Get to know your potting mix: Vermiculite and perlite

Look up the word "vermiculite" in Henry Beard and Roy McKie's hilarious "A Gardener's Dictionary" and you'll see it defined as an "obscure order of nuns devoted to gardening." In fact (as opposed to in fun), it is a silicate material similar to mica that is sometimes found in the potting mixes gardeners use to start seeds in spring. The word, from Latin, means "breeds worms."

I think I can explain that. When my sisters and I were little, we would take a handful of our parents’ vermiculite, pour water on it and watch in awe as the dry, compressed flakes expanded to form wormlike columns. This is what vermiculite does in potting soil: Because it is spongy and absorptive, it holds water, so you don’t have to water a container so often. This is especially important with plug trays or soil blocks into which seeds are sown, where the mix can dry out quickly and put tiny seedlings at risk. Organic matter plays a similar role in soil, but vermiculite, mineral by nature, is sterile and inert, thus protecting the seedlings against a fungus that causes sudden collapse — damping-off — and other ills.

Gardeners don’t use vermiculite quite as much as they used to, in part because of an environmental disaster. Vermiculite mined at a plant in Libby, Mont., once the main source of the material, was found to be contaminated with asbestos fibers. Though the plant was closed and the industry reformed, people have, to some degree, made the switch to perlite.

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Perlite is made from a mined volcanic glass of the same name. As a raw material it contains water, trapped by the rapid cooling of lava. The moisture vaporizes explosively when heat is applied. The result is a much-expanded mineral popcorn, white in color thanks to light reflecting off tiny bubbles on the surface of its particles. It has a texture that retains water on that surface (though not in the volume that vermiculite does) but retains air in the spaces between. That lightens your potting mix considerably, as well as providing valuable oxygen for plant roots, along with better drainage than vermiculite. Most potting mixes contain at least 25 percent perlite, which is why they look as if a takeout container had been chopped into bits and stirred in. But it’s a harmless mineral and, like vermiculite, sterile and inert.

Each material has its uses. For seed-starting, I go with a vermiculite mix for my germination but a perlite mix for growing in pots. A mix containing both can also be valuable. It is easy to mix your own, but make sure you buy horticultural-grade vermiculite and perlite. Both are available from places that sell gardening supplies, bricks-and-mortar and online. If you are using either in quantity — to lighten soil in large containers or even in small garden beds — you’ll want to shop for prices.

Some gardeners use them to store dahlias or bulbs over the winter. Or, as vermiculture mavens consistently do, employ vermiculite as a medium for, yes, breeding worms.

Damrosch's latest book is "The Four Season Farm Gardener's Cookbook."

Tip of the week:

Final call to prune rosebushes: Use thick gloves and lopping shears to remove dead, weak and conflicting canes. Keep about half a dozen canes around an open center and cut them back to about 18 inches, just above an outward facing bud. Remove any suckering canes emerging from the soil.

— Adrian Higgin

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