Rainwater, dirt and debris will all discolor decorative rocks in a garden over time. Techniques for cleaning rocks vary, depending on surrounding vegetation. Better to play it safe and use a mild, soapy liquid dishwashing detergent with extra elbow grease than to risk damaging or killing garden plants with harsher cleanser that might work faster, such as chlorine bleach. A pressure washer is risky, as it can blast smaller rocks out of position, causing plant damage. Stronger cleaning methods can be used on smaller rocks that can be moved easily to a safe location away from flora.
Cleaning Large Rocks
Use a leaf blower to remove debris and leaves from around the rocks in the garden.
Prepare a bucket of soapy water using liquid dishwashing detergent.
Scrub the rocks with soapy water and steel wool or a grill brush to removed caked-on dirt.
Rinse the rocks with the garden hose and leave them to air-dry.
Cleaning Smaller Rocks
Move smaller rocks from the garden to a metal wash tub.
Mix 1/2 pound of oxalic acid (sold as wood bleach in hardware stores) in a bucket filled with 1 gallon of water. Wear rubber work gloves.
Cover the rocks in the metal tub with the water and oxalic acid solution. Let the rocks soak overnight.
Scrub the rock surfaces with steel wool or a grill brush.
Rinse the rocks with a hose and replace in the garden.