I have tested in a small experiment whether lava rocks can really deter slugs.
Lava granules | Field trial
To test the effect, I spread the granules in a semicircle and placed slugs in the middle.
Lava granulate was used, which is explicitly sold as a slug barrier.
The slugs were freshly collected from the garden. Among them were some leopard slugs and several Spanish slugs.
The following video shows what happened next—in time-lapse, at about 10 times the speed.
Video: Lava granules against slugs in the test
The video clarifies that most slugs are not very impressed.
They simply crawl over the lava stones without being disturbed.
But you can also observe individual slugs that turn away after coming into contact with the granules and take a different direction.
However, these are clearly in the minority.
Conclusion
Lava granulate was not convincing in the test. It was only slightly to not at all effective in deterring the snails.
Therefore, I cannot recommend this product. This is because the stones are relatively expensive and not worth the effort.
Certainly, lava stones can be used as mulch or added to planting soil. It can help suppress weeds and improve soil health.
It is also good as a drainage layer in flowerpots. The volcanic rock further serves as a heat accumulator and increases the oxygen content in the soil.
Natural Lava Rocks | Granules | 100% Pure Volcanic Rock
I have tested numerous mulch materials, but only sheep’s wool and mint have shown success. Coffee grounds, eggshells, sawdust, etc. have proved almost completely ineffective.
