Lava rock
Also known as Scoria, Volcanic cinder, Pulverized lava
In short
Horticultural lava rock is usually scoria, a dark volcanic rock full of gas bubbles. Crushed, it adds permanent drainage and air to mixes for bonsai, succulents, orchids and carnivorous plants, and holds some water in its pores.
How it looks
Dark red, purplish-red, brown or black irregular pieces with many open bubbles (vesicles). It is heavier than pumice and usually sinks in water.
Signs
Mixes with lava rock drain quickly and do not compact, and they need watering more often than peat- or bark-rich mixes.
What favours it
Scoria forms when gas-rich magma erupts and chills, trapping gas bubbles (vesicles). It is mostly volcanic glass with minerals such as plagioclase, pyroxene and magnetite and holds water both in its vesicles and between grains.
Prevention
- Rinse crushed lava rock to remove dust before use.
Treatment
Start with the first step and move down only if the problem persists.
- For orchids in lava rock, the AOS suggests watering once or twice a week because it holds some water in its pores.
Pros and cons
Pros
- Permanent structure that does not decompose or compact.
- Holds some water in its pores while draining freely.
Cons
- Heavy, adding weight to large containers.
- Holds no useful nutrients for most plants, so feeding is needed.
- Mined, non-renewable material.
Look-alikes
Typical uses
Bonsai, succulents and cacti, orchids and other plants needing a very open mix; also landscape drainage.
Related
Sources
- Scoria — Wikipedia
- Beginner’s Series — Part 2: Media Mania Revisited — American Orchid Society
- Bonsai cultivation and care — Wikipedia
Written in our own words from these sources. Check a product label before using any treatment.