Suppliers sell lava rock by the cubit foot, cubic yard and by the truckload. Use this porous stone to create effects ranging from a tropical paradise with colorful plants surrounded by black lava pebbles to water-saving hardscapes in place of a lawn. Lava rock lasts much longer than organic mulch, saving on labor. This versatile rock serves as garden mulch, erosion control, shrub underlay and rock garden material.
Check Your Choices
When the supplier specifies a size, such as 3/8-inch lava rock, the measurement refers to the average size of the rocks. These small rocks, similar in size to pea gravel, work well for mulch, paths and ground covers. For decorative areas, 3/4-inch lava rock has a chunkier appearance that adds texture to the landscape. These larger stones make for unsteady footing, so don't use them for pathways or seating areas.
Plan and Measure
When you design, avoid lava rock under trees that drop needles or other sources of debris. They're challenging to keep clean. If your climate has high temperatures, don't use lava rock near plants in full sun. The rocks reflect heat up to the plant and raise soil temperature. Lava rock works well under decks, around fire pits, for shade plantings and for other bedding areas in temperate climates. Measure the areas where you want coverage and note the dimensions to calculate how much lava rock you need to buy.
For ground cover, plan to apply the rocks 2 to 3 inches deep. Each 1 cubic foot of lava rock will cover a plot about 6 feet square with a 2-inch layer of rocks. When you buy lava rocks, buy landscape fabric to lay under them. Landscape fabric helps with weed prevention and keeps the rocks from sinking into the soil. Landscape fabric also makes lava rock far easier to remove if you redesign your yard in the future. Add edging to your shopping list, too. Flexible metal edging works well on curved beds and lasts longer than plastic. Bricks, larger lava rocks and wood also work. Edging keeps lava rock in place so it doesn't float off during heavy rain or watering.
Shop and Order
Suppliers often sell small quantities for garden use, such as bags containing "0.5 cubit foot," meaning 1/2 cubic foot, or loose rock by the 1/2 yard, meaning one-half of a cubic yard. Some stone suppliers and landscape suppliers will deliver the rock. If you order it by the truckload, check how much lava rock you'll be getting in each load. For a large project, having the rocks delivered can be worth it. Lava boulders make eye-catching landscape features. Lava rocks called "red" tend to be brick colored. Black lava rocks may have a gray or burgundy cast to them.