Watering your houseplants - and why they don’t like tapwater

It’s the main point of interaction we have between us and our houseplants, so it makes sense that we are obsessed with watering. How much should we give them? How often do we water? What sort of water should we use? (And yes, there are different types of water…) The questions mount up. They really do. But we’re here to help simplify this topic and figure out how you should be watering!

The Basics

First off, let’s look at the basic biology and physics of houseplant watering.

Just like you, your plant needs water to live. Unlike you, your plant gets water by osmosis (no really), between the roots and the soil. This means that, when there is enough water in the soil, it travels into the roots. So that this happens at the right time (and so already-saturated roots aren’t sat in super soggy soil, which would make them rot), you need to time your watering right.

The other aspect is how water sticks around in soil. For roots not to go mouldy, there needs to be Oxygen in the soil too, so your houseplant soil needs to let excess water drip out while soaking up the liquid the plant needs. Letting it drip through means a soil is well-draining, while soaking up water is referred to as being moisture retentive. Different plants need different levels of both - e.g. a fern used to a damp woodland setting would need soil that is very moisture retentive, while a cactus used to quick-drying desert sand needs its soil to be well-draining.

Now you know how your plants actually drink, let’s look at what they should be drinking…

Tapwater

The most readily-available water for us is tapwater. Unfortunately for our houseplants, tapwater is designed for us, not for them. Tapwater is usually treated with Chlorine and other chemicals, such as Fluorine. Exactly what it’s treated with depends on whereabouts in the country (or in the world!) you live, but in a nutshell this is good for us, as we get clean, bacteria-free drinking water, and sometimes it’s even beneficial for our teeth. But plants don’t need it to be as clean, and they don’t have teeth, so it’s not what they’re after.

Rainwater

This is what plants are used to drinking. Their drink of choice, if you will. Preferably at rainforest temperature, filtered through the tree canopy (and not your gutter), and soaking straight into their soil. If you have access to a water butt, or other rainwater collection techniques, this is still preferable (even if it’s been on your roof and through the gutter). Just let it warm up if it’s been outdoors in winter; you don’t want to shock your plants. If the water is too cold, it’s basically like giving them brain freeze, but they react more slowly and dramatically!

Distilled Water

Some plants, like Venus Fly Traps, are SO picky that you’ll need to water them with distilled water to get the most of them. In short, they do this because in their natural habitat there are so few chemicals in the ground that one, they have to turn to eating flies for nutrients, and two, they have had to adapt to having water with NOTHING in it. So, in the wild, they’re pretty hardcore. Unfortunately, this means they’re maladapted (not well adapted) to tap water.

So what do I do about it?

And here’s the bit you’re waiting for: the solution. (Little pun for the chemistry geeks out there) There are a few things you can do, and I suggest you take whichever option you can work into your watering routine, and fit around your life. Here goes, from most to least effective:

1. Collect rainwater. I know this isn’t possible for everyone, but if you can do it, even sometimes, it will be great for your plants, and even cuts down on water waste, so it’s better for the planet too. A water butt is definitely on my Dream Home Wishlist.
2. You can filter or treat your water. Tap water dechlorinator reacts with Chlorine in water to make Chloride, which is less bad for your plants. Filters for drinking tapwater will also do the job, reducing the amount of chemical ions in the water. If you keep your Brita in the fridge, though, remember to let the water warm up before you use it for your plants!
3. If you and your plants are less fussy, leaving tapwater to stand for 24 hours allows some of the chlorine to evaporate, enough that less picky plants will be totally fine. I also find doing this means I always have water ready to go - by refilling my watering can when it empties, it’s ready when I next need it.
4. In joint fourth, I have listed some expensive (and largely less sustainable) methods of getting water for your plants:

You can buy distilled or purified water. This is probably not feasible for too many plants due to the expense, but if you need to tide a flytrap over between rains it may be useful!

Other expensive ways of watering plants include boiling it (if you’ve noticed limescale in your kettle you’ll see why - Calcium ions can be removed by this method, turning them into limescale!).

You can also use water from a dehumidifier, which has been distilled as it was collected in gas form. Water collected in a tumble dryer is the same. But again, these devices can be expensive to run, so I’d say only use these if you’re using them already!

If you know you will struggle with using anything but tapwater, you can also select your plants based on this. Plants like Cacti, Spider Plants, Philodendrons or those tough-as-nails ZZ and Snake Plants will be basically fine with tapwater. They just aren’t that fussy. Steer clear of Calatheas, Carnivorous Plants, Orchids and Ferns if you can only give them hard tapwater. It might sound brutal, but there’s no point buying something that will be more likely to end sadly when you have a choice! 

Hopefully this has been helpful and informative - while you’re editing your plant shopping list, I’ll be busy working out if my mop bucket can collect enough rainwater for me to keep one of those beautiful Calatheas happy…