Tony O’Neill, gardener and author of the popular “Composting Masterclass” and “Your First Vegetable Garden,” combines lifelong passion and expert knowledge to simplify the art of gardening. His mission? Helping you cultivate a thriving garden. More on Tony O’Neill
As I sit in my slapped-together backyard garden, I try to remind myself what it was like before ‘perlite,’ ‘vermiculite,’ and ‘drainage’ became words as commonly used in my household as ‘breakfast’ and ‘television.’ I wondered If I could use perlite for drainage instead of gravel.
Perlite is a mined volcanic Glass that is heated until it explosively pops into a round white and light material and is perfect for use in place of gravel. It performs the same when drainage is required, but weight is an issue. It is ideal for soil mixes, including seed starter mix and has many uses in horticulture.
Perlite for drainage
Like many of you, I have decided to develop my green thumb during these uncertain times. If you’ve also become an amateur gardener like me, you’ve likely heard the word ‘drainage’ at least a dozen times.
You may have read that some plants thrive in wet environments, but most prefer ‘fertile, well-drained soil’ – or so it says on those little seedling tags we get from the store.
Everyone wants good drainage while ensuring their plants get sufficient water for good health and growth. Sound complicated? It turns out it’s not.
What is Drainage?
So what is drainage, and why do we need it? Well, proper soil drainage is crucial to avoid plant roots sitting in water unless we’re growing water lilies or other aquatic plants.
This can lead to rot and, essentially, drowning those plants you’ve so lovingly tended to from seed or carefully transported from the store without tipping over more than once.
Like bakers, gardeners also thrive on clever substitutions to save money and reduce overall waste, not to mention the opportunity to brag about the nifty new use we’ve found for pantyhose.
Two words: watermelon hammock!
In today’s life, we can find an alternative for eggs, milk, cheese, meat, gluten, face cream, and energy sources; you name it! So what about in our very own backyards?
Can we use perlite instead of gravel?
To answer this question, it’s necessary to explore first the uses of each around the home.
Gravel or fine sand has many uses in the garden and around your home, from paths and walkways to water features to uses as a mulch layer to suppress weeds and prevent water loss.
When spread in thin or thick layers, the weight of the tiny rocks makes it difficult for weed seeds to germinate and push through.
This technique can even be used around plants if we carefully ensure a 4 – 6 inch clearance around the stems of long-lived perennials. The main downside to using gravel between plants is that it is tough to remove once laid.
To test this theory, try spilling a pile of sand on your garden soil, then try removing it. This is why many resourceful gardeners choose it as a long-term mulch layer around trees, shrubs, and succulent gardens that they don’t intend on altering or moving around too much.
There is an old gardening wives’ tale that adding gravel to the bottom of your potted plants increases drainage. This theory has been tested repeatedly, and now the gardening community mostly agrees that this only adds weight and reduces the growing space in a pot, essentially making it smaller.
Adding tiny rocks or sand to your garden soil is only beneficial if you have dense, clay-like soil that holds moisture like a bog.
Amending it with gravel or sand can be the difference between being able to grow absolutely nothing to becoming farm-‐to-‐table. In this case, the rocks break up the clay, allowing water to run through and down to the ground below.
Using Gravel In Landscaping
What about landscaping? This is where gravel has over a hundred and one uses, such as for pathways, in between bricks or rocks, in trenches for drainage, or as edging around water features, to name a few.
Some people even use gravel for their backyard, forgoing the constant need to mow grass that never stops growing!
This, of course, is a much lower maintenance option than a lush golf-course- an expanse that must be mowed weekly to avoid looking like a nightmare.
Don’t try to move any of that gravel once it’s down. Your best option would be to cover it like the pimples on an adolescent face for prom.
So now that we know a little more about gravel’s uses, let’s move on to perlite. Despite sounding like an obscure shade of white among the visual arts community, perlite is quite common and useful to the modern gardener.
Using Perlite In The Garden
As you may or may not know, potting soil is used for, yes, you guessed it potted plants. Why? Because it accomplishes three main things:
It has good drainage and aeration and retains water longer than traditional soil mixtures.
What gives potting soil better drainage and aeration than other garden soils? Usually, it’s perlite, an insulating Styrofoam-like volcanic glass that pops like popcorn when heated.
It is chopped into small pieces and mixed with several other ingredients to create sterile potting soil.
At first glance, it looks like a pile of light-colored gravel.
When heated, perlite pops and expands up to thirteen times its previous state, consisting largely of air. Besides lightening your load when carrying your potted plants, this creates good aeration in the soil, insulating your plants from temperature fluctuations.
It also absorbs moisture like a sponge, meaning your pots take longer to dry. If you live in an area where the cost of water is high or if you have a long distance to lug your watering can to and from the rain catcher. Just me? Moving on, then!
The next time you see a potted plant, take a gander inside the pot it’s sitting in. You’ll likely find the dark soil mixture sprinkled with little white dots like a cloudless night sky.
That would be perlite or vermiculite, but that’s a different article! Due to the high air content, perlite rises to the top of the pot as the soil gets watered, which is why older potted plants often look like they have a thick layer of foam at the top.
Substituting Perlite for Gravel
Now that we know some of the uses and benefits of gravel and perlite, we can ask whether they can substitute for each other.
Honestly? The answer to this depends on what the project is.
You’d like to create a walkway that winds through your garden. You have no plans to use the walkway area as a cultivatable ground in the future and don’t want weeds popping up all over the place faster than beads at Mardi Gras.
Gravel may be your best bet here. In this case, the weight of the gravel is significant, so would perlite be a good substitute? Because it is very light, it would not be able to suppress any weeds and likely last a very short time.
Just think about how a thick layer of finely chopped Styrofoam would behave in the wind. This is why replacing the gravel with perlite as a mulch layer around your shrubs or hardy perennials wouldn’t be feasible.
Perlite as Drainage
What about creating a drainage trench in a wet area or surrounding a water feature? The idea here is to create a deep trench and fill it with gravel to encourage water to drain instead of sitting on the grass, soil, or whatever ground cover you have.
Creating a muddy bog. This strategy effectively keeps the soggy, wet ground from bleeding into gardens and walkways.
Gravel is the solid first choice here, but could perlite be? Unlikely. The lightweight volcanic rock would absorb water like a sponge and trap water in your trench, negating the entire point of the project.
With everything considered, what about using perlite between bricks or stones like dry mortar to create a pathway or landscaping feature?
It would only expand in the rain and push the bricks or stones further apart from each other, ruining whatever painstaking time you’ve taken, leveling every single brick.
Is there any situation in which perlite can be substituted for gravel at all?
Yes! As mentioned above, many gardeners swear by using gravel or sand mixed in with their regular garden soil to improve drainage. While gravel is best used as a top mulch layer for drainage, mixing it with other soil can help break up the composition and allow air pockets so water can travel downwards in difficult soil.
Now that sounds like something Perlite can do. Although some consider it controversial, if you don’t have gravel or fine sand to mix into your garden soil and if you notice your soil seems to feel more like dense river clay, you can mix in that old bag of perlite that’s been sitting in your grandmother’s greenhouse for over a decade collecting dust.
Like in potting mixes, it will aerate the soil, improve downward drainage and, ultimately, make your hard ground fertile to grow your food.
Although perlite, in some instances, can replace gravel as suggested, each has its special characteristics and is suited to individual scenarios.
Consider perlite when lightening the soil in heavy containers or for young plants. But as far as landscaping goes, I would suggest sticking with gravel.
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Remember, folks, You Reap What You Sow!