After a particularly wet spell of weather, you may notice an unattractive green, powdery film or deposit developing on the stone or wood surfaces in your garden. This slimy coating could be algae. Once algae starts to collect on a path, it may render it slippery when wet. In areas with large trees or next to ponds and puddles, you will notice that shade and moisture create an environment where algae thrives, especially in the late winter and spring.
Mix a solution of bleach and warm water. Use 3/4 of a cup of bleach for each 4.5 litres of water. Household chlorine bleach will do the job. If you have concerns about the environment, use oxygen bleach instead. Avoid bleaches that gel or become thick since it tends to sit in one spot. This will make it harder to give the surface an even cleaning.
Pour the solution onto the path and let it sit. After about five minutes use a broom or scrubbing brush with thick, hard bristles to work the solution into the path's surface. If you have a large path, work in sections so that you can control the amount of bleach on the surface.
Rinse the bleach solution from the path with water from a garden hose or pressure washer.
Repeat the process two or three times to remove all the algae.
Prevent algae from colonising your path by repeating the cleaning process every two or three months. Eliminate shady areas where algae thrives whenever possible. Brush the surfaces with a broom on a regular basis to prevent the algae from taking hold.
If you have a wooden deck path, use oxygen bleach. It will not remove the colour from the wood. Dissolve the powder in water and use it the same way as you would the chlorine bleach on a stone path.
When laying a new path, make it slope so that water will drain away. This helps deter the growth of algae. Dig channels around the edges of paths and fill them with gravel to absorb the run-off.
Pressure washers can erode the mortar joints on stone paths. Use a garden hose instead to avoid damaging the path's surface.
Bleach can also kills other plants, such as the adjacent lawn. Always dilute the bleach with clean water before you apply it to the path. Keep a garden hose handy in case you spill the solution where you do not want it.