Best Water for Plants: Tap Water versus Rain Water

Tap water will keep most houseplants alive. However, distilled or rainwater is always the better option for plant longevity and optimal health.

Using rainwater on your houseplants has benefits beyond maintaining the health of your plants. It is an environmentally sound practice that saves water usage and bills. 

What’s wrong with using tap water on houseplants?

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Carnivorous plants, air-plants, and micro-orchids require distilled water or rainwater to stay alive. Bromeliads strongly prefer it. For other species, collecting rainwater or buying distilled water might require more effort, but it will pay off. This is particularly true if you are raising delicate, fussy, or young plants. 

The minerals contained in tap water varies across different regions but will be some combination of fluoride, chloride, magnesium, and calcium. Micro-quantities of magnesium, chloride, and calcium are essential for plant growth but any additional amounts from tap water can kill them.

Some parts are particularly sensitive to specific chemicals found in tap water. Fluoride is damaging to spider plants, corn plants, prayer-plants, dracaena, and peace lilies. It is taken up through the soil and accumulates in the leaf margins, creating brown marks (necrosis). It inhibits photosynthesis by reducing the synthesis of chlorophyll, degrading chloroplasts, and inhibiting the Hills reaction, which is the way that plants convert light into energy. 

Chloride is particularly damaging to Tradescantia. Chloride toxicity produces similar symptoms to fluoride toxicity, such as leaf necrosis. Like excess fluoride, chloride also damages photosynthesis and may interfere with the uptake of nitrogen and phosphate which are key macronutrients for the plant. An easy way to remove much of the chloride in tap water is to sit in an uncovered container of tap water for 24 hours.  

It may take some time for harmful minerals to build up in the soil and damage the plant. Once they do, however, your plant may deteriorate quickly. If you see a thin white crust on the soil, this indicates excess minerals, it’s time to flush the soil or repot using a fresh potting medium. 

General symptoms of plant distress such as browning leaf tips and curling leaves can sometimes be traced back to tap-water mineral build-up. If you are noticing browning and curling in plants and are struggling to pinpoint the reason, particularly if you’ve had the plants for a few months, flush the soil and switch to distilled or rainwater. 

Bottle-spraying with tap water present problems too. If you bottle-spray your plants with hard tap water, you will notice white mineral residue on your leaves over time. You may want to switch to rain or distilled water for misting as well as watering.

What is the difference between hard water and soft water?

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Hard and soft water refers to the different types of minerals found in regional water supplies. While both can be detrimental to plants, some plants will be more sensitive to either hard or soft water.

Water in hard water areas will contain calcium and magnesium. These elements form the white encrustations you find on showerheads and taps. Hardwater is particularly detrimental to acid-loving plants like azaleas, orchids, African violets, and begonias. 

Water in soft water areas will contain lots of sodium, which inhibits water uptake. Hardwater tends to be more damaging to plants than soft water, but sodium can be worse for orchids.  

How do you collect rainwater? 

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If you need rainwater for just a couple of houseplants, you should be able to collect enough rainwater by placing a few containers outside. Cover the containers with insect mesh cover to prevent insects, leaves, and other detritus from falling inside. You should try and place the containers in a north-facing position.

If you want a large, consistent supply of rainwater, you should consider fitting a run-off collection system, otherwise known as a rain harvesting system. This installation will collect any rainwater that falls across a large surface area, such as a roof. The drainage system deposits this run-off into a barrel. You can also get the water collection barrels fitted with a hose spout or tap for easy release. 

Remember that the quality of rainwater is affected by local environmental conditions. Air pollution and run-off from industrial works may add harmful chemicals to rainwater. If you are collecting the surface run-off of a roof, make sure that the roof is made of non-corrosive materials and that it is clear of animal feces. 

Rainwater should not be collected off treated rooves and asphalt. The best material for clean rainwater will be a metal roof with a baked-on or powder-coated finish. 

If you want to check that the rainwater around you is safe, get a water testing kit. Various kits will test for different types of contaminants. 

How do you de-mineralize tap water?

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Fill a pot halfway with tap water. Place a bowl inside so that the bowl floats on the water. Cover the pot fully with a lid. The lid should be placed curved side down. Place a bag of ice on the lid. Boil the water.

The boiling tap water inside the pot will turn into steam, leaving any minerals behind. Once the de-mineralized steam hits the cold lid, it will turn back into liquid. As the lid is curved, the droplets will drip into the floating bowl below. 

This is not a very efficient or cost-effective process if you want a large amount of water.