I’m sure you’ve heard of vermiculite! Is it appropriate for indoor gardens? What about outside gardens? My years of experience with this growing medium have taught me how and where it can be used in your garden!
Vermiculite can be used as a soil amendment and as a topdressing for indoor houseplants as well as outdoor gardens. It works best in a 1:3 vermiculite-to-soil ratio. It can also be used as a germination starter, stem cutting substrate, and insulation for bulb and tuber storage. Vermiculite should be used in dry regions to increase water retention only.
Where can you buy vermiculite? How much vermiculite is best for your garden space? Keep on reading to find out the answers to these intriguing questions!
Vermiculite is an excellent seed-germination medium because it is sterile, can retain water, and provides enough space for the seed to grow. Thus, it is a good medium to use for seedlings.
Seeds require an abundance of water to grow. Always bear in mind that water access is critical during the germination stage! Vermiculite, which holds water for a long time, can meet this requirement for our seeds.
Another advantage is the sterility of vermiculite. We must grow our seeds in a clean environment. So by using vermiculite, we can ensure that our seed is not exposed to the development of microorganisms that could infest the seeds in the future.
When seeds germinate, they must have enough space to expand their roots and growing stems. Vermiculite’s lightweight and spacious structure meets this requirement.
Rooting of stem cuttings is favored in vermiculite because it is a sterile growing medium and offers enough space for roots to grow.
The sterility and spacious structure of vermiculite allow the roots of stem cuttings to grow better.
Same with growing seeds, sterility is also a consideration in the propagation through cuttings. We do not want bacteria and fungi to infest our cuttings as they will prevent proper growth!
Additionally, roots can also grow better in vermiculite since the medium is porous and not compact.
Vermiculite improves drainage for compact soils due to its large particle sizes, ranging from 2 mm to 8 mm. The physical space they occupy allows more tiny spaces to loosen the soil structure, thus providing ample space for air and water to flow through.
Vermiculite can be mixed with compost or garden soil. It is a common soil amendment, particularly for clay-like and compact soils. It acts as a soil loosener, hence the recommended 1:3 vermiculite-to-soil ratio.
Perlite and vermiculite are often compared in this aspect. What are their differences? Do they have any similarities?
Read further about this in our article on perlite vs vermiculite.
Vermiculite has excellent insulation and hydration control properties. These characteristics make it an effective medium for storing bulbs such as garlic and tubers like potatoes.
I believe that, just like me, you are also shocked by this fact!
Bulbs and tubers, when exposed to heat stress, have higher respiration rates. This means they degrade and perish faster.
Vermiculite works well with heat! It can redirect the heat instead of absorbing it. Thus, storing them in a box filled with vermiculite will lengthen their shelf-life.
Compare vermiculite’s usage with other growing mediums in our article on best & worst substitutes of vermiculite.
Vermiculite is absolutely safe for garden use and will not damage the plants. It is used in gardening due to the following characteristics: being lightweight, heat insulating capacity, high cation exchange capacity, high water-holding capacity, aesthetic, odorless, and sterility.
Because vermiculite is lightweight, it facilitates the development of seeds and roots when used as a growing substrate. Its use as a soil amendment is anchored to the fact that it can be blown by air—making mixing necessary.
As seen in the figure above, vermiculite flies away easily. Does this mean they could not be used as mulch? We will discuss more of this later!
Fun Fact: A hundred liters of vermiculite only weighs 10 kilograms!
Vermiculite’s ability to not absorb heat supports its usage in extending the shelf-life of commonly used vegetables such as potatoes, garlic, and onion bulbs.
To use vermiculite as a heat insulator, you just need to fill a container (can be a box or a tub) with vermiculite, press the vegetables towards the vermiculite, and cover this setup.
Because of its excellent cation exchange capacity, vermiculite can store nutrients well. This feature allows the roots to have easier access to nutrients that promote plant growth and development.
Cation exchange capacity is the ability of a growing medium to hold the nutrients and make them ready for plant use.
Learn more about this concept in our article on vermiculite features.
Although vermiculite seems to float in water, they actually absorb the water inside each particle. Unlike perlite that floats above, vermiculite is filled with water and feels mushy when exposed to water.
Let us go back to our question a while back: Can you use vermiculite as mulch?
The answer is: Yes, you can use vermiculite as mulch. This feature makes that possible. Once vermiculite absorbs water, it becomes heavy and difficult to blow away.
In other words, vermiculite will stay on the soil once used as top dressing or mulch.
Pro Tip: For better performance, experts suggest mixing vermiculite to the top 3 inches of your garden bed! This will efficiently control the water supply, hydrate the roots, and let them spread.
Vermiculite has a golden sheen which elevates the aesthetics of gardens. This aspect would be useful for houseplants in indoor gardens.
