While many of us might simply use the hosepipe to water our bonsai, there are actually a range of options for recycling or collecting water for this purpose. I’ve looked into some of these below to understand how suitable water from different sources is for watering your trees.

Dehumidifier water – good unless you have a dessicant humidifier and toxic air

Dehumidifiers remove water from the air in a few different ways. One common type is a compressor dehumidifer. This type pulls air through a filter and over cooled metal coils which cause water in the air to condense onto the coils and drip into a reservoir.^ref Since this is effectively distilling the water, it should be relatively pure. Where contamination could come into play is if the coils or the water reservoir are not kept clean, but for bonsai tree watering, this shouldn’t be an issue.^ref You should be cleaning the reservoir anyway to avoid Legionnaire’s disease (see below). The water from a dehumidifier won’t have any minerals or nutrients in it (unlike rainwater), so fertiliser would be needed.

A dessicant-type dehumidifier pulls water through a dessicant material – such as zeolite^ref (if you’re wondering where you’ve heard that name before, it’s used as a bonsai soil additive^ref). Water is absorbed in the dessicant then this material is heated and the water drips out into the reservoir. It’s not exactly the same as distilling because the water is in contact with the dessicant as it is condensed and could hold dissolved compounds. Researching the properties of zeolite will take you down an entirely new internet rabbit hole (including 1.24M research paper results on Google scholar). But this substance is known for extracting heavy metals and other contaminants from liquids^{ref1 , ref2}. So in theory the zeolite could hold other molecules which could be released into the water as it condenses. This might be an issue if you are using dehumidifier water from a location with particularly toxic or polluted air. If not, the risk to your bonsai should be fairly low. As above cleaning the water reservoir and dessicant regularly is important.

Tumble dryer water – good if condensing, less good if venting

Tumble dryers also work in slightly different ways but the main mechanism for extracting water is that warm air evaporates water from the clothes, then this air either passes over a condenser or is vented outside, in both cases water condenses from the air as it cools.

With a condensing dryer, the process is another form of distillation so should be relatively pure water and fine for use on bonsai.

If you are using a vented machine however there may be microplastics, particulates and lint from the drying clothes coming through the venting pipe. In 2021 tumble dryers were found to be a leading source of microfibre air pollution.^ref You might not think this would affect your tree very much, but it has been found that nanoplastics and microplastics can enter plants through their roots, carrying a range of toxic substances including pesticides, polybrominated diphenyl ethers (PBDEs), endocrine-disrupting chemicals (EDCs), polycyclic aromatic hydrocarbons (PAHs), phthalates, and bisphenol-A. PLA microplastics have also been shown to negatively impact arbuscular mycorrhizal fungi diversity and community structure.^ref If you want to avoid microplastics in your pots and tree roots you probably don’t want to use vented tumble dryer water.

Air conditioner water – good

Air conditioners work similarly to dehumidifiers and dryers – they pull air through a mechanism which cools it, and in doing so water is condensed from the air. So it’s fine to use for watering (although as above will contain no nutrients).

Boiler condensates – not good

I’m not sure how many people would be trying to use boiler condensates for watering but just in case you think of it, boilers which use fossil fuels produce condensates containing carbonic acid, sulphuric acid and nitric acid, all of which reduce the pH of the water coming from the system.^ref Although slightly acidic water (in the range 5.5-6.5) has been show to optimise plant growth^ref, boiler condensates can be as low as 3 which is toxic to plants.^ref

Rainwater – good – maybe the best

Rainwater is a different proposition to the previous water sources. Whilst it is distilled from air just like the others, it has to fall through the atmosphere to reach the ground. As it does this, rain absorbs compounds present in the air in particulates and gases. It also runs off roof surfaces, down drains and pipes and into storage tanks. So rainwater collects contaminants along the way. As atmospheric carbon dioxide is one of the molecules rain collects, it has an average pH value of about 5.6 (just at the lower end of preferred plant water pH which is 5.5-6.5)^ref

The chemical composition of rain varies geographically even before it hits the ground. For example in Samoa the rainwater composition is highly influenced by marine sources, which makes sense since it’s an island.^ref Locales near the ocean have rainwater with a similar (diluted) composition to sea water^ref but those inland vary depending on natural and manmade influences such as industry, topography and weather. Raindrops have also been found to contain airborne bacteria and fungi.^ref

Rainwater in the UK is monitored for ions of sodium, calcium, magnesium, potassium, phosphate, nitrate, ammonium, sulphate, sulphur dioxide and chloride, and for acidity/pH and conductivity.^ref The latest measurement at my nearest monitoring station showed the following contents from a 3mm sample – nutrients are there but nowhere near the same levels as a seaweed fertiliser.

The other factor to consider is where you store your rainwater. Outdoor water reservoirs are usually colonised by bacteria, fungi and other organisms, and have rotting plant matter, bird faeces, dead insects and other detritus. This may also be a good source of fertiliser, depending on how concentrated your detritus gets, or a source of toxic microbes and algae, particularly if there isn’t much flow and replenishment of the water.

But overall rainwater is a good choice for watering bonsai. It has a favourable pH for plants, and is a mild fertiliser containing a range of macro and micro nutrients. As it doesn’t have carbonates like groundwater, using rainwater can help you avoid limescale marks on leaves & pots if you live in a hard water area. Just try to avoid leaving it standing or stagnant for long periods particularly if the temperature is above 20^oC (see Legionnaire’s disease below).

