What Is Perlite Made of?
Perlite is made from a naturally occurring volcanic glass formed by the rapid cooling of lava. It has a significantly higher concentration of water trapped within its structure than most rocks. When the volcanic glass is heated to extremely high temperatures, the water it contains turns to steam that pops the volcanic material like popcorn. This creates the familiar-looking, puffy, Styrofoam-like material known as perlite.
Why Is Perlite Good for Plants?
When added to potting mix, perlite creates a lighter, fluffier soil. With its low density, it helps create air pockets and channels, reduces compaction, and allows for improved water and air movement in the soil.
Better water movement means improved drainage for plants so soil conditions don’t stay overly wet. This allows better air circulation around roots, which need oxygen to stay healthy. If they can’t access it due to soggy conditions, the plant suffocates.
Are Perlite and Vermiculite Interchangeable?
Though perlite and vermiculite are both commonly added to potting mixes to improve drainage and moisture retention, they are not the same thing.
Vermiculite is best suited for enhancing moisture retention in potting mixes, although it also has a secondary benefit of improving soil structure and drainage. Perlite is best suited for creating lighter, well-drained potting mixes. It keeps the soil loose and reduces compaction, but also provides a secondary benefit of modest moisture retention.
Choose perlite for aeration and drainage, and vermiculite for water retention. For a memory aid, the V in vermiculite and the W in water are next to each other in the alphabet. A good general-purpose potting mix often includes both perlite and vermiculite.
How to Use Perlite
Perlite is easiest to use when preparing a new potting mix for your potted plants, seedling starts, or cuttings. It wouldn’t make much sense to attempt to add it to existing plant pots by working it into the soil, as this would damage the plant’s roots in the process. Use it to create a lighter potting mix with excellent drainage.
Perlite in Potting Mix
There are many recipes out there, depending on your available ingredients and the plants you’ll be using it for, but most potting mix recipes call for about one-quarter to one-third of the material by volume, not by weight, to be perlite. For example, use two quarts of peat or coir, one quart of compost, and one quart of perlite to make a gallon of potting mix (add a little slow-release fertilizer, too).
Perlite can also be added to improve store-bought potting soil. Since purchased potting soil may already have some shredded bark for drainage, you’ll likely need to add less perlite than when making your own potting soil from scratch. For most bagged potting soils, try adding approximately 10 to 15% by volume and mix well. For example, in a one-gallon container, add approximately half a quart (one pint) by volume of perlite. An old 16-ounce yogurt or sour cream container makes a good measuring cup.
Perlite for Seed Starting
Seed starting mixes should be light and loose to allow for good seedling root growth. They should also be well-draining, yet still hold a bit of moisture. Add perlite to increase drainage in either store-bought products or DIY recipe. A popular formula is to mix peat or coir, perlite, and vermiculite in a 2:1:1 ratio.
Perlite for Propagating Plants
Taking and rooting cuttings is an inexpensive way to multiply your plants. More so than seedlings, however, cuttings need steady moisture, good drainage, and a growing medium with a light, fluffy consistency. Cuttings can be rooted in pure perlite that is kept consistently moist, or in a mixture suitable for starting seeds. An easy way to propagate softwood cuttings is to place them in a sealable bag containing perlite and enough water to moisten it, then seal the bag to retain moisture while the cuttings are rooting.
Should You Add Perlite to a Garden?
Given all the soil and plant benefits of adding perlite, you may be tempted to use it for your garden beds. Except for a very small raised bed, buying perlite at retail prices to add to garden soil would be prohibitively expensive. To incorporate 10 to 20% by volume of perlite into the soil mixture of a standard 4x8-foot raised bed with a depth of 10 inches would require about three to four cubic feet of perlite.
Frequently Asked Questions
Can perlite go bad?
No, perlite won’t go bad. It is made from volcanic rock, which won’t go bad or expire, and neither does the perlite. It won’t rot or mold, although if you leave moisture in the bag or have other materials in it, mold could grow.
Will perlite stop fungus gnats?
Fungus gnats need moist soil to lay their eggs. Adding perlite to your potting mix will improve drainage. However, a layer of perlite on top is not likely to do more than slow them down unless you apply it very thickly, which could become messy.
Is perlite dust harmful?
Perlite in a bag is quite dusty, and inhaling the fine white dust is unpleasant and irritating to the respiratory system. To limit dust, use a scoop or do the pouring of the contents outdoors. Another option is to wet it down by pouring a small amount of water into the bag, holding it shut, and shaking it up before removing the perlite.
