Lava rock landscaping, known scientifically as scoria, is popular for its distinctive color, lightweight nature, and durability. This highly porous, volcanic material creates a beautiful aesthetic in garden beds and walkways. However, the air pockets that make the rock lightweight also trap fine dust, soil, and organic matter like decomposing leaves or fungal spores. Over time, this trapped debris encourages the growth of moss, algae, and mildew, necessitating a specialized cleaning approach to restore the rock’s original appearance.
Initial Preparation and Debris Removal
Before introducing any moisture, physically remove surface debris. Clear large organic materials, such as fallen leaves and pine cones, using a rake or a powerful leaf blower. This prevents the debris from dissolving into a slurry once water is applied, which would spread stains and soil the porous rock further.
For effective cleaning, the rocks must often be separated from the landscaping bed, especially if the layer is thick or heavily contaminated. Shoveling the scoria onto a durable tarp or into a wheelbarrow allows access to the entire volume of rock, not just the visible surface layer. This separation also provides an opportunity to remove any underlying soil or broken-down landscape fabric contributing to the contamination.
Once separated, a shop vacuum equipped for dry use or an industrial blower can be employed to remove fine dust and sand trapped within the rock’s texture. These fine inorganic particles become mud when wet, making the cleaning process more difficult and leaving a permanent haze on the rock surface. Starting with a completely dry removal of particulates makes subsequent washing methods significantly more efficient.
Techniques for Removing Organic Growth and Stains
The actual washing process begins after all loose debris has been cleared. Wear appropriate safety gear, including gloves and eye protection, when handling cleaning agents. Use a high-pressure garden hose or a pressure washer set to a low-to-medium setting. Direct the stream of water at the rocks from a distance of at least one foot to prevent chipping the lightweight material while dislodging superficial dirt.
For stubborn patches of dirt or compacted soil, a stiff-bristle brush, preferably plastic or natural fiber, provides the necessary manual agitation. This mechanical action breaks the surface tension holding contaminants to the rock’s texture, allowing the water to wash them away. A simple rinse with clean water is sufficient for rocks that have only accumulated dust and light surface grime.
When dealing with mild algae, moss, or general discoloration, natural solutions offer gentle cleaning power. Apply a solution made from one part white vinegar mixed with three parts water directly to the affected areas. White vinegar contains acetic acid, a mild organic acid that disrupts the structure of minor organic growth.
Alternatively, mix a mild dish soap solution, formulated without heavy degreasers or brighteners, with water and apply it to the scoria. Allow both the vinegar and soap solutions to sit for 10 to 15 minutes to let the cleaning agents penetrate the porous surface. Following the soak, a thorough, vigorous rinse with clean water is necessary to remove all residue.
Severe instances of black mold or mildew require the use of a more potent chemical solution, specifically a highly diluted sodium hypochlorite, or household bleach, mixture. Because scoria is exceptionally porous and absorbent, the dilution must be extreme, one part bleach to ten parts water. This high dilution minimizes the risk of bleaching the surrounding soil or harming nearby plant life upon runoff.
The diluted bleach solution should only remain on the rock for five to ten minutes, as the oxidizing agent works quickly to destroy the microbial structure of the mold. The most important step when using bleach is the subsequent exhaustive rinsing. Rinse the rocks multiple times with fresh water to ensure all residual chlorine is neutralized and flushed away before the rock is returned to the landscape bed.
Ongoing Maintenance and Prevention
Once the lava rock is clean, proactive maintenance prevents the need for frequent, intensive deep cleaning. Installing a high-quality woven landscape fabric beneath the scoria layer is a significant preventative measure. This fabric acts as a physical barrier, preventing the underlying soil from migrating up and mixing with the rock, which is a major source of long-term contamination.
A routine schedule of light maintenance greatly extends the clean appearance of the landscaping. Periodically using a leaf blower, perhaps once a month, quickly removes freshly fallen organic matter before it begins to decompose and stain the porous surface. This simple action interrupts the cycle of organic buildup that leads to algae and moss growth.
For maximum longevity, some gardeners apply specialized rock sealants after deep cleaning. These clear, acrylic-based sealants penetrate the scoria and reduce its porosity and water absorption rate. This decreased porosity limits the environment where mold and algae can thrive. Sealants can sometimes subtly darken the rock’s natural color and require reapplication every few years to maintain their protective barrier.