How To Make Tap Water Safe for Bonsais

Watering your bonsai is essential for proper care and to keep it alive. However, you can’t merely use any water. There are some dangers involved, and it may not be receiving all the nutrients it needs. We’ll show you how to make tap water safe for bonsais so that you don’t harm them.

How To Make Tap Water Safe

The Water Treatment Process

To understand what the water consists of when it reaches your tap, you need to know the process. Before it can be piped into your home, water needs to be quite thoroughly treated. The treatment process typically has four steps:

  • Step 1 – positively charged chemical compounds are added to the water. Because dirt and most dissolved particles found in groundwater have a negative charge, the addition of positively charged chemicals neutralizes the overall negative charge of the water. Once the charge of the water has been neutralized, the dirt and dissolved particles in the water bind with the added chemicals, forming larger particles.
  • Step 2. These larger particles are too heavy to float in the water, and they settle on the bottom as sediment. As long as the water is not churned around too much, this sediment stays behind when the water goes to the next stage: filtration.
  • Step 3: The sediment-free water is filtered through different types of filters, which filter out smaller and smaller particles at each stage. This removes pathogens like bacteria and viruses, as well as tiny parasitic organisms.
  • Step 4: The final step involves the addition of chemicals (usually chlorine) to get rid of any remaining impurities.
How To Make Tap Water Safe

Potential Dangers of Tap Water

While you can use tap water for your bonsais, here are a few aspects to consider to ensure proper care.

We have a detailed bonsai tree care guide with all the relevant information. Check the guide out here!

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Chlorine is a very vicious chemical. It’s commonly used by homeowners to kill off algae in swimming pools, but chlorine was also used as the main agent of chemical warfare in WWI.

If you breathe in chlorine in its gaseous form, the chlorine gas, combined with hydrogen in the air, reacts with water particles in the lungs to form hydrochloric acid. This acid eats into the lung tissue and causes death by suffocation within a few minutes, even at a concentration of as little as 0.1% chlorine in the air.

Chlorine is also used in drinking water. Of course, it’s a good thing if all the pathogens in the water get killed off before you drink it: for example, the chlorination of drinking water has been effective in almost completely wiping out cholera, which is a very nasty water-borne disease.

On the other hand, though, the idea of putting such a dangerous agent of chemical warfare into one’s body – not to mention, using it to water one’s precious bonsais – is quite off-putting.

But actually, having chlorine in your tap water is not a very serious problem. The amount that gets added to your municipal tap water varies – at times, there is even enough to smell and/or taste – but it is seldom sufficient to do much harm to plants. When chlorine does damage plants, the damage is usually done to the tips of the roots; this prevents them from being able to absorb nutrients properly and may affect their overall growth.

Because bonsais are so tiny, it’s even more important for each one of their tiny roots to be able to absorb enough nutrients. Fortunately, chlorine is not very long-lived. If you leave your watering can full of water to stand for 24 hours, the chlorine will take on its gaseous form, and dissipate completely into the air by the next day.


Fluoride is a mineral that occurs naturally in some rocks; it’s released from the rocks into the soil, and can therefore also make its way into groundwater, and from there, into tap water. However, the amounts of fluoride found naturally in water are typically pretty low.

Moreover, different cities use various types of purification and filtration for the water that they then pipe into people’s homes – and some of those purification and filtration systems also remove the fluoride. But fluoride is incredibly effective in protecting teeth against decay and cavities.

Therefore, in order to enable their communities to benefit from the excellent effects of fluoride on dental health, most municipalities (in the USA and around the world) add extra fluoride to tap water before they pipe it into people’s homes.

Using tap water that has added fluoride to water plants can lead to an unhealthy build-up of fluoride in the plant’s tissues (especially the leaves). Fluoride moves through the plants as they absorb the fluoride-rich water; when water evaporates out of the leaves, the fluoride from the water stays behind and builds up at the edges of the leaves.

At the very least, this can reduce the plants’ ability to carry out photosynthesis, and it can even lead to fluoride toxicity. This causes portions of the leaves to get dry and brittle, turn yellow or brown, and even die. In fact, fluoride toxicity affects plants in much the same way as salt toxicity or even drought stress. Some areas on the edges and tips of the leaves can die; however, the plant itself is not usually killed.

The plants that are sensitive to having too much fluoride in their water all seem to be the ones with long, thin leaves (like spider plants, or dracaena). These leaves display brown spots or scorch marks, especially at the tips and along the margins, if they have been damaged by fluoride.

Young plants seem to be more vulnerable to fluoride damage than more mature plants, and again, bonsais might be more susceptible to the effects of fluoride than full-sized trees. However, fluoride dissipates into the air within about 24h, just as chlorine does. If you have a problem with fluoride in your water, just put it in a watering can, or any container, and let it stand overnight before use.

How To Make Tap Water Safe

What is ‘Hard’ Water?

Even if your water doesn’t have particularly high levels of chlorine or fluoride, it may not be very healthy for your bonsais. ‘Hard’ water refers to water that contains a high level of calcium, magnesium, and other ions. These minerals are absorbed into groundwater as it filters through limestone, chalk, or gypsum deposits in the earth.