Aquarium water – maybe depending on your tank

The subject of aquarium water almost warrants its own post. Anyone wanting to better understand the chemical and biological parameters inside a planted aquarium should read the brilliant book “Ecology of the Planted Aquarium: A Practical Manual and Scientific Treatise” by Diana Walstad.

The water in your aquarium is likely to be completely fine for bonsai if it’s fine for fish. They have high standards – and can’t handle large pH ranges, excessive levels of nitrites, ammonia or excessive nutrients like heavy metals. In fact this water can be an excellent source of nitrogen since fish in aquariums excrete ammonia, which is the main source of nitrogen for most fertilizers. Usually when doing a water change on an aquarium the levels of rotting organic matter have accumulated and ammonia levels are at their highest – one of the main reasons for doing water changes is to reduce them. Planted aquariums which use soil as a substrate and have fish (and fish food as an input) also contain other macro and micronutrients, to the extent that fertiliser isn’t necessary. So aquarium water could be a good addition to your bonsai, as a fertiliser.

On the other hand, if you have certain plants in the aquarium, some are known to be ‘allelopathic’ – that is they produce compounds to inhibit other organisms. For example the water lily Nuphar lutea kills duckweed and lettuce seedlings (Walstad, 2012). Aquatic algae and bacteria can also behave allelopathically, to the extent that Diana Walstad keeps her prized plants in their own substrate and even in separate tanks to stop them being killed by competitive organisms. So there is a risk with water from planted aquariums or aquariums with algae that there may be allelopathic compounds which damage your tree. It’s hard to make a recommendation since it’s impossible to know without testing whether toxic compounds are in your aquarium water. If you have no algae or plants, then the water is probably fine.

Greywater – not unless you treat it

Grey water is waste water from your bath, shower, washing machine, dishwasher and sink.^ref It can contain detergents, oil, dirt, organic matter like food, skin particles, microplastics, bacteria, fungi or anything that you wash off yourself, your dishes or your clothes. As such greywater isn’t suitable for watering your bonsai. Some larger properties like hotels recycle their greywater, but it’s treated first using UV light, chemicals or serious filtration systems.^ref So unless you have access to a high quality greywater filtration system, do not use this to water your trees.

Blackwater – no

The things you learn about when researching bonsai websites. We won’t go there – just – no.

Seawater – not unless you have a desalination plant

Unless you are growing kelp forests, do not water your bonsai with sea water. Excess salt is extremely detrimental to plants, and can kill them.^ref

Lakes, streams, rivers, boreholes, wells or ponds – it depends

Natural water sources such as these are not treated and can contain all sorts of things aside from water, but this really depends on what runs into them, what happens and lives on and in them. Some lakes & streams are very clean, others have industrial runoff, pesticides, sewage overflow^ref, algal blooms from excess fertiliser runoff and worse. What you want to avoid with a water source is dissolved contaminants which might harm your tree – usually these will derive from human activity going on upstream. So before using such a water source, it would be wise to investigate what might be entering it and perhaps to invest in some testing.

It’s thought that water obtained from aquifers underground (eg. via wells or bore holes) is of higher quality if it comes from a deeper confined aquifer than from a shallow one.^ref Shallow aquifers are more open to contamination from pollutants.

pH is a consideration when using a natural water source. According to the Kentucky Geological Survey: “Streams and lakes in wet climates such as Kentucky typically have pH values between 6.5 and 8.0. Soil water in contact with decaying organic material can have pH values as low as 4.0, and the pH of water that has reacted with iron sulfide minerals in coal or shale can be even lower. In the absence of coal or iron sulfide minerals, the pH of groundwater typically ranges from about 6.0 to 8.5, depending on the type of soil and rock contacted. Reactions between groundwater and sandstones result in pH values between about 6.5 and 7.5, whereas groundwater flowing through limestone strata can have values as high as 8.5.”^ref Since you want to keep water pH between 5.5 and 6.5 for plant watering, it would be a good idea to test the pH of any natural water source you are using.

A note on Legionnaire’s disease

Legionnaire’s disease is a potentially fatal form of pneumonia caused by inhaling mist or droplets of water containing Legionella bacteria^ref. This bacteria can grow in any non-sterile water^ref, feeds on algae, rust, scale or sludge, and thrives in temperatures between 20^oC and 45^oC.^ref So it could be present in some of the above sources, particularly when water is left standing. It’s worth being more careful when temperatures are above 20^oC as you really don’t want to get this disease, which may already be a risk to bonsai practitioners since another source of Legionnaire’s is compost & potting soil.^ref In warmer weather it may be safer to use tap water for bonsai.

So to wrap all of this up from a bonsai perspective – feel free to use rainwater or water which comes from the distillation process of condensing humidifiers, tumble dryers or air conditioners. Be careful though if this water has been standing for a long time in temperatures above 20^oC. Feel relatively free to use aquarium water (but keep an eye on your trees to monitor for allelopathic effects) or dessicant-type dehumidifiers. Test the water if you’re using an untreated natural water source like a river, lake or well. Avoid using water from vented tumble dryers. And definitely do not use sea water, boiler condensate, untreated grey water or black water. Happy watering!