Who would want a dirty-looking pot, right? This is something that I have always taken into consideration when putting plants in my indoor garden.
With vermiculite, you can add greenery to your home without having to deal with unsightly plant pots filled with dirt!
Because vermiculite has no odor, it is suitable for indoor use.
There will be no unpleasant odor from decomposing medium components because it is an inorganic growing medium.
The sterile nature of vermiculite makes it suitable for germinating seeds and propagating stem cuttings.
Using vermiculite as a starting substrate is beneficial since seeds and/or cuttings are exposed to a clean environment. Therefore, there are lesser chances of plant pathogen development.
The best ratio of vermiculite and soil is 1:3. This amount of vermiculite can aerate the soil and improve its structure for root growth and development. Adding too much vermiculite will result in a waterlogged medium which can harbor plant diseases.
Vermiculite has a high water absorption capacity. But keep in mind that this does not imply that you should use them as much as you would like.
What will happen if you add too much vermiculite to your garden soil?
Remember: A growing medium that absorbs water well can hold water for a long time.
This is not ideal for gardening because it results in a soggy growing medium. This environment is ideal for microorganism reproduction. As a result, it can infest your plant’s roots, causing softening and, eventually, death.
Explore more about vermiculite’s features in our article on what is vermiculite.
So how is vermiculite used in gardening?
Vermiculite can be used outdoors as a soil amendment for potting mixes and raised bed vegetable gardens. It can also be used as topdressing when mixed with soil.
You can use vermiculite in your vegetable garden by mixing it with soil and using the mix as the top 3 inches of your raised bed! By doing this, you are elevating the air and water spaces in your veggies’ roots.
In mixing, use at most 20% of vermiculite to the soil. This is intentionally lower than our recommended 1:3 ratio because outdoor plants are watered more often.
|Amount of Vermiculite (kilograms)
|per cubic meter
|per cubic yard
|per cubic foot
If you use more than this, your raised bed is at risk of being soggy! We do not want that!
Vermiculite can be used for potted houseplants as a soil amendment or topdressing.
If you are planning to mix vermiculite in your potting mix for indoor gardening, you can mix it with garden soil or a soilless potting mix (those typically made with carbonated rice husks or coir).
In indoor gardening, you can also use at most 25% of vermiculite mixed with garden soil or soilless potting mix.
Make sure your soil is not filled to the brim if you are planning to use vermiculite as a topdressing! Allow 1-2 inches for the distance between the tip of the pot and the soil. This will ensure that the vermiculite will not overflow when watering.
Can you add vermiculite to Fox Farm’s Happy Frog Potting Soil?
Most gardeners find that adding 25% vermiculite to Farm Fox’s Happy Frog Soil improves aeration, water-holding capacity, and retention of nutrients.
Vermiculite comes in different packaging sizes on Amazon. One can purchase them in 1, 2, 4, and 8-quart bags, depending on their specific gardening needs.
The one below is a 1-quart bag weighing 10 ounces on Amazon. You can purchase this if you only need a substrate to germinate seeds in seedling trays.
The weight of this 2-quart vermiculite bag on Amazon is approximately 20 ounces. You may choose this if you are topdressing more than 5 pots of indoor plants.
This 4-quart package of vermiculite on Amazon weighs approximately 2 pounds. Choose this if you are looking for a soil amendment for 3 to 5 pots!
An 8-quart package of vermiculite is available on Amazon and it weighs 5 pounds! This is the best option if you are creating a soil mix for many indoor plants or outdoor gardens.
Can you reuse vermiculite?
Because vermiculite is an inorganic material, it can be reused year after year. If used as the sole medium in germinating seeds, it can be sterilized using hydrogen peroxide after use. If used as a soil amendment, it will remain there to improve the drainage continuously as it does not decompose.
Should you soak vermiculite before using it?
There is no need to soak vermiculite before using it in the garden. This is because it can readily absorb water once exposed.
Will vermiculite hurt worms?
Worms will not be harmed by vermiculite. Because worms are accustomed to these types of minerals in the soil, they will continue to play their ecological role despite the addition of vermiculite.
Vermiculite can be used in both indoor and outdoor gardens. It can be a substrate for germination, cutting propagation, topdressing, and soil amendment. It can also be used as insulation for storing bulbs and tubers.
Vermiculite can be used in gardening because it is lightweight, heat-insulating, has a high cation exchange capacity, high water-holding capacity, is aesthetically pleasing, odorless, and sterile.
- “Horticultural Vermiculite” by N/A/ in J.P. Austin Associates, Inc.
- “Inorganic and Synthetic Organic Components of Soilless Culture and Potting Mixes” by Papadopoulos, A.P. et al. in Soilless Culture
- “Post-Heat Stress Respiration Pattern of Tulip Bulbs in Storage” by Liou, S.S. et al. in Acta Horticulturae