This post may contain affiliate links. Read the full disclosure here.
This is an extremely valid question that I don’t see being answered anywhere other than Facebook groups, so let’s sort it out.
When one first starts getting into house plant collecting, one of the first things we learn I that overwatering is far and away the best way to kill your plant within a month. We accept this, and we move on.
Perhaps we buy ourself a little moisture metre to help out.
I mean, it makes sense. Drowning any living thing is going to kill it pretty sharpish.
But then we delve deeper into the hobby. We start looking into self watering pots, and semi hydro and keeping plants in leca, or even just water, forever.
It’s pretty confusing, no? You can kill a plant by overwatering it, but that VERY SAME PLANT can 100% thrive with its roots fully submerged in water.
MAKE IT MAKE SENSE.
Ok, I will.
Why does overwatering kill plants?
I’m really going to get my money’s worth out of that drowning analogy, so remember it.
Plants need the same basic three things humans do:
I mean, their food is light and our food is chips, but they boil down to be pretty much the same three things.
We’re often told that plants absorb carbon dioxide and release oxygen (all true), but they need oxygen to start them off with their respiration.
So how do plants get their oxygen?
They absorb it through their roots.
You know how I recommend adding perlite of orchid bark to your potting mix? That’s to add in aeration and help the plant with it’s oxygen uptake.
If your plant is in a very dense, heavy mix, a lot of extra water will end up compressing the soil and squeezing all of the oxygen out, leaving your plant with no air to breath.
Why do overwatered plants get root rot?
As if a lack of oxygen wasn’t bad enough, waterlogged soil and lack of air pockets in the soil can cause a buildup of bad bacteria.
These bad bacteria can increase the speed that root rot develops. See? Overwatered plants have it rough.
There have been reports (read it here if reading about worm poo is your thing) that worm castings can carry these same bacteria. I’m not worried to be honest, but I am careful to add fewer worm castings than I might once have, and I make sure to mix them in with the other ingredients well so I’m not creating any pockets of bacteria.
Worm castings are quite moist to mix well!
As well as the issue of bacteria from outside sources, bacteria that causes root rot will flourish in warm, damp environments.
This all very straightforward. Overwatering can lead to bacteria, oxygen starvation, and then death.
But then…what about keeping plants in water?
Why aren’t plants kept in water overwatered?
Firstly, you can absolutely cause root rot if you pull a plant out of soil and stick it straight into water. It’s a technique I use regularly to pull plants back from the brink of death (read about it here) but it’s very much a kill-or-cure deal.
So provided you transition your plant slowly, most plants (even succulents, though it can be tricky) can live in water.
SO WHY THE HECK DON’T THEY GET ROOT ROT??
Back to the whole drowning analogy:
Imagine someone chucks you into a lake. You can’t swim and there’s no one there to help.
What do you do?
Well, speaking as a human, I’d drown. The reason plants don’t is that they can grow new roots that can absorb oxygen from water.
It’s the equivalent (kind of) of a human being chucked in a lake and coping with the situation by growing a pair of gills.
So in short, plants can live in water or soil because they can grow roots to adapt to whatever environment.
There are very very few plants that can live in waterlogged soil, and those that can are real picky. Carnivorous plants, I’m looking at you.
Why don’t self-watering pots cause overwatering and root rot?
I’m not sure if I’m a bit slow or it is actually confusing, but it took me an embarrassingly long time to come to terms to how self-watering pots work.
I’ll try to explain it, but I’m just going to clarify that self-watering pots CAN cause overwatering, so don’t use them on plants like succulents or snake plants unless you have a firm grasp of what you’re doing.
Self-watering pots are basically a fancy way of bottom watering plants. You pour the water down a hole in the side of the pot* into a reservoir in the bottom of the pot.
This reservoir is usually filled with water-absorbing substrate, such as LECA or Pon. Over time, the water will be wicked up by the substrate (which is in contact with the soil) and will very slowly creep up towards the roots.
The idea is that the substrate at the bottom releases water very slowly, so the soil doesn’t become too saturated. We’re basically bottom watering very slowly.
Self-watering pots still require a level of house plant knowledge. Some plants are happy to have their reservoir topped up, whilst others would prefer to 100% dry out.
*There are self-watering pots that require you to drench your soil, but I don’t recommend them. If you have them, try to add some sort of pipe so you can bet the water to the reservoir in the bottom with saturating the soil.
Important things to remember re. overwatering
- Whilst plants can be transferred back and forth between water and soil, it takes time for plants to transition between soil and water roots. Your plant may decline or even lose all its leaves.
- Keeping plants in water isn’t easy. It’s definitely possible (I have an article about it here) but it can be a lot of upkeep because you have to keep on top of changing the water and getting rid of algae. A cheap air pump is a great addition.
- If you have an aquarium, try putting a couple of plants in there (unless you need to keep the lid on). I propagate my plants in my boyfriend’s aquarium and they root super quickly due to the air flow, warmth and lighting.
- When moving your plant into water, try to clean as much soil off the roots as you can. Every crumb of dirt can hinder your plant’s ability to grow soil roots and it can lead to bacteria buildup and root rot. Different plants have differing levels of sensitivity to this. In my experience, syngoniums, philodendrons, and Monstera aren’t that picky about it, but hoya are.
- If you want to try out growing plants in water but you have no faith in your root-cleaning skills, maybe try rooting a cutting in water rather than risking your whole plant.
Plants need oxygen, and usually they get it from the soil. If the soil is waterlogged the oxygen is removed and the plant dies. Put a plant in pure water though, and it can grow a different type of roots that can absorb oxygen from water.