Using Perlite for House plants – FAQ

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Perlite is one of those things you can go your whole life never knowing about…and then all of a sudden it’s a major part of your life.

Perlite is quietly game-changing when it comes to house plant care.

It can elevate your store-bought house plant soil into something that can make your plants happier and your life easier (to which we say hurrah).

Also, it’s cheap. We stan.

What is perlite?

Perlite looks like fine white gravel, but it’s REALLY light.

I have a massive bag of the stuff, and it’s so light it inevitably goes flying upwards when I go to pick it up because I overestimate how heavy it’s going to be.

Perlite starts its life as volcanic glass called pyrite (hence its other name, expanded pyrite), but it has a rather unique property – it pops (think popcorn) when it’s heated. Once it’s popped, it becomes extremely porous, making it a great addition to potting soil.

(by ‘heated’ we’re talking 1600oC – this isn’t something you can do at home with your hob and an old pan)

The other weird property perlite has is that it retains moisture on the outside, not the inside, which makes it really good for roots to absorb.

It’s to house plants what hyaluronic acid is to skincare – it traps water.

However, the porous nature of perlite means that the water does drain away pretty quickly.

For those of us that are chronic underwaterers, vermiculite, which retains more water, might be a better option. I’m an underwaterer, but I prefer to use perlite, you know, just in case.

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What are the benefits of using perlite?

Perlite aerates the soil

Keeping a good amount of oxygen around your plant roots is the key to keeping root rot at bay (read why here), and perlite is a super simple way to keep your soil aerated.

The size irregular shape of the granules (is that what we’re calling them? I’m getting gravy vibes from ‘granules’) can trap a lot of air, and allow air movement through the soil.

Perlites reduces soil compaction

Soil compaction occurs when the soil, er, compacts, and can cause root rot due to lack of oxygen (because lack of oxygen can mean an increase in anaerobic bacteria).

Perlite is very lightweight, has pretty large granules, and doesn’t clump together like compost or dense house plant soils.

Compacted soil can also lead to inadvertent underwatering since after a while the soil can become hydrophobic.

This basically means that the soil is so compacted that water can’t penetrate (sorry) it, aaaand since water always follows the path of least resistance it’ll simply run down the gap between the pot and the soil and out of the drainage hole at the bottom.

Adding perlite to soil really helps soil remain uncompacted, and is less invasive than using a chopstick or similar to break up the soil. If you already have compacted soil, soak the soil in water (just set the whole pot in a bucket) to soften it before adding perlite into the potting mix.

Perlite aids humidity

Remember how moisture is trapped on the outside of the granules? As it’s released, it can contribute to the humidity around your plants – great news if you have plants that love humidity, such as calatheas and hoya.

Just remember that the humidity from perlite isn’t going to have a MASSIVE effect on the ambient humidity of a room, so if you suspect you have dry air you may still need a humidifier as well.

I always recommend that people get a hydrometer (hygrometers are pretty cheap – this is the one I have from Amazon) before they get a humidifier, just so they know how much additional humidity they need.

Perlite helps with drainage

This is key if you’re an overwaterer.

Yes, perlite locks in moisture for a bit, BUT it’s very porous and allows for water to drain quickly from your soil.

It’s for this reason that perlite is still recommended for use in plants that prefer dry soils like succulents.

Perlite is a cheap soil additive

Especially if you buy in bulk.

You can buy perlite from Amazon – it’s quite cheap when buying smaller amounts BUT if you have a tonne of plants, you can often pick up big bags from garden centres pretty cheaply.

It’s about £9.99 for 4 litres (garden centres and Amazon are similarly priced when buying this volume), but I bought 20 litres for £20. Bargain.

If you only have a few plants you don’t need 20 litres of perlite. It’s a lot.

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What are the disadvantages of using perlite?

Perlite can leach fluoride into the soil

Ok, to be honest, I’ve only learnt about this today, but it’s worth mentioning.

Perlite is relatively inert – it won’t add or take nutrients from your soil. But apparently, it can cause fluoride burn. Great.

I don’t know how common this is. I don’t typically have a lot of brown tips on my house plants, but now I’m looking around my office suspiciously.

