Watering is of critical importance in the care of potted orchids indoors, typically in terms of how much is too much and how much too little. But specifically what water is best for the care of orchids in the home? Is there a difference? There are orchid keepers who insist upon watering their orchids with distilled water, and there are those who only use tap water. Which is best? In truth, rain water, distilled water, and tap water all have the potential to be used to water orchids, at least if treated correctly. But the choice of water has a knock on effect on orchid care in general.
In the cloud forests and tropical jungles where most houseplant orchid species naturally inhabit, rain water is an important source of moisture. But there is huge amounts of humidity in the air itself. This comes from clouds (at higher altitudes), but also from the continual evaporation of water. Both rain and this water vapor are very pure sources of water; i.e. they have very low levels of dissolved impurities. So surely rain water is the most natural way to water a rainforest orchid? Actually, orchids are fertilized by detritus washed down from the canopy above. Although rain water is pure, by the time it reaches the orchid's roots, it may be anything but.
But when we consider what to water our orchids with, it is not just the needs of the plant that have to be considered; also the availability of different water types. Some people, for example those without gardens, or in arid climates, may be unable to collect rainwater. Likewise other people may find distilled water is not readily available in their local stores. Tap water, or even well water, may be the only option for some orchid growers.
Rain water is not absolutely pure H2O, but it is, pretty close. Obviously the container it is collected in may allow the addition of soluble and insoluble particles, microbes etc, plus the chance of the odd invertebrate thrown in. If you live in an industrial area with notable environmental pollution, rainwater may contain contaminants and be unsuitable. But in general the use of rain water is an excellent way to water orchids. Of course, it may not be available year round, depending on local conditions. Rain water needs no preparation beyond being allowed to reach the ambient temperature of the orchids that will be watered.
Distilled water has been purified by a process of heating and evaporation, until it is effectively pure H2O. This can be bought in certain larger stores, though this is perhaps not practical, especially if you have many orchids. Water can also be purified in the home using a reverse osmosis unit. This purifies water by passing it through a membrane that only lets the smaller water molecules through, and none of the larger salt molecules that were dissolved within it. After the initial cost of the unit, this allows the production of unlimited pure water at no expense. Both distilled and reverse osmosis water are excellent choices for watering orchids. As with rain water above, the lack of any nutrients means plants must be fertilized with regularity.
When we talk about tap water, we are making a number of general assumptions about it's quality; i.e. about the dissolved salts within it. But tap water varies hugely from place to place. Neither is it of consistent quality year round. Typically, tap water contains simple salts, such as calcium and magnesium carbonates and chlorides; plus a long list of other chemicals occurring in differing quantities. Everything from biological molecules: protozoa, bacteria, and hormones; to inorganic particles: heavy metals etc. For tap water to be safe for human consumption chlorine and occasionally chloramine are routinely added. So is chlorine safe for my orchids? The answer is probably yes. At least in the concentrations typically seen in most tap water, and for most hardy orchids. But there could be exceptions. Is tap water in general safe for my orchids? Again the answer is probably yes, but again, there could be exceptions.
At certain times of the year, water companies may briefly increase chlorine/chloramine concentrations and even add other chemicals to tap water. This is in order to clean pipes, control algae, prevent bacterial blooms and so on. These occasional additions, may have a detrimental effect on orchids.
But one factor that really should be considered is the level of nitrates (NO3-) in your water. A compound of nitrogen and oxygen, nitrates are basically plant food. If you put a closed container of clean tap water on a bright window sill, algae will grow, feeding off the dissolved nutrients. The dissolved nitrates are impossible for water companies to remove, and may be added to from a number of sources, most significantly run-off from farmland, or gardens. Nitrate levels in tap water can get quite high. Have you ever noticed an aquarium, even though understocked, lightly fed and well planted can still have severe algal blooms? This is usually down to nitrate-rich tap water. And a reason serious aquarists make pure water with a reverse osmosis unit, adding solutes back in as required.
But nitrate rich tap water is fine for orchids. However, if used, it may be a good idea to reduce the quantities of fertilizer added to the plant when it is fed. Many orchid keepers using tap water routinely halve the recommended fertilizer dose, just to be sure not to overfeed.
Of course not all tap water is the same. Some water is incredibly hard with huge amounts of dissolved solids. Generally such water will have a high (alkaline) pH value. This water is great for keeping African Rift Lake cichlids, but less appropriate for orchids. Some people use the term 'liquid rock'. The successful orchid growers that use tap water exclusively most likely don't have extremely hard water. Whilst it may be the case that some of the most hardy orchid species tolerate such water, many other species will struggle. If you believe you live in such an area, and other water sources are unavailable to you, it may be worth talking to other local orchid growers. Find out which orchids can thrive with your local water.
Like any water used to hydrate our orchids we should ensure tap water is not too cold. A significant temperature change can shock delicate plants. The best regime is to draw tap water a day in advance and let it reach room temperature before it is used for watering. Even though chlorine does not seem to be a huge issue for most species it will dissipate over several hours. It is not present in aged tap water. Chloramine does not disappear so easily. If you are concerned about it's presence in your tap water, you can add a tiny volume of dechlorinator, in the same way aquarists make their water safe for fish.
Nitrates can be removed, though not easily. A better approach is to simply keep track of the levels (they may vary) and fertilize accordingly. You can find out about nitrate levels from your water company, local aquarists or by the use of a simple nitrate test kit.
Tap water can also be filtered through activated carbon. This will not reduce nitrates, but will remove larger impurities such as the aforementioned heavy metals.
The truth is, orchids are adaptable plants. Commonly available varieties have been maintained by horticulturalists, sometimes for decades. These plants are generally well-adapted and will thrive whatever water we use.