The sight of green, slick growth across a patio or a dark, crusty film forming in the lawn is a common frustration for property owners. This “yard algae” is often a mix of true algae, mosses, and cyanobacteria, which are photosynthetic bacteria sometimes called blue-green algae. These organisms thrive in specific conditions, quickly turning outdoor spaces unsightly and potentially creating slip hazards on hard surfaces. Addressing this pervasive growth requires a multi-step approach that combines immediate removal with environmental modification to prevent its return.
Identifying Algae and Environmental Triggers
Yard algae typically manifests in two main forms: a thin, vibrant green, slippery film on hard surfaces, or a dark, greenish-black, slimy crust on soil and thin turfgrass areas. The crust in lawns is often cyanobacteria, which dries into a hard layer that repels water and suffocates the grass underneath.
These organisms require a specific environmental recipe to flourish. The three primary triggers are persistent moisture, lack of direct sunlight, and poor air circulation. Low-lying areas that retain standing water or surfaces constantly shaded offer the ideal damp environment. Excessive nutrients, particularly nitrogen and phosphorus from fertilizer runoff, can also accelerate their proliferation.
Immediate Removal from Hard Surfaces
For hard surfaces like driveways, patios, and decks, immediate removal involves mechanical action combined with chemical treatment. Mechanical removal begins with scraping or scrubbing the growth with a stiff-bristled brush. Pressure washing can quickly clear large areas, but use caution; high pressure can damage wood fibers or force water into cracks, potentially causing structural issues.
Chemical solutions destroy the organisms’ cell structure, making them easier to rinse away. Diluted white vinegar, which contains acetic acid, is a common household option. For a stronger approach, a solution of household bleach and water can be used, though this risks discoloring wood and harming surrounding plants if not rinsed thoroughly.
Commercial algaecides often use compounds like oxygen bleach (hydrogen peroxide) or potassium salts of fatty acids, which are formulated to be safer for nearby vegetation than chlorine bleach. Always apply these solutions on a cloudy day, allow sufficient dwell time for the chemical to work, and rinse the area completely to remove all residue and dead growth.
Treating Algae in Lawns and Soil
Treating algae and cyanobacteria in turfgrass requires a different approach, as harsh surface chemicals would damage the lawn. The thick, black algal crust on the soil surface must first be physically broken up to allow the turf to recover and improve water and air penetration. This is achieved through light raking, spiking, or core aeration, which alleviates the soil compaction that contributes to the problem.
Once the crust is disrupted, turf-safe treatments can be applied. Products containing copper sulfate are effective algaecides, though repeated use requires monitoring, as copper can accumulate to toxic levels. Applications of ferrous sulfate, or iron, can also dehydrate and darken the algae, causing it to die off. Applying lime or gypsum can help adjust an acidic pH, which algae often favor, making the conditions less hospitable for growth.
Changing Conditions for Long-Term Control
The most effective long-term strategy involves eliminating the environmental conditions that allow algae to thrive: poor light exposure, poor air circulation, and poor drainage. Trimming overhanging tree branches and thinning out dense shrubs will increase direct sunlight, helping surfaces dry quickly and inhibiting the growth of shade-loving organisms.
Improving air flow is accomplished by pruning low-hanging branches and ensuring plantings are not overly dense, which reduces humidity and speeds up surface drying. For persistent moisture issues, improving drainage is necessary. This can involve regrading the soil to ensure water flows away from structures or installing subsurface solutions like French drains. Routine maintenance, such as sweeping hard surfaces and ensuring downspouts direct water away, will also reduce standing water and nutrient sources.