What kind of water is best for your cannabis plants?

The best water for cannabis is clean, low-ppm water: reverse osmosis or distilled water is ideal (with a cal-mag supplement), while rainwater and dechlorinated tap water also work if you keep an eye on ppm and pH. Plants need water, we all know that, but the type you use matters if you want the cleanest, purest buds.

To understand water quality, you need to know what parts per million (ppm) are and why they matter. PPM measures total dissolved solids (TDS), meaning every organic and inorganic substance dissolved in the liquid. In short, ppm tells you the total mineral content of your water.

Those minerals count as impurities, especially at high concentrations. The higher the ppm, the more "contaminated" the water. To keep cannabis plants healthy, it's best to water with a low ppm reading.

Types of water: which is best for cannabis plants?

Water falls into two broad camps: hard and soft. The concentration of calcium and magnesium, measured in ppm, tells you how "hard" the water is. The higher the mineral content, or ppm, the harder the water.

For watering cannabis, your water should contain between 100 and 150 ppm of calcium. If it's "very soft," around 50 ppm, you'll want to make up the difference with a cal-mag supplement.

Use this table to make sense of ppm readings:

Hardness / PPM
Very soft: 0 to 70 ppm
Soft: 70 to 125 ppm
Moderately hard: 125 to 200 ppm
Hard: 200 to 300 ppm
Very hard: 300 to 500 ppm
Extremely hard: over 500 ppm

Tap water: Most growers use tap water. Whether it works for you depends on where you live, since water varies a lot from place to place. Tap water can be hard (EC above 0.8), medium (around 0.4) or soft (below 0.4), it usually has a pH above 7.0, and it often contains treatment chemicals that keep it clear. Depending on the area, it can also carry lime, chlorine and fluoride, which can kill off the beneficial life in your soil or growing medium and drag down its quality.

If you water with tap water, you can let it sit for 24 hours so the chlorine can evaporate, or get a reverse osmosis filter to clean it up.

AC condensate: The water that drips from an air conditioner can be great for cannabis because it's close to distilled, though you should always check it with an EC meter, since it can pick up metals from the unit. Its pH tends to sit above 7.0.

Rainwater: You can use it on almost any plant. Plants in the wild grow on rainwater, and it's one of the cleanest sources of fresh water on Earth, as long as you collect it in clean containers. It's usually slightly acidic, often somewhere around pH 5.5–6.5.

Distilled water: Distilled water is made by heating water until it turns to vapor, then cooling the vapor back into liquid, leaving every other component behind. It's perfect for plants, and people can drink it too, though since it has no minerals at all it's not a great choice for everyday drinking.

Reverse osmosis water: RO water is similar to distilled but not quite as pure, since it doesn't remove 100% of the minerals (lime, chlorine, etc.) and other impurities. It's made with filters that trap minerals and other unwanted stuff. Depending on the filter and model, the water will be more or less pure, and so higher or lower in quality.

Bottled mineral water: Better than tap water or water from an unknown river or well. Mineral water is guaranteed to be free of harmful substances and usually has a pH around 7.0, but it can carry large amounts of minerals, like calcium, that may affect plant growth and how your soil behaves.

The best water for growing cannabis is clearly reverse osmosis or distilled water. Both make sure your plants don't take up toxic minerals.

Why watering your cannabis plants right matters

Knowing how to water your cannabis plants properly is crucial for their health and development. Irrigation water plays a key role in nutrient uptake, water balance and the plant's metabolic processes.

You need to give your plants the right amount of water, avoiding both overwatering and underwatering. Too much water can lead to waterlogged soil, roots starved of oxygen and fungal diseases. Too little can cause dehydration, water stress and a general decline in plant health.

Your choice of water matters too. In general, use good-quality water that's free of contaminants and has the right pH to avoid nutrient lockout. Mineral and distilled water are popular choices for cannabis because they tend to be more neutral and free of unwanted elements.

Keep in mind that watering needs change depending on the growth stage, plant size, growing medium and environmental conditions. Adjust your watering to each plant's needs and check soil moisture regularly so it never gets soaked or bone dry.

To sum up, watering your cannabis properly is essential for healthy growth. Your irrigation water, whether mineral, distilled or another good-quality source, plays a crucial role in nutrient uptake and water balance. Avoiding both over- and underwatering and adjusting to each plant's needs are the keys to a healthy grow.

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