Oak trees (Quercus spp.) can be quite messy, especially in the autumn when they are dropping acorns. Large oaks produce a prolific amount of them, and they are rich in tannins, which are responsible for the brown stains they leave. Oak leaves are also full of tannins, so if you have a paver patio or walkway under an oak tree, it's going to get discolored, and there's not much you can do to prevent it short of sweeping the pavers daily.
Tannin stains are unsightly — no doubt about it — but if you're patient, most of them will probably disappear by the time spring rolls around, and those that remain will make your pavers look aged, which isn't such a bad thing. You may prefer to have a pristine and unblemished paver surface, though, and if so, you can remove stains from pavers caused by oak trees with a minimal amount of effort.
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Tannin Stains Are Organic
Organic stains, such as those caused by tannins, sink deep into the pores of the concrete, but unlike some stains, they aren't a product of a reaction with the concrete, so they aren't permanent. You can try scrubbing them with a solution of dish soap and water, but that's going to involve a lot of scrubbing, and you may not be successful. You'll have much better luck if you use a cleaner that can actually dissolve them, and that isn't hard to find.
To be really effective, your cleaner should contain bleach or ammonia (but not both because the combination produces toxic fumes). Household chlorine bleach is not a good option because it's toxic to plant life and can damage your lawn or garden, but oxygen bleach is environmentally friendly and works just as well if not better. You can use the stuff you put in the washing machine, but to be really thorough, you should use a product designed for outdoor cleaning.
How to Clean Tannin Stains
Power washing is a good way to remove stains from solid concrete, but it isn't as good for pavers because the high-pressure spray displaces the packing material between the pavers. You can use a power washer to clear off debris prior to the stain removal procedure, being sure to use a nozzle with a wide spray pattern and keeping the tip at least a foot away from the pavers. You can also use a garden hose and a standard spray nozzle.
Mix a cleaning solution by following the recommendations on the label if you're using an outdoor cleaner or by adding 2 or 3 tablespoons of laundry oxygen bleach per gallon of water. Pour the solution on the stains or apply it with a plant sprayer and let it soak in for several minutes because it needs time to work. Put on some rubber gloves and scrub the stains with a stiff scrub brush until they come out and then rinse the area with plenty of clean water. Some stains might be stubborn, so just follow the old advice: If at first you don't succeed, try, try again.
How to Prevent Tannin Stains
Your oak trees aren't going to stop dropping leaves and acorns, so if you're not a fan of tannin stains, you have to sweep the pavers often. If you have a large paver patio, you might want to invest in a leaf blower. A cordless one is easy to maintain and not quite as noisy as a corded or gas-powered one.
Once you've cleaned your pavers to look as good as new or if they are new, sealing them with an acrylic sealer will prevent the tannins from penetrating. Stains will still occur, but they'll be on the surface and much easier to remove.