The Pros and Cons of Lava Rock As Mulch

by Deborah Green
Lava rock is a product of volcanic activity.

Lava rock is a product of volcanic activity.

A layer of mulch, whether organic or inorganic, gives your landscape an attractive, finished look. Lava rock is one of many commercially available inorganic mulching materials. Inorganic mulches won't add nutrients to your garden or improve soil characteristics, but have the advantage of lasting longer than organic materials. The relative permanence of inorganic mulches makes it important to weigh if a material such as lava rock is a good choice for your mulching needs.

Pros for Gardeners

Red lava rock, technically called scoria is a colorful addition to a garden; some suppliers also sell black lava rock, which garners its visual impact from texture, alone. The red coloration is formed during eruption due to the oxidation of iron. A favorite for desert landscapes, lava rock will work best where it mirrors the surrounding terrain or is designed to be a standout feature, such as an island bed. Relative to other rock mulches, lava is lightweight, so less costly to transport and less onerous to move into the garden for spreading.

Pros for Plants

Mulching serves a critical function in temperature and moisture moderation. Lava rock absorbs heat during the day and releases it during the night. Rock mulches generally slow evaporation from the soil, although if days are warm, dark mulch means higher temperatures, with faster evaporation. If installed with a fabric weed barrier underneath, lava rock creates an effective weed suppressant.

Cons for Real Estate

Aesthetics are an important component of mulching. If you are concerned about curb appeal for enhancing real estate values, consider that red lava rock may be unappealing to some -- and if you don't like how lava rock looks after you've installed it as mulch, it is difficult and costly to remove. The rocks can also be used by neighborhood vandals to scratch graffiti onto surfaces.

Cons for Gardeners

Rock mulches don't break down as quickly as organic mulches, but lava rock can wash or blow away if you use it in exposed locations. New Mexico State University Extension specialists recommend installing lava rock at least an inch below sidewalk level to reduce its movement out of planting beds. Despite being lightweight, lava rocks can damage mower blades if pieces end up on your lawn. Even if they remain where you place them, lava rocks' sharp edges make the material difficult to work around, so it is best used in permanent plantings or other locations where maintenance tasks are minimal.

Cons for Plants

Lava rock may create an inhospitable environment for plants to grow and spread, especially if installed over a weed barrier. Such an installation may also inhibit watering if the fabric slopes away from the plants. Soil or sand that sifts in between the rocks from above may also interfere with watering or support weeds that will compete with plants.

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