What type of Water is best for plants?

What type of Water is best for plants?

Hand watering plants - header image

Water is one of the most crucial building blocks for plant growth. The rest of the ‘Big 5’ consists of oxygen, light, favourable temperature and nutrients. Interestingly enough for the purpose of this article, there can be a slight bit of overlap between the type of water you use and a couple of those other factors. But first things first - when we talk of ‘types’ of water, what exactly do we mean? Here we will go through the different choices you have when it comes to plant irrigation.

Tap Water
Tap water is probably the best place to start: many gardeners and food-growers irrigate their plants with tap water, and a lot of the time it will do fine. It’s a bit like you or me drinking tap water: we do it because it’s convenient and inexpensive, but we’re also aware that it might have some ‘impurities’ in it that aren’t so good for our health and that can build up over the long-term. Now maybe this applies to Ireland more than the UK: here in the west of Ireland we’ve had frequent boil-water notices in recent years, whereas the UK seems to have a more reliable clean water system. But even if tap water doesn’t have a high concentration of harmful substances, it does contain minerals such as magnesium and calcium.

How much of these minerals are in your tap water can depend on where you are in the country, as well as your water supply and source. You’re probably familiar with ‘hard water’ and ‘soft water’. Hard water will leave limescale deposits in your kettle and (I find anyway) can make your tea taste a bit…well, rubbish. To be more technical about it, hard water means that there’s a comparatively high concentration of natural minerals that have seeped into the water from rocks and soil. On the other hand, soft water has low concentrations of calcium, magnesium and other minerals, and is more acidic.

So what does this all mean for plants?, I hear you ask. The higher concentrations of minerals in hard water can gradually build up in the soil as you irrigate with tap water. In turn they can alter the pH of the soil to its detriment. However it’s worth bearing in mind that if you live in an area with regular rainfall, this buildup will often be ‘diluted’ or washed away somewhat by the rain. Houseplants or plants being grown under cover are more reliant on the type of water you provide for them.

Chlorine is added to tap water to kill bacteria. However, frequent irrigation with chlorinated tap water can lead to chlorine toxicity: the signs of which include burnt leaves and Another ingredient to be aware of in tap water is fluoride. National water fluoridation is actually fairly rare in Europe, to my surprise. Here in Ireland it’s fairly normalised, but in the UK only certain regions have added it to the water supply. Fluoride can be harmful to some plants if it builds up over time, interfering with photosynthesis and so on.

Distilled Water
As the name might suggest, distilled water is water treated to remove any chemicals and minerals. This is done by boiling the water into a vapour and then condensing it back into a container. This method of ‘purification’ results in a softer water. Distilled water is available in some shops, or it can also be done at home. Distilling water shouldn’t be confused with filtering water: the latter process retains some nutrients in the water, but distillation aims to boil off everything other than H₂O itself.

Does this result in a better irrigation option for plants? While you obviously won’t have to worry about excessive mineral content, the flipside of distilled water is that it doesn’t contain any nutrients that the plants can use. This means that it’s not quite the ideal choice for watering plants in comparison to some other options: there’s such a thing as being too ‘pure’, after all. When used regularly it can even potentially result in stunted growth. If using distilled water it’s important to also provide adequate fertilisation, according to the needs of the crop - particularly if it’s a period with not much rainfall.

Do Plants Grow Better With Filtered Water?
These days, more and more houses have some type of water filter in place, whether it’s a simple Brita filter or ‘whole house’ systems that filter the water at all points of entry. It can give you peace of mind about the quality of water that’s coming into your home. Depending on the kind of filter you use, it will remove certain ‘undesirable’ impurities from water (such as lead and bacteria), while leaving minerals like calcium, magnesium etc. intact. The different types of filters available include carbon, reverse osmosis and ion exchange.

That’s the science out of the way; how does filtered water measure up when it comes to watering your plants? We’d argue that filtered water is a better option than distilled, for the above reason that it retains minerals and nitrates that plants can benefit from. An activated carbon filter is very effective at removing chlorine and mercury - and it’s relatively cheap compared to other options.

Softened Water
While we’ve outlined some of the possible issues you might have with other water types above, many plants will do fine with the water you use, whether it’s filtered or from the tap or whatever else. Softened water, on the other hand, is the big no-no of the bunch. Water can be naturally soft or it can be artificially softened. Softened water refers to the end-product of a process where sodium bicarbonate is added to hard water. It’s no coincidence, then, that soft water often tastes ‘salty’. This excess sodium is not good for your crops. It can dehydrate plants, lead to nutrient deficiencies and slow down growth. So really: this is the type to avoid when it comes to watering.

We’ll end with what’s widely regarded as the best option of all when it comes to watering your plants. Even without knowing anything about the science, many food-growers will attest to their garden looking much more vigorous after a spell of rainfall. It’s not an exact science as to why this is, but there are some factors that go some way to explaining it. Natural rainwater contains higher levels of oxygen, carbon dioxide and nitrates. It’s in the ideal pH range for most plants, which in turn contributes to a slightly acidic soil that crops thrive in.

Rain water can also flush salt down through the soil and past the plant roots, helping plants to grow more effectively. You also won’t have to worry about added ingredients like chlorine and so on, which can kill microorganisms in the soil.

So now that we’re aware of the benefits, how do we gain access to a regular, reliable source of rainwater that we can use as and when we need it? Option one is to move to the west of Ireland, where you’ll never want for rainfall. Just kidding…Setting up a water butt or tank to catch and store rainwater is the way to go. These typically collect rain water by being connected to a gutter downpipe, and come in various sizes (some can store as much as 650 litres of water). You’d be surprised at just how much rain water you can harvest from your rooftop or the roof of a garden shed. This water can then be used to irrigate your crops - it’s particularly handy during hot and dry weather conditions, or as a back-up in the event of a hosepipe ban. Expect to see a few more of those over the coming decade.

Hot and Cold
Another important factor to keep in mind when watering plants is the temperature of the water you’re using. Overly hot or cold water can potentially shock or stress sensitive plant roots. If it’s cold enough it can even trigger dormancy in plant roots! The ideal temperature for watering plants is somewhere between 60 and 70°F: typical tap water falls some way short of the lower end. This is another area where rain water establishes its superiority: with its naturally warmer temperature plants will have no trouble absorbing its goodness.