Q. Can you use lava rock as a groundcover around any shrubs? I am thinking particularly of Japanese boxwood.
A. Lava rock was used extensively in Las Vegas landscapes as a surface covering before desert landscaping became popular. Desert landscaping uses specialty rock mulch of different colors and sizes and its use has pushed lava rock out of this niche. Lava rock has some interesting qualities, different from the rock we have available to us today. It can be a good alternative to rock mulch we have available to us today.
Lava rock, or any rock mulch for that matter, would not be a good choice around Japanese boxwood. Japanese boxwood grows better in soils amended with compost and the soil covered in organic wood mulch, not rock mulch or lava rock.
Using rock mulch or not depends on the plants. I am frequently asked which plants can tolerate rock mulch and which ones cannot. That’s difficult to explain unless you know where the plant originated. Plants that originate from desert climates can generally handle rock mulch better than those which don’t. Sometimes that information is difficult to find.
Boxwoods come from non-desert environments. They don’t come from the “deserts of Japan”. They come from wetter and cooler climates than ours. This suggests to avoid placing them in West and South exposures, but amend the soil at the time of planting with compost and cover the soil with something that decays such as wood chip mulch, which adds organics back to the soil.
Lava rock falls into the category of a rock mulch. It does break down over time adding minerals to the soil but adds no “organics”. The same problem occurs with other rock mulches. Over a few years, the compost added during planting is gone, replaced by the minerals without organics. These soils may be rich in minerals but lack the physical properties needed for good drainage, root growth and the chemistry required for nondesert plants to thrive.