Moss, a non-vascular plant, thrives in damp, shaded conditions, making asphalt surfaces vulnerable to its growth. Moss is detrimental because it acts like a sponge, retaining moisture directly against the pavement structure. This constant dampness accelerates the degradation of the petroleum-based asphalt binder over time. In colder climates, the trapped water can lead to freeze-thaw expansion, causing existing small cracks to widen. Addressing moss growth quickly is important for maintaining the structural integrity of paved surfaces.
Preparing the Surface for Treatment
Before applying any treatments, the asphalt surface must be prepared to maximize the effectiveness of the moss killer. Begin by using a stiff-bristled broom to sweep away loose debris, such as leaves, dirt, and accumulated sediment. This removes the organic matter that provides a foothold and nutrients for the moss to grow.
Next, physically remove the bulk of the moss growth by scraping or firm brushing. Carefully use a scraper to avoid gouging or tearing the asphalt aggregate, which creates new areas for water to pool. Removing this top layer ensures that the subsequent chemical solution can penetrate directly to the remaining moss roots and spores.
Asphalt-Safe Moss Killing Solutions
Effective moss-killing solutions that do not harm asphalt often involve common household ingredients. A highly effective DIY option is a diluted white vinegar solution, typically mixed with water in a one-to-one ratio. The acetic acid in the vinegar alters the pH level of the moss, quickly desiccating its structure and killing the plant cells.
For a gentler approach, a mixture of water and a surfactant, such as dish soap, can be used. The soap functions by breaking down the moss’s waxy cuticle, allowing water to penetrate the cells and cause dehydration. This method is generally safe for surrounding landscaping, provided the runoff is not overly concentrated. Apply both the vinegar and soap solutions generously to saturate the moss entirely.
When considering commercial moss killers, check the product label for asphalt compatibility before purchase. Many commercial products contain zinc or iron compounds that are effective against moss and safe for petroleum-based surfaces. Avoid products containing harsh solvents or high concentrations of chlorine bleach, as these degrade the asphalt binder. Do not use rock salt or high-concentration salt mixtures, which can corrode the asphalt and surrounding concrete and are detrimental to soil health.
Application Guidelines and Safety Precautions
Once the moss-killing solution has been prepared, apply it using a garden sprayer to ensure even and complete coverage of the affected areas. Completely saturate the moss and the surrounding asphalt, allowing the solution to soak in for several hours or even a full day. The moss will typically turn a brown or yellow color, indicating that the solution has successfully killed the plant structure.
After the moss has died, use a garden hose with a standard nozzle to thoroughly rinse the area with low-pressure water. Avoid using a high-powered pressure washer, as the force can strip the aggregate and binder from the asphalt surface, creating premature wear. If a pressure washer must be used, keep the pressure below 1,200 PSI and utilize a wide fan tip (40-degree or greater). Protecting nearby landscaping is important; pre-wetting plants with plain water before application and rinsing them immediately afterward helps dilute potential chemical runoff.
Long-Term Prevention Strategies
Preventing moss from returning requires addressing the environmental factors that encourage its initial growth, primarily shade and moisture. Trimming overhanging tree branches or shrubs will increase sunlight exposure, which naturally inhibits moss proliferation. Improving drainage around the edges of the asphalt is beneficial; ensure that water drains quickly and does not pool in low spots or along curbs.
Regular sweeping and maintenance are effective preventative measures, as removing debris eliminates the organic material moss spores need to germinate. Applying an asphalt sealant is a proactive step, as a freshly sealed surface is less porous and offers fewer microscopic footholds for moss spores to take root. A smooth, sealed surface dries faster after rain, making the environment less hospitable for future growth.