Beginners sometimes confuse vermiculite and perlite. Indeed, there is significant overlap between them, because both are classified as either growing media (for starting seed) or soil amendments. In some cases, there may be little harm in using them interchangeably. But because each has its own unique properties, one will work better than another for certain applications.
Distinguishing between vermiculite and perlite (and knowing when to use one rather than the other) can really pay off. Here's how vermiculite and perlite differ, and how to know when to use each.
What Is Vermiculite?
Both vermiculite and perlite are made up of small, granular-type pieces, but a close examination will reveal how they differ.
Vermiculite is a silvery-gray, flaky material sold by the bag at home improvement stores. A popular size is 2 cubic feet, which is somewhere between 10 to 20 pounds when dry.
Vermiculite is mined from the earth in its raw form. It is then exfoliated and subjected to extreme heat and pressure. This treatment both sterilizes it and forces it to expand. The result is a clean, fluffy product that can serve a number of functions for gardeners. Vermiculite also contains minerals that are helpful to plants, although it does not contain any added fertilizer.
Uses for Vermiculite
As a soil amendment for the garden, vermiculite helps loosen the soil and retain water and nutrients. But vermiculite isn't just for growing things. It's also an effective medium for storing tender bulbs over winter (it absorbs any moisture that could cause the bulbs to rot).
Its top use, though, may be as a growing medium when starting seed (both indoors and outdoors). Plant roots readily rap around its nooks and crannies, thereby anchoring themselves, and its fluffiness allows it to retain water like a sponge. So whether you're starting annual seed in a greenhouse or starting a lawn from seed, a thin layer of vermiculite covering the seed helps prevent it drying out. Plus, the sterility of fresh vermiculite ensures you aren't exposing your seed to pathogens.
What Is Perlite?
Like vermiculite, perlite is mined and then subjected to extreme heat and pressure until it expands (rather like popcorn). After this treatment, it becomes a gardening product valued as a soil amendment. But its appearance is distinct from that of vermiculite: Perlite is white and chunky. Even its texture is different: Perlite feels like styrofoam.
Perlite is sold in bags at home improvement centers. Common sizes are 2 cubic feet and 4 cubic feet. Professional growers buy perlite wholesale in larger bags, but these can be a bit bulky for the consumer. A bag that is 2 cubic feet weighs about 8 to 16 pounds when dry. The 4 cubic feet size is more manageable in this case than with vermiculite. The difference in weight comes from the fact that perlite is airier than vermiculite. This airy quality furnishes a clue as to its uses.
Uses for Perlite
Its airiness gives plant roots greater access to oxygen and promotes drainage. Perlite contains no nutritional value for plants, so its function is not to feed plants but to alter the structure of the soil in a beneficial way. It is used as a soil amendment both for containers and planting beds.
Waterlogged soil leads to root rot for many plants, which makes perlite useful as a readily-available soil amendment that promotes drainage. Most gardeners are familiar with the little white chunks you find when you dump the potting mix out of a hanging pot at the end of the growing season.
Can You Use Vermiculite and Perlite Interchangeably?
While there is some overlap between vermiculite and perlite (both generally improve drainage), their different properties often help you decide which is better to use in a particular circumstance.
Both help retain water, but they do so in different ways and to different degrees. Vermiculite absorbs it like a sponge. Its flakes actually expand when it sops up water. Perlite doesn't so much absorb water as it does hold water within the tiny crevices that pockmark its surface. Of the two, vermiculite does a much better job of retaining water.
Contains minerals beneficial to plants
Holds water in surface crevices
No nutritional value for plants
When to Use Vermiculite vs. Perlite
Vermiculite is the logical choice when you want a soilless potting mix that must retain water. Use it, for example, in hanging pots with plants that need a lot of water, such as Impatiens. In planting beds, it's a great choice for sandy soils, which lose water rapidly.
For certain garden tasks, you may very well end up using both vermiculite and perlite. Vermiculite is unrivaled when starting seed indoors, so you may use it indoors in the early spring if you want to get a jump on the growing season. Then, once it's warm enough to transplant seedlings to the garden, you may use perlite outdoors (in the planting holes) to ensure that your plants have good drainage.