Vermiculite, perlite’s lesser known cousin.
This naturally forming mineral may not be the go-to plant soil/substrate amendment for people yet, but it’s actually better than perlite at some things.
Honestly, you just need to know its uses.
It’s a hit for growers with the thirstiest of plants, so maybe it’s just what you’re missing?
Personally, I’ve used vermiculite in a variety of ways with my tropical houseplants with great success.
So, in this article we’re going to deep dive on this fascinating material, finding out what it is and how to best use it to grow your plants.
Let’s dig in.
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What is Vermiculite?
At first glance, vermiculite looks like a load of tiny accordion sponges with an earthy golden hue.
And, in reality… it’s exactly that.
Technically it’s a name for a group of “hydrated, laminar magnesium-aluminum-iron silicate minerals” (usually biotite or phlogopite) but we’ll stick with the term “vermiculite” for obvious reasons.
Made from volcanic rock – or more specifically a variety of minerals from it – it’s been expanded under intense heat to form inert, sponge-like mineral flakes.
It’s this spongy texture that makes it such a versatile material.
Able to hold on to enormous amounts of water, it’s a great fit for moisture-loving plants and a variety of horticultural processes that need consistent moisture, i.e. propagation.
Plus, it’s a literal mineral, which is full of all kinds of important elements for plants.
What is Vermiculite Made Of?
At an atomic level, vermiculite looks a bit like a stacked ice cream sandwich.
The concertina ripples representing a repeating set of calcium layers, each sandwiched between two planes of aluminium and silicone sheets – with some interspersed calcium and potassium for good measure.
The thing is, all of these elements are great for plant growth.
So, alas not the tastiest sandwich in the world for us, but your plants will love it.
Vermiculite vs Perlite
A quick primer on these two because the question will be inevitably asked.
Both perlite and vermiculite are both expanded volcanic rock, but the difference in the type of rock that’s used produces very different qualities.
Perlite is like polystyrene balls, fully fluffed out and fixed in position. It can hold on to some water on its porous surface, but it cannot expand to absorb water. The stability of perlite makes it poor for water retention, but fantastic for drainage as it’s always going to resist compaction.
On the other hand, vermiculite is able to expand dramatically to absorb moisture – making it excellent for water retention.
Though, its ability to expand and compress makes it not as effective at providing soil structure or root aeration (vs perlite, but it still helps!).
In short, perlite is better for drainage, vermiculite is better for water retention. Though, there is some crossover.
Now, back to vermiculite!
Benefits of Vermiculite
1. Vast water retention ability – vermiculite is able to hold 3-4x its weight in water, so they can act as a consistent reservoir of moisture for your plants to access at any time.
2. It’s full of beneficial nutrients – plants need their minerals too! Plus, the ease of which both roots and the water itself can pass through the material makes it very easy for plants to access these nutrients.
3. It can regulate pH – vermiculite itself has a neutral pH, but it has a high buffering capacity. Meaning it can neutralize any undesired acidic or alkaline effects.
4. It won’t rot, degrade or mold – because vermiculite is chemically inert and inorganic, it’s able to hold up extremely well in moist settings. It also means it can be used to block pests when used as a top dressing.
5. Holds on to liquid fertilizer – being the sponge that it is, vermiculite is able to absorb the water-soluble nutrients from liquid fertilizers that would otherwise flow straight through the soil and out the bottom.
Convinced? See vermiculite on Etsy.
What is Vermiculite Used For?
In an Indoor Plant Mix
Any moisture-loving tropical plant is going to benefit from the addition of vermiculite to its substrate mix.
It’s not just about being able to access a large volume of water, it’s generally more about the consistent presence of it.
Ferns in particular tend to suffer if they are allowed to dry out for any amount of time, and so a heavy vermiculite blend can (just about) ensure they never go without water.
It can also help with rapid growers by absorbing the excess nutrients from liquid fertilizers and releasing them over time.
I like to use around 15-30% vermiculite as a nicely balanced ratio for a typical tropical blend, but you can go as high as 50% if you really need the extra moisture..
In a Terrarium Mix
For all the same reasons above, vermiculite can be a helpful addition to terrarium mixes too.
Terrarium substrates must maintain a consistent level of moisture – as you can’t really let them dry out – and so the absorptive and rot-resistant qualities of vermiculite can bring a lot to the table.
Vermiculite is a cheaper and much more sustainable material than sphagnum, but admittedly it’s not as natural looking – so I don’t see it used all that often.
Check out my full guide to terrarium substrates for more detail.
Plant Propagation and Seed Germination
Again, vermiculite offers a viable alternative to sphagnum moss.
When rooting a plant cutting or germinating a seed, you need to provide a consistently moist environment with plenty of airflow to allow for healthy root development.
Of course, vermiculite can provide both.
The granular shape of vermiculite allows for plenty of space for roots to grow between, and the fact that it’s so lightweight means that the roots and sprouting plants can easily penetrate it.
You can go 100% vermiculite here if you like, or mix it with other light materials like coco coir.
Much better than a heavy mix!
In the Garden
I’m a purely indoor gardener (somewhat by choice, partly because I don’t have a garden) but I know vermiculite is regularly used as a soil conditioner outdoors.
Much like indoor propagation, vermiculite can be effective in seedling beds to really promote early growth.
It can also be helpful if you’re dealing with soil pests (like knats – urgh) as they won’t be able to penetrate the vermiculite if you use it as a top dressing. If they can’t get into the soil to lay their eggs, you solve the problem.
Where to Buy Vermiculite (and What Type to Get)
Vermiculite is actually used in a variety of industrial settings too, for insulation and filling etc.
So, be careful what kind you’re buying because they can differ quite a bit.
Avoid buying “construction grade vermiculite” as it can reportedly contain other undesirable substances like pesticides and chemical toxins (there’s even been issues with asbestos-contaminated vermiculite).
Stick to horticultural vermiculite and you can’t go wrong!
You’ll find it in various “grades” with different particle sizes. The smaller grades are perfect for seeds and small plant propagations, and the larger grades can be a little too big so I tend to opt for medium grade.
Vermiculite is a pretty common material found in most garden centers and the like, but as with most of my supplies I like to buy from Etsy.
Over to You
Have you had success growing with vermiculite?
I’m especially keen to hear if you’ve used it as part of a terrarium blend.
Let us know in the comments!