Volcanic lapillus often shows up on the surface of a pot or at the bottom of a planter, yet its role is rarely explained. It does a precise job: it gives structure, drains, and protects the substrate without breaking down over time. Let’s look at what it actually does, which plants benefit from it, and its pros and cons.
What is volcanic lapillus?
Lapillus is a rock of volcanic origin: small porous fragments produced by eruptive activity, then selected and screened into different grades (for example 3-5 mm for finer uses and 5-10 mm for drainage and mulching). Its defining trait is porosity combined with inertness: it holds a little water in its micropores but lets excess water drain away, and it does not decompose the way an organic material would.
That stability is what sets it apart. A material that doesn’t degrade keeps its properties for the whole growing cycle and beyond, without compacting or losing volume.
What is volcanic lapillus used for in plants?
When you add it to a substrate or use it on the surface, lapillus works on three fronts:
- Drainage and air to the roots — its porous, irregular structure creates room for excess water and oxygen, reducing the risk of waterlogging in pots and beds.
- Stable, lasting structure — being inert, it doesn’t compact or wear down: it keeps the substrate’s porosity constant over time, where an organic material would tend to collapse.
- Surface protection — used as a mulch, it insulates the substrate from temperature swings and limits evaporation, keeping moisture steadier below the surface.
Volcanic lapillus as mulch: how to use it
The most common use, especially in pots and gardens, is as a mineral mulch. A layer of lapillus spread over the substrate surface does several jobs at once: it slows water evaporation, suppresses weeds by starving them of light, and buffers the day-to-night temperature swing, protecting shallow roots.
Unlike organic mulch, a lapillus mulch doesn’t decompose and doesn’t need redoing every season: it stays in place and adds a decorative function too, with its typical dark colour. It’s a practical choice wherever you want stable, low-maintenance cover.
Which plants is volcanic lapillus suited to?
Lapillus fits a range of situations. It’s useful for potted plants prone to waterlogging, where a layer at the base or a fraction mixed in improves drainage. It suits succulents, cacti and Mediterranean species that want a free-draining substrate and are prone to root rot. It also works well in gardens and beds as a decorative, functional mulch, and in landscaping projects that need stable, lightweight materials, such as green roofs.
The logic is always the same: lapillus doesn’t feed the plant, but it creates the physical conditions — air, drainage, protection — in which roots work better.
Volcanic lapillus: pros and cons
Like any material, it has strengths and limits worth knowing before you choose it.
- In favour: it’s inert and doesn’t degrade, provides good drainage, doesn’t compact, lasts over time, is decorative, and needs no seasonal maintenance.
- To weigh up: it’s a mineral material, so it adds no organic matter or nutrients; it’s heavier than materials like perlite; and, like all structural materials, it should be chosen for a function, not used indiscriminately.
In practice, lapillus is the right choice when the goal is drainage, stable structure and surface protection; it’s not the material to expect nutrition or extreme lightening from.
pH and characteristics to read on the label
Volcanic lapillus is an inert, stable material, with a reaction that tends toward neutral, which is why it blends well into peat-based substrates without noticeably shifting their balance. Grade, pH behaviour and specific usage guidance vary from product to product, though: those are the values shown on the label and technical sheet, and that’s where to check them before use.
Volcanic lapillus in the Arber range
Arber offers selected volcanic lapillus in different grades, as a structural and mulching material to pair with the substrate according to the crop. The same logic applies to the other aeration and drainage materials in the range, such as pumice: fraction and function are chosen for the plant and the stage, not to fill out a recipe.
Always read the product label and technical sheet for grade, pH and specific usage guidance.
Not sure whether you need lapillus, pumice or an already-balanced substrate? Explore the volcanic lapillus in the catalogue and the professional substrates, or talk it through with our technical team: get in touch.
Frequently asked questions
What is volcanic lapillus used for?
It’s used to improve a substrate’s drainage and aeration and to protect its surface as a mulch. Being an inert mineral material, it doesn’t degrade and keeps its structure over time: it creates the physical conditions in which roots breathe and drain better, without adding nutrients.
Which plants is volcanic lapillus suited to?
It’s particularly suited to potted plants prone to waterlogging, to succulents and cacti, to Mediterranean species that want free-draining substrate, and as a decorative mulch in beds and gardens. It’s also used in stable landscaping projects such as green roofs.
How do you use volcanic lapillus as mulch?
Spread it in a layer over the substrate surface, in a pot or bed. This slows evaporation, limits weeds by depriving them of light, and protects shallow roots from temperature swings. Unlike organic mulch it doesn’t decompose, so it doesn’t need redoing every season.
What are the pros and cons of volcanic lapillus?
Pros: it’s inert, drains well, doesn’t compact, lasts, and is decorative. Cons: it adds no organic matter or nutrients and is heavier than materials like perlite. So it’s best chosen when the goal is drainage, structure and protection, not feeding.