Why driveways under trees turn green and black in Austin

If your driveway is darker on one side than the other, look up. The dark half is almost always the half that sits under a tree. Around Austin, where live oaks and cedar elms shade a lot of front yards, it is one of the most common reasons people call us, and the cause is simple.

Concrete is porous and holds water. A driveway in full sun dries out within hours of rain or a sprinkler cycle. The same concrete under a tree stays damp much longer, and damp concrete is where algae and mildew grow. The tree feeds it too. Leaves, live oak pollen tassels, berries and bird droppings land on the slab and break down in the wet, which gives that growth something to live on. Leaves that lie on wet concrete can also leave brown prints of themselves behind from the tannins they release.

The growth starts as a faint green film you mostly notice when the driveway is wet. Left alone it darkens to grey and then black, and it follows the shade line almost exactly. That pattern is the giveaway. Oil sits where a car parks. Rust shows up orange, often in a fan shape near a sprinkler head. A dark stain that matches the outline of a tree's shadow is organic growth.

Cedar Park is a good example. Most of the calls Nathan gets there are for driveways, and