What is Vermiculite? How to Use it in the Garden

Vermiculite and perlite are in many potting soil mixes, but you may not be aware of why. If you have water-loving plants, want vigorous seed germination, or need to help cuttings develop strong roots, vermiculite is essential!

So, what is it? Vermiculite is a naturally occurring mineral mined in several places around the world.

Horticulturists love this stuff because it’s sterile, expands as it absorbs water, and helps soil retain water. Your plants’ roots stay moist longer, and you don’t have to water as often. It’s also used in other industries, such as construction and vehicle production.

You may have heard at some point that vermiculite isn’t safe for any growing medium. It was once hazardous due to a specific mine, but you don’t have to worry these days. There are processes put in place to prevent contamination, making it perfectly safe to use in your soil and even in your home.

Let’s get into it so you can learn how to make use of this in your garden!

What Is Vermiculite?

Vermiculite is a sterile mineral mined from the earth and treated to be used in different ways, depending on the industry. It’s always a shade of brown regardless of its form. The kind used in soil is usually a golden or greyish brown. This blends in with the soil well and can be difficult to spot when it’s mixed with soil or compost.

After it comes out of the mine, it undergoes different treatments based on the way it’s going to be used. When it’s going to be used in gardening, it is treated with heat to make exfoliated vermiculite. It expands to make it absorbent and lightweight with a neutral pH.

Expanded vermiculite is the kind you’ll see in garden soil. It will look a bit like worms rather than the flakes it used to be when it was in the mine.

This mineral is used for moisture retention and is often found in soil mixes made for vegetables, tropical plants, or those that offer moisture control. Unless a plant needs dry soil, you can assume most plants will benefit.

However, you can definitely have too much of it in a growing medium. You have to be careful when using it, even in heavy clay soils.

What Is Vermiculite Made Of?

Vermiculite is a naturally occurring silicate mineral. It’s a completely natural, sterile, non-toxic mineral that comes from the earth. It’s safe to use in organic gardening and doesn’t contain any chemicals that would harm your organic gardening efforts.

Some mistakenly think the bits they find in their potting mix are little styrofoam balls. That isn’t the case.

Since vermiculite is found in the earth, it has to be mined. There are several mines around the world that contain this mineral.

What Is It Used For?

Vermiculite acts like a sponge that soaks up water, expands, and slowly releases the water as the soil dries out. That’s why it is used for water retention or to keep plants watered.

Vermiculite shouldn’t be used for plants that prefer to dry out between waterings. Since it can absorb up to four times its weight in water, it can negatively affect soil aeration and change the soil structure. This causes roots to get waterlogged in the growing medium and develop root rot. It’s great for plants that like damp soil, but don’t use it for cacti or other desert plants. 

There are also uses beyond your garden. You may have it in your attic as loose-fill insulation if you have an older home (since modern homes usually use fiberglass). It is also utilized in building safety to prevent burning buildings from collapsing. And it’s used to reduce greenhouse gas emissions in kilns, furnaces, and wood-burning stoves as well.

Vermiculite can be used for concrete screeds. The fine particles are light, insulate the concrete, and provide some fire resistance. In that same vein, it is used to make ‘logs’ in gas fireplaces.

Some of these uses require it to be treated in such a way that it wicks away water rather than absorbs it. If you want to use it for plants to improve moisture retention, be sure you get a version intended to be used in the garden.

Is It Safe?

You may have heard that vermiculite contains asbestos which is dangerous for your lungs and your plants’ roots. Don’t worry—that’s outdated information! 

There was a mine in Libby, Montana, that had an asbestos deposit in it. This mine produced vermiculite from the 1910s until it shut down in 1990. Modern-day mines have strict industry protocols in place that test the minerals for asbestos. You can trust that contaminated products will never make it to the store shelves.

The only chance you have today of coming into contact with toxic vermiculite is if you find an old bag of it from the 20th century that was mined and produced in the Libby mine. Most likely you won’t come across one of those, so there’s no need to be fearful.

Although pure, uncontaminated vermiculite is non-toxic and perfectly safe to use in your garden, there are a few safety measures to take to prevent irritation.

Dry versions may create a cloud of dust—it’s a mineral that came from the earth, after all! Breathing in the dust may cause temporary irritation, like sneezing. The dust may also irritate severe allergies or asthma. You can wear gloves and a mask to prevent any irritation.

How To Use Vermiculite in the Garden

There are several ways you can employ vermiculite to retain water.

Combine vermiculite with other soil amendments for seed germination. Mix half vermiculite with half peat moss or compost to keep the moisture and nutrients in check while your seeds germinate. It’s also a wonderful inclusion to your homemade potting mix or preferred material or growing medium. If you’re working with heavy soils, add more vermiculite to lighten them.

Vermiculite is also good to use when you transplant new seedlings and young plants. Retaining moisture is key to keeping growing plants healthy while they develop strong root systems. When you move seedlings to their new spot in the garden, provide them with vermiculite and a weak fertilizer solution to help the soil stay moist.

Vermiculite isn’t just limited to garden use. You can apply it to your lawn after you plant grass seed to help keep the soil moist. This is especially important if you live in a hot climate or have a yard that receives full sun. You won’t need to water as much if you’re generous with vermiculite.

If you want to breed worms directly in garden beds, vermiculite is excellent for soil aeration as well. This benefits worms as well as seedlings, and seeds in seed starting too. Because it retains water, it brings in worms that consume soil nutrients and create castings in the process. 

You can also store root crops in vermiculite to prevent moisture problems. Place the vegetables in a container and surround it with vermiculite. It will absorb excess moisture in the container or from the vegetable to prevent mildew or mold from forming, allowing you to store root veggies longer. 

Grades

Different grades of vermiculite serve different purposes in gardening. Coarse vermiculite has larger chunks and allows for better soil aeration. However, the water retention won’t be the best. Many people use this kind for hydroponics or for plants that don’t like soggy soil.

Medium-grade vermiculite is good for cuttings. It holds plenty of water but allows air to circulate, which is necessary for plant roots to develop and grow. Put a cutting into a container of medium-grade vermiculite until the roots develop to promote anchorage. Then move it into soil, mixing the original media with the nutrients in the current one.

Fine grades of horticultural vermiculite are typically employed for horticultural use, added to soil mixes with other materials. Keep in mind that the small granules don’t provide good aeration. Perlite can be used to help aerate the soil if needed.

When it comes to germinating seeds and other seed-starting practices, use fine vermiculite. Any coarser grades can make it difficult for seeds to push through.

If you’re not sure which grade you need, the medium grade may be the best vermiculite to use. The water and nutrient retention and soil aeration are pretty even.

Frequently Asked Questions