The source I linked above has a little table of plants that are especially sensitive to fluoride, and OF COURSE Marantaceae are on there because what aren’t they sensitive too?

Also no surprise re. peace lilies.

Dracaena and spider plants are also plants that don’t like so much fluoride, and they’re also renowned for getting brown tips on their leaves. Hmm.

If you’re worried about fluoride burn, this article gives instructions on how to (in their words) ‘sweeten the soil’, i.e. raise the pH.

Perlite looks pretty crappy

I mean, it looks like styro foam. Or mould. Or eggs.

It’s just not aesthetically pleasing (unless you’re JUST using perlite, in which case it looks pretty cool).

And OF COURSE it’s super light so it floats to the top.

Great.

There’s not really a lot you can do about this other than keep either stirring the perlite back in or re-top dressing your soil with worm castings.

Personally, I just accepted it and got used to them.

Perlite is very low weight and might blow away

I have an article here about why you shouldn’t put rocks in the bottom of plant pots and someone left me a common regarding something I hadn’t even considered – rocks in plants pots stops them from blowing away.

I mean…yeah.

As you may know, I’m a great advocate for putting my plants outside in summer, after being too scared for ages, and plants blowing away can be a real problem ESPECIALLY WHEN PERLITE IS INVOLVED.

Seriously, it’s quite hard to appreciate how light something is when to all intents and purposes, it’s rock.

Solution?

Put rocks on top of the soil. Stops the plants and the perlite from blowing away, and it can help keep moisture in the pots since plants dry out much quicker outside.

Perlite is a non-renewable resource

I’m yet to find a definitive answer on how much pyrite there is in the world and how environmentally friendly it is.

One of the reasons I switched to leca is that it’s is reusable. Whilst I do still use perlite to make my soil, I also add orchid bark for drainage.

Perlite is very dusty

As someone who has used perlite a LOT, please, please, please, use a mask and safety glasses.

I’m a very messy person, so I make my soil outside, but I still wear both protective eyewear and a mask. Perlite is VERY dusty and appears to have a magnetic attraction to the insides of human lungs.

Protect your lungs and eyes, and the lungs and eyes of any humans/animals around you.

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Can you grow plants in just perlite?

Yes, you can use perlite as a soil-less medium – it’s an alternative to leca.

I can’t find any information on why leca is better than perlite, so I’m just going to go with my experiences.

Perlite is great for propagating cuttings (check out the leca article I linked above for instructions – it’s the same process) and it could be great for growing plants semi-hydroponically IF it were a little more practical.

But it isn’t really, because it’s too light.

It’s bad enough keeping plants upright in leca, when they’re first transplanted, never mind a medium like perlite that’s considerably lighter.

Also, from an aesthetic standpoint, algae is a very real thing in hydroponics. It doesn’t look great on leca, and would look a million times worse on the pristine, crisp whiteness of perlite.

If you have solutions to either of those problems, go ahead – plants should grow perfectly happily in just perlite.

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Can you use styrofoam instead of perlite?

Bizarrely…yeah. Kind of.

If you have the right kind of styrofoam (the really thick stuff, not the thin stuff that you use for packing gifts) it works pretty well.

I mean, it’s not ideal. Perlite and styrofoam may look pretty similar (and they’re both light af) but if you’re thinking of ways to get rid of some styrofoam you have lying around, it might not be the best idea. It could have been treated with chemicals, or just have picked up general grime on it’s way to you.

You COULD use styrofoam, but I wouldn’t recommend it. Apart from anything, it’s a disaster for the environment, and if you use old stuff you have laying around at home, you could introduce something nasty into your plants.

I’d stick with perlite.

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Is perlite toxic to humans?

Perlite isn’t toxic, but as i mentioned earlier, it’s incredibly dusty.

The dust in itself isn’t the worst thing in the world, but that paired with the fact that perlite is so light mean that the dust hangs around in the air for a long time.

Wear a mask, eye protection, and make everyone around you do the same.

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Should I add perlite to my soil?

Yes. For those of you that don’t want to make your own soil, I still highly recommend adding a bit of perlite to store-bought house plant soil.

