Vermiculite shows up in countless substrate recipes without anyone ever explaining what it actually does. Yet a handful of expanded vermiculite in the right place changes how a seed tray holds water, or how a young root system finds air and nutrients. Let’s look at what it’s used for, when it’s worth adding, and how it compares with the other structural materials.
What is expanded vermiculite?
Vermiculite is a mineral from the mica family. Heated to high temperature it “exfoliates”: the layers open up like an accordion and the granule expands, becoming light, porous and able to act like a tiny sponge. What you find in horticulture is expanded vermiculite, in different grades (for example 0-2 mm for finer uses and 0-4 mm for mixing into substrates).
Unlike perlite, which works almost entirely on air, vermiculite does two things at once: it holds water and nutrients while still keeping some porosity. That dual nature is what makes it useful at specific stages of the crop.
What does vermiculite do for plants?
When you add it to a substrate, the main functions are three:
- Water retention — the layered structure absorbs and releases water gradually, smoothing out moisture swings in the substrate. That’s what keeps a seed tray evenly moist without waterlogging it.
- Nutrient holding — vermiculite has a good exchange capacity: it intercepts feed elements and releases them slowly, limiting leaching in the first weeks.
- Lightness and structure — while holding water, it stays light and keeps air spaces, helping young rootlets explore the substrate without hitting compacted zones.
Vermiculite for seeds: how to use it
The most common use is in sowing and propagation. Here vermiculite works two ways. Mixed into the seeding substrate, it raises water retention and keeps moisture more constant around the seed during the most delicate phase, germination. As a covering, a thin veil of vermiculite over the seed keeps the seedbed moist while still letting air and light through where needed, and reduces surface crusting.
The practical result is more even germination and a lower risk that some cells of the tray dry out before the others. That’s why vermiculite is a natural ally of professional seeding substrates, where cell-by-cell consistency drives the final result.
Vermiculite or perlite: when to choose which
It’s the most common question, because the two materials look interchangeable but aren’t:
- Choose vermiculite when the job is to hold water and nutrients gently: sowing, young transplants, crops that suffer from moisture swings.
- Choose perlite when the job is to drain and aerate: substrates prone to waterlogging, long pot cycles, roots that mainly need air.
Often the best answer isn’t “one or the other” but a balance between the two, calibrated to the crop and the stage. Always start from the intended use, not from habit.
pH and specs to read on the label
Expanded vermiculite has a broadly neutral reaction, which is why it integrates well into peat substrates without noticeably shifting their balance. It’s also an inert, stable material that doesn’t break down over the crop cycle. Grade, pH behaviour and specific use directions do vary from product to product, though: those are the values on the label and technical sheet, and that’s where to check them before use.
Vermiculite in the Arber range
Arber offers selected expanded vermiculite as a structural material to mix in, and it’s among the components that go into the formulation of its professional growing substrates. The logic is always the same: the grade and function of a material are chosen for the crop and the stage, not to fill a recipe.
Always read the product label and technical sheet for grade, pH and specific use directions.
Not sure whether you need vermiculite, perlite or a ready-balanced substrate? Explore the vermiculite in the catalogue and the professional substrates, or talk it through with our technical team: get in touch.
Frequently asked questions
What is vermiculite used for with plants?
Mainly to hold water and nutrients while keeping the substrate light and porous. Added to a substrate, it reduces moisture swings, limits leaching of feed in the first weeks, and helps young roots develop in an even medium. It’s especially useful in sowing and propagation.
How do you use vermiculite for seeds?
Two ways: mixed into the seeding substrate, to raise its water retention and keep moisture constant around the seed; or as a covering, a thin veil over the seed, to keep the seedbed moist and reduce surface crusting while still letting air through.
Vermiculite or perlite — which is better?
It depends on the goal. Vermiculite holds water and nutrients and suits sowing and young plants; perlite drains and aerates and suits situations where the risk is waterlogging. They’re often used together, dosed to the crop and the stage.
Does vermiculite change the pH of the substrate?
Expanded vermiculite has a broadly neutral reaction and usually doesn’t noticeably shift the balance of a peat substrate. The precise pH behaviour should still be checked on the technical sheet of the specific product.