From The Old Farmer's Almanac.
First published March 6, 2008
There is one common garden product that has not only revolutionized container gardening, but also helped keep our backs from slipping out of alignment. It’s name? Perlite…although at the greenhouse, it occasionally answers to godsend.
Most of us recognize perlite as those round, white pellets commonly blended into potting soil mixtures—you know, the stuff that looks like Styrofoam beads. But while perlite isn’t as lightweight as Styrofoam, it still has a fairly low density, which has a huge impact on the size of containers we choose to fill and whether or not we can straighten up our backs after a day of moving them around the garden.
Step aside, sand
For many years, sand was the particle of choice for containers because it was one of the few readily available materials that provided the drainage necessary for plants to grow properly. But sand has one major drawback: weight. At a whopping 2000 kg per cubic meter, sand can transform even a relatively small pot into a back-breaking heavyweight. And the fun doesn’t stop there. Large hanging baskets that contain sand as a base component are not only tough to hang, they’re also tough on the old cranium should they come crashing down.
Interestingly enough, the perlite that our backs have come to love wouldn’t be much lighter than sand if it weren’t super heated and transformed from its natural state—a volcanic glass-like rock. Once the raw perlite hits a temperature of about 870 C, it explodes into a low-density popcorn-like particle. At about 30 kg per cubic metre, perlite is only a fraction of the weight of sand yet equivalent in its ability to improve soil drainage. Even when dealing with clay-rich garden soil, sand still isn’t the best choice for improving drainage. It might seem like the right choice, but because it takes such a vast amount of coarse sand to change the texture of clay soils, the addition of small quantities often does nothing more than transform clay to concrete. No seemingly good deed goes unpunished!
Getting it right
In standard potting soil mixes, perlite is added to counterbalance the sponge-like, water holding capacity of peat moss. Translation: peat moss tends to absorb large amounts of water, and the perlite lets it move through the soil and out the container’s drainage holes. Good quality potting soils should contain anywhere from 10 to 20 per cent perlite. I tend to like those that nudge the 20 per cent side because roots breathe best in a more porous mix. Having said that, the reality of many “professional” potting soils is that they contain what I suspect is no more than one per cent perlite—a mere window dressing. Unfortunately, a few bits and pieces scattered throughout a bag will have virtually no effect on the structure of the potting soil. The solution: take a good look at what you’re buying. Authentic professional soilless mixes have light-coloured, long-fibred peat moss and lots of coarse perlite scattered throughout.
A second option is to buy all of the components individually and mix them yourself. The advantage, of course, is that you can customize the blends. The disadvantage, however, is that custom blending can be a bit messy and dusty. Perlite dust in particular can irritate the eyes and throat, so if you blend your own mix, do so in a well ventilated area or wear a mask.
That’s perlite in a nutshell—a fantastic product that allows gardeners to grow plants in much larger containers, which in turn produce better looking, larger plants. I know I wouldn’t be without it. After all, besides amending my soil, it also allows me to enjoy my containers up close rather than longingly through my living room window whilst I lie on my couch with a hot water bottle on my back.
The photo is of two Polyanthus Blue Shades plants. I love the way the reflexive flowers are held in the air, saying please pollinate me to passing insects.