe for humans, it can be dangerous to cannabis. You will have to correct or “soften” hard water before use, making it more suitable for the plants.

Can I water cannabis with soft water?

Soft water lies on the other end of the water hardness scale. It contains low concentrations of mineral ions – or sometimes none at all. Generally, it is considered soft at about 70 to 125 ppm and regarded as very soft at less than 70 ppm. Unlike hard water, it has a more neutral to acidic pH. It also tends to have higher salt content – but not enough to be harmful to humans or plants.

Rain is a fantastic example of soft water – and it’s also one of the best types of water for marijuana plants. Note, however, that it often becomes charged with minerals upon hitting the ground and after moving through soil. Other excellent examples include distilled, ionized and purified water.

How to source clean water for your marijuana garden

After knowing the water requirements of cannabis plants, it’s time to look into possible water sources. For many, tap water is the only option. Some, though, might own water collection systems. In either case, you will most likely have to use filtration systems to make the water usable. Alternatively, you can simply buy distilled bottled water although it can be costly in the long term (to both the environment AND your wallet).

1. Can I water cannabis with unfiltered tap water?

You might have heard that using unfiltered tap water isn’t advised – and a surefire way of killing cannabis plants. But the truth is, it’s not so bad. Tap water is cheap and widely available. If adjusted properly before use, it can be one of the most reliable water sources for marijuana growers.

Keep in mind, though, that the quality of tap water varies per area and depends on the water-treatment protocols employed by the distribution company. Some municipalities have extremely hard water packed with calcium, magnesium, and chlorine. This can harm or even kill the plants. In other places, tap water might have fewer contaminants. While the concentration is usually too low to be lethal, it still runs the risk of hindering the biological activity in the soil.

The bottom line – do not use tap water as it is. Make sure you test it first to identify the mineral content. Doing so will help you plan the best approach to filter out the impurities. For instance, letting the water sit for 24 to 48 hours is enough time for the chlorine to evaporate. If the water is treated with chloramine, it can also be removed using activated charcoal, and fluoride via an activated alumina filter.

2. Can I water cannabis with distilled water?

Distilled water is a type of purified water that has been stripped clean of both contaminants and minerals, making it a great source of pure, uncontaminated water. You can buy it in many grocery stores, convenience stores or supermarkets. You could also have it delivered.

Aside from availability, another major appeal of distilled water is that it does not need any extra filtration. In low quantities, it won’t burn a hole in your pocket, making it ideal for watering a few plants.

The downside, though, is that the costs will accumulate over time. It can be expensive in the long run, especially for large-scale growers. Distilled bottled water isn’t sustainable OR environmentally-friendly. The production and transport alone uses substantial resources, such as fuel. The plastic bottles create unnecessary waste, usually ending up in landfills or the ocean.

3. Can I water cannabis with rainwater?

Rainwater collection systems – also called rain catchers – are a sustainable and eco-friendly way of sourcing water. You can buy them, or, even better, you can make them yourself.

The drawback is that using and maintaining water collection systems can be quite laborious. And since rainwater is not as clean as it once was, due to pollution, you’ll still have to test and adjust it before use. That said, rainwater IS your most reliable water source, provided it rains where you live!

4. Can I water cannabis with a water filtration system?

If you have the budget, you should consider investing in a water filtration system (especially if you are running a large-scale operation). These systems significantly reduce the mineral concentrations in the water, making it safe for the plants. The best thing about filtration systems is that they allow you to have an unlimited supply of pure, clean water in your garden. There are several types available and the price can range from hundreds to thousands of dollars. Needless to say, the initial startup cost is enough to scare away most beginners and small-time growers.

Here are some of the most common water filtration systems used in cultivation.

Activated Alumina Filtration. This filtration system is made of aluminum oxide – a highly porous material. Activated alumina filters are specifically designed to remove up to 99% of fluoride, making it ideal for fluoridated tap water. It can also get rid of arsenic, selenium and thallium. Due to its limited contaminant removal capacity, however, you will have to use it alongside other water filters.

Activated Carbon Filtration. As the name suggests, this system uses activated carbon to separate the impurities from the water. It comes in two types – activated carbon block (ACB) filtration and granular activated carbon (GAC) filtration. ACB uses a solid chunk of carbon, whereas GAC uses powdered carbon. Both can effectively remove chlorine, sediments and volatile organic compounds (VOCs) – but not minerals, dissolved solids or salts.

Distillation. This water purification process involves boiling impure water to let it evaporate, capturing the steam, then condensing it into liquid form. The result is a powerful filtration system that can remove most of the contaminants found in water, such as dissolved fluids and excess minerals. Unfortunately, a distillation apparatus is not only pricey but also high-maintenance, expensive to operate and time-consuming. It’s not ideal for the average home grower.

Ion Exchange. This filtration system entails replacing unwanted dissolved ions in the contaminated water with other ions of the same electrical charge. The ion exchange process can remove dissolved inorganic contaminants (including calcium, magnesium, fluoride, arsenic, nitrates and sulfates), but is weak against sediments, organic pollutants and microorganisms. While cheaper than other water treatment systems, it can be costly to operate.

Reverse Osmosis (RO) Filtration. Of all the filtration systems, an RO system is the most widely used among cannabis growers. It uses a semipermeable membrane to filter out around 99% of contaminants, including heavy metals and minerals, but not chlorine and some pesticides and solvents. To get rid of chlorine, some use an activated carbon filter as a pre-filter. The RO system also tends to produce a lot of wastewater, but installing a permeate pump helps avoid this problem.

How to Measure the PPM of Your Water

Do not skip this step, especially if you are planning on using unfiltered tap water. By knowing the water hardness, it will be easier for you to balance or correct as needed.

If you want to know the specific substances in your tap water, though, you will have to visit your local government unit and ask for the water quality records. Most municipalities use chlorine, chloramines and/or fluoride to treat tap water. As mentioned, chloride will naturally evaporate, whereas chloramines and fluoride call for a more aggressive purification.


Measuring the ppm is simple. All you need is a glass containing the water you want to test and a good-quality TDS meter, which only costs around $25. You can’t go wrong with the AP-1: AquaPro Water Quality Tester from HMG Digital – cheap, easy to calibrate and accurate. If you’re using it for the first time, consider calibrating it with a 1000 ppm calibration solution. You can buy one online for about $10.

Refer to the instructions manual on how to calibrate the TDS meter. After calibrating, remove the cap from the end and turn it on by pressing the ON/OFF button. Wait until the reading is zero. Carefully dip the electrode into the water. Do not plunge it too deeply – half an inch will do. If the device has a hold button, be sure to press it.

Wait until the ppm value is displayed on the screen. Take note of it. Turn off the TDS meter and set aside.

Remember, the optimal range for cannabis plants is 100 to 150 ppm. If the ppm reading of your tap water is higher than that, you need it to run it through a filtration system. Measure it again afterward. If the water ends up too soft, use cal-mag supplements to bring the ppm to the ideal range.

Do not forget about the pH level. Grab a digital pH meter to test for the pH value, then adjust as needed using pH up or down solutions. Soil-based grows require a pH that sits between 6.0 to 6.8 (5.5 to 6.5 for soilless systems).

High-Quality Water for Robust Cannabis Plants

The importance of water quality cannot be stressed enough. Without good-quality water as a starting point – be it for watering the crops or for hydroponic grows – the cannabis plants will not reach their full potential.

Aside from the ppm, be sure to keep the pH level within the optimal range at all times. If you do, you’re one step closer to healthy cannabis plants and big, bountiful harvests.