What is perched water table (re. house plants)?

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A perched water table is one of those things that made me go ‘oooooohhhh’ when I first got into house plant care.

If you have drainage holes in your plant pot, and use a decent, chunky soil mix, you don’t need to read this.

But if you’re new to plant care and aren’t sure why drainage holes are so important, and why you can’t put gravel at the bottom of a plant pot in lieu of drainage holes – this is written for you.

There are many people out there that use hole-less pts and gravel as drainage with no worries. I know you exist.

But please don’t advise other people to do the same thing, because it’s setting them up for failure. Drainage holes are much easier to deal with compared to having to work out exactly how much water to give to stop your plant from rotting.

What is a perched water table?

Here is a diagram, drawn EXTREMELY professionally (by me) showing a perched water table:

On the left, we have a plant that’s just been watered. The soil at the bottom is saturated (perhaps it’s been bottom watered, perhaps it’s just gravity), but it doesn’t matter because the roots aren’t in the saturated zone.

If they were down there, they’d be absorbing the water, because there’s only so much water in the soil – the excess went out of the drainage holes.

There’s no water table – there’s no standing water at all.

On the right, we have the potential for a perched water table. Gravel has been added to the bottom of a holeless pot. When we water, the water collects that the bottom.

The water just sits there, because where else would it go? It can’t be wicked up by the soil because gravel isn’t absorbing it.

This is sometimes fine, by the way. If the water evaporates completely between waterings, great.

But if you keep watering and the water doesn’t have a chance to evaporate, the cavity filled with gravel at the bottom will fill up – standing water.

Once the cavity is full, the water table will be in the soil, resulting in a perched water table – the water table is perching in the soil. And the soil is oxygen-less mud just waiting to rot your precious roots.

All the gravel is doing is reducing the amount of soil that can be kept in the pot.

If you look at the diagram, there’s less soil in the image on the right.

How does a perched water table form?

If you keep watering a plant that has no drainage holes, the pot will fill up with water.

This is especially likely to happen to newbies, because they might interpret the overwatered plants symptoms as underwatering and water more.

All you need to do to stop a perched water table from forming is use a pot with drainage holes. You can continue adding gravel if you like, but it isn’t doing anything.

Water that’s being held in the saturated area of the soil won’t drain into the gravel – gravity isn’t strong enough to squeeze water out of soil. All that will go into the gravel is the excess water – which we could really do with getting rid of.

Why do house plant people care?

Overwatering is one of the most common ways of killing house plants, especially for beginners.

If you aren’t aware of how and why overwatering is so dangerous to plants, then you might not consider that water building up at the bottom of your pots is concerning. Plants love water, don’t they?

Using gravel as drainage is one of those old wives’ tales that is still doing the rounds.

For some plants – it’s fine. The excess water sitting in the gravel will evaporate before the next watering. If your plant is in a drainage hole-less terracotta plant and you live somewhere warm, you’ll probably be fine.

The issue is, unless you’re using a clear pot, there’s no way to see if there’s a water table or not. You can’t tell if the water table is now sitting at the plant’s roots, potentially rotting them.

It’s also harder to thoroughly water when you’re worried about a perched water table. You might end up only giving your plant a little bit of water at a time, which isn’t great for it.

How do you drain a perched water table?

If it were me, I’d take the whole shebang apart. Take out the plant, remove the soil, and drill some drainage holes in the pot.

If you don’t want to damage your pot, or don’t have a drill, then buy a nursery (plastic) pot that will fit inside the pot and pot your plant in that.

If you suspect that damage from the perched water table has already been done, then you need to check the plant’s roots. Cut off anything brown and mushy. If there’s not much left in the way of roots, then read this article on rooting plants quickly.

It’s more about propagating, but propagating is basically just growing roots, which is what you need to do (I mean, get your plant to do. You don’t need roots).

I know it’s a ballache, especially for big plants, but it’s better to get it over with rather than risk the plant.

By the way, if you have a big plant that’s being kept with gravel and no drainage holes, but it’s been living like that for years and it’s fine…leave it. You probs hit on the right amount of water. Just keep doing what you’re doing.

How to stop a perched water table from forming

Use pots with drainage holes, and use a well-draining soil mix.

You do not need gravel in plant pots.

Gravel in plant pots serves two purposes, neither of which have anything to do with the health of the plant and gravel does not aid drainage. Leave me a comment if you can think of more.

1 – Putting gravel in pots can help to weigh them down

If you put your plants outside in summer, gravel can help to stop them from getting knocked down by the wind.

HOWEVER

Put it on top of the soil. That way it can also help to stop the soil from drying out so quickly.

2 – Gravel can bulk out a pot that’s too big

If your pot is too big and you fill it with soil anyway, you can end up overwatering because the soil won’t dry out quickly enough. Filling some of the pot with gravel (or something else non-absorbent) will stop this from happening.

I get a bit annoyed when people say that it’s fine to have pots without drainage holes because whilst I know that it totally can be fine, it’s waaaay easier to have drainage holes.

Fin.

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