You are likely already growing plants and herbs in a volcanic rock product without ever knowing it. The tiny white granuals in your potting soil -- vermiculite -- are pieces of volcanic rock that were heated until they popped. Pumice, a pale gray, lightweight product that resembles gravel, is crushed volcanic rock. Lava rock, reddish-brown or gray nuggets of lightweight rock, is commonly used as a mulch, but it can be used in several ways to grow edible herbs.
Function of Lava Rock
When used as a soil amendment, lava rock improves drainage by adding space for air and water within the soil. It also holds heat and nutrients. This is particularly important for Mediterranean herbs that like dry, warm conditions, such as rosemary (Rosmarinus officinalis),suitable for U.S. Department of Agriculture plant hardiness zones 7 through 10, oregano (Origanum majoricum), hardy in USDA zones 5 through 9, and annual basil (Ocimum basilicum).
Uses of Lava Rock
Lava rock can be mixed with soil, oyster shells, sand and other amendments to simulate Mediterranean conditions or used at the bottom of herb containers to improve drainage for edible herbs. As a mulch -- also often combined with other materials -- it reflects warmth, prevents disease by discouraging water splashback, and can control slug damage. Because it is lacking in nutrients, lava rock is only used alone to grow herbs in hydroponics, where lava rock provides a substrate that holds the roots of herbs in a liquid nutrient solution.
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