One important reason for sweeping leaves off your concrete walkway or patio regularly is that, the longer you leave them, the more likely they are to leave their signature in the form of brownish tannin stains. Tannins are complex chemicals -- insoluble and resistant to decomposition -- that give flavor to wine and tea, and have been used for centuries to tan leather hides. While not permanent, tannin stains -- which can also come from walnuts and the needles and cones of conifers, such as redwoods -- remain in concrete for a long time, and your patio gets a little more unsightly with each new one that appears. Sweep off the debris and follow a simple procedure to get rid of the stains.
Getting Rid of Tannin Stains
Sweep all the leaves into a pile, bag them and remove them to prevent the wind from blowing them back onto the patio. It's best to do this on a dry day; if any leaves are stuck to the concrete, loosen them with a rake.
Power-wash the concrete to get rid or surface dirt and mold. By itself, power-washing won't remove the tannin stains, but it will make the removal process easier my making them more visible.
Mix 2 to 5 ounces of dish detergent in a 5-gallon bucket of warm water, spread the mixture on the stains and scrub with a scrub brush. Use a stiff fiber scrub brush -- not a wire brush. Scrubbing with a wire brush will abrade and damage the surface. Rinse off the soap solution with the power washer.
Spread powdered detergent on stubborn stains while the concrete is still wet. Leave it for about 10 minutes, then scrub it. It's best to use a powdered -- rather than a liquid -- detergent to take advantage of the abrasive action of the powder. Use a product recommended for removing organic food stains.
Rinse well with the power-washer after you've removed all the stains.