Putting Lava Rocks Around Hostas to Control Slugs

Slugs, small night-feeding garden pests, consume rotting organic matter, debris and succulent leaves of live plants, such as hosta. Leaving a trail of slime behind, these soft-bodied mollusks can do serious damage to garden plants, eating large holes in hosta leaves. Unlike their close cousins, the snails, slugs have no shell, exposing them to the elements. During the day, slugs hide under plants, sticks, boards, bricks or any available shelter. By night, slugs come out to feed. One way to control slugs and keep them away from your prized hostas is put lava rocks around the plants.

Lava Rock

Under the earth's surface, temperatures are great enough to melt rock. Tiny bubbles of gas infiltrate liquid, igneous rock -- the obsidian, glassy type that flint arrowheads are made of. At times, pressure from gases builds to the point that the hot liquid rock, called lava, spews to the earth's surface. As molten rocks is thrown into the air by pressure from below, it cools quickly once it reaches the atmosphere, solidifying with tiny holes throughout its mass. Called cinder, this whipped obsidian makes a good garden mulch, blocking out light to prevent weed germination, but allowing moisture to pass through, and conserving moisture. The texture is rough and sharp.


Slugs are terrestrial mollusks with rough, slimy skin and tentacles. The brown field slug (Deroceras panormitanum), also called the long-neck field slug, is found in gardens and other vegetated sites in California and other worldwide locations. The gray field slug (Deroceras reticulatum) causes serious damage in crops and gardens, preferring juicy plants such as lettuce and succulent hosta leaves. In total, 17 species of slugs are native to California.


Hosta, commonly called plantain lily, is a tender perennial with broad, succulent leaves emerging from fleshy roots in springtime in U.S. Department of Agriculture plant hardiness zones 3 to 9. Considered an understory plant that thrives in shade, gold and chartreuse varieties are sun tolerant. Green and variegated hostas also tolerate some sun exposure in locations with cool summers. Primary pests are slugs and snails. However, hosta varieties with heavily ribbed or textured leaves are less susceptible to slug damage.

Slug Deterrent

Slugs have rough skin, but very little protection from the environment, except for slime that keeps the skin moist. Iridescent trails are evidence of slug infestation, even though slugs are seldom seen during daylight hours. With exposed bellies, slugs are particular about the materials they crawl over, avoiding sandy, gritty surfaces. Lava rock, with its highly irregular and sharply gritty, cinder surfaces deters slugs and protects plants mulched with this material. For adequate protection, lava rock mulch must extend beyond the leaf canopy and it must remain free of debris or soil that could create a bridge to the hosta plant.

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