In fact, because limestone is the main source of calcium and magnesium, saying ‘hard water’ very nearly amounts to the same thing as saying ‘water that comes from somewhere with a lot of limestone in the soil’.

Farmers or breeders of aquatic animals – including fish, reptiles, amphibians, and even mammals that live in or around water – may need to raise the ‘hardness’ of the water by adding calcium carbonate and/or magnesium sulfate (aka Epsom salts). Crop farmers or gardeners, and especially growers of delicate bonsais, may need to take steps to remove or reduce the mineral salts found in hard water.

You can get ‘softened’ water, from which the calcium and magnesium have been removed – but softened water often has a high salt content. You can even say that the salts found in ‘softened’ hard water cause more damage than the lime found in the original (unsoftened) hard water.

Both ‘hard’ and ‘softened’ water may contain unacceptably high levels of different minerals and salts, which can stop plants from being able to absorb as much moisture as they need. Moreover, apart from the detrimental effects they can have on the health of the plant, a whitish crust of salts on top of the soil of your beautifully presented bonsai doesn’t have a great aesthetic appeal.

How To Make Tap Water Safe

How To Make Tap Water Safe for Bonsais

Most tree species prefer slightly acidic soil, or at least neutral soil, to an alkaline one. Some – including many junipers and coniferous species, including spruces, larches, and cedars – do particularly poorly in soils containing high levels of lime. The same is true of azaleas, bougainvilleas, camellias, and ilexes.

They are known as ericaceous, or lime-hating, plants. For these species, the build-up of alkaline salts from hard water can be severely detrimental. You can usually identify plants suffering from a build-up of alkaline salts from hard water because too many alkaline salts lead to a deficiency in iron, and a deficiency in iron leads to the leaves appearing pale or yellowish.

If you live in a hard water area, an easy solution might be to collect rainwater (which is often slightly acidic, and certainly never alkaline/‘hard’), and use that for watering your bonsais. But unless you live in a climate that has plenty of rain all year round, this will be tricky to do in the dry season (or if you have a very large collection of bonsais, requiring very large quantities of water).

Alternatively, using a fertilizer that reverses alkalinity in bonsai soil, about once every two weeks, will usually be sufficient to keep your bonsais healthy.

Cleaning Lime-Scale

If you have ‘hard’ water (containing too much calcium, magnesium, or other metals), you will notice a build-up of mineral deposits in the water pipes. This is usually brownish or reddish in color, and builds up in a thicker and thicker layer – eventually, the mineral salts can actually eat right through steel pipes, and eventually cause leaks.

It’s very difficult to remove the reddish-brown stains, caused by limescale, from your bonsai pots (not to mention, from baths, toilets, and sinks) using normal cleaners. However, normal vinegar has a magical effect against this type of stain.

To get the stains of your bonsai pot, you can just put the vinegar straight on – or better yet, wait until you are repotting your bonsai, and put the whole pot directly into a container with vinegar in it. The acid vinegar reacts with the alkaline scale, and can even produce a visible fizzing effect, like hydrogen peroxide, as the scale dissolves.

If you have limescale on your bonsai itself, a strong vinegar solution (e.g., 1:20 vinegar to water) can be used directly on the bark to soften the scale. You can then brush the scale loose with a soft brush like a toothbrush, and wipe it away with a soft cloth or paper towel. If the scale really doesn’t want to come off, you can even leave the vinegar solution to work for minutes or even hours before brushing and wiping.

If you spray the foliage of your bonsai, and you are using hard tap water, a similar scale build-up can occur on the leaves. Whereas limescale on pots usually appears reddish-brown, limescale on plant leaves usually appears milky-white.

Unless you are dedicated enough to wipe down each leaf individually, you can get away with spraying a weaker vinegar solution (1:50 vinegar to water) thoroughly over the foliage. Saturate the soil with tap water (or rainwater, if you have it) immediately afterward to rinse out the residual vinegar; and then, after 5-10 minutes (before the vinegar gets dry), spray the leaves with clean water.

Other ways to promote plant health when using tap water

About once a month, add a small amount of vinegar to your bonsai’s water (about 1 tsp vinegar to 7l water) to counteract the alkalinity of ‘hard’ water.

Fertilize plants at the height of the growing season, when they are growing most vigorously. If you add fertilizer in fall or winter when most plants become at least partially dormant, the fertilizer will not be fully absorbed, and this can lead to even more build-up of mineral salts in or on top of the soil.

 Re-potting your bonsai yearly may be necessary if you have a really bad hard water problem. Clean salt deposits off the pot, and refill it with fresh soil.

Final Thoughts

In some cases, the wrong kind of water can cause your bonsai to be really miserable. But do not despair! There are simple steps you can take to make almost any kind of water safe for use.

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Jane grew up in rural Kwa-Zulu Natal, South Africa. From a young age, she learned to love tending to various farm animals and pets, climbing trees, playing on her own in the garden, and exploring around the small-holding, as well as around the nearby stream and dam. She loves the natural world, and she does not love the big city.

Although she is a big outdoors person, Jane is also an academic. She has recently completed her Ph.D. in Philosophy and is working on a book project based on her Ph.D. thesis. She hopes that researching and writing about bonsais may be a good way to begin integrating herself slowly back into the real world, after so many years of writing about philosophy!

Jane Anderson

Jane Anderson


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