As for how much, it really depends on how much attention you give your plants.

As I mentioned above, you can grow plants in 100% perlite, so you can use anything from 0% perlite to all perlite.

I tend to recommend a 70:30 soil/perlite mix, but if you’re an overwaterer that likes to fertilise (more perlite means fewer nutrients, simply because there’s less soil) feel free to do 50:50.

I just eyeball it (and use a scoop, because in my experience perlite comes out of the bag slow…slow…ALL THE PERLITE).

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Can I use sand instead of perlite?

Perlite and sand aren’t the same, and don’t have the same properties.

Sand can help with easing soil compaction and improving drainage because sand doesn’t absorb water and doesn’t clump (as long as you use non-clumping horticultural sand).

It’s great to add to the soil of plants like succulents that don’t like to sit in damp soil BUT you might find you need to water more frequently.

It’s a myth that succulents don’t need a lot of water – they like a lot of water, but they do NOT like to have damp soil. More water = more growth but the soil MUST dry out between otherwise you end up with root rot.

But sand doesn’t hang on to water, and it doesn’t really improve soil aeration, so I wouldn’t recommend using it for plants that like their soil to retain a bit of water (i.e. most plants except for succulents).

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Can I use perlite instead of leca?

Yeeees, but I outlined above why it’s not the best idea.

It’s great in theory, and for propagating cuttings, but it would be impractical over time simply because of how light perlite is.

Also, algae growth would make it look grim and green pretty quickly. Algae isn’t a huge issue (although it can steal nutrients away from your plant) but it grows really quickly in clear containers and its better to, you know, not have it.

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Does perlite decompose?

No, it remains pretty much the same over time.

If anyone knows of a way to separate it from soil, so we could easily reuse it, let me know. I mean, other than just picking it off when it rises to the top.

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Does perlite lower pH?

Perlite is pretty much neutral BUT (we’re back to fluoride here) it can go up to 7.5 due to the release of fluoride.

To be honest, it’s not alkaline enough to worry about unless you have a super fancy-pants plant (and I see plant people using perlite on super rare plants all the time) and you can always lower the pH using something like leaf mould.

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Can egg shells replace perlite?

No.

I really don’t like to recommend people use eggshells for indoor plants, no matter their advantages, because you’re extending an open invitation to every fungus gnat and fruit fly in a hundred-mile radius.

It. Is. Not. Worth. It.

Gnats aren’t that big a deal when it comes to the health of your plants, BUT once you know the hell of not being able to have a quiet glass of wine without having someone SWIMMING IN IT you’ll know why I vetoed all DIY food-related soil accoutrements.

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Does perlite contain asbestos?

I HOPE NOT

And according to this article, most likely not.

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Does perlite hold water?

Yes, but not like, amazingly well. Which is a good thing!

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What’s the difference between vermiculite and perlite?

Vermiculite is an aluminium/iron/magnesium silicate that, like perlite, is heated until it expands.

Vermiculite can soak up to 4 times its volume in water, so it retains far more water than perlite BUT it isn’t as good as aerating the soil.

I prefer to use perlite – I’ve found that vermiculite is too good at retaining the water, and the soil remains damp for longer than I’d like.

There is no right answer here – if you find that your plants dry out too quickly, and you’re watering every few days, vermiculite could really help.

If you live in a warm country, vermiculite would be a great soil additive.

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What’s the difference between perlite and pumice?

I much prefer pumice to perlite because pumice has all the positive properties of perlite (increases soil aeration, retains water, improves drainage) but has two key advantages:

  1. Pumice is heavier than perlite, so it doesn’t blow away or float to the surface
  2. Pumice doesn’t look as naff as perlite

Unfortunately, pumice is more expensive than perlite and I can’t quite justify the cost.

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Where to buy perlite

I highly recommend trying out your local garden centre before buying online. Some things (terracotta pots for example) are considerably cheaper to get from a garden centre, and found that bulk buying perlite is one of them. Take a friend and split the bag (WEAR A MASK)!

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I hope I answered all of your perlite questions – if you have anything else to ask or add, please leave me a comment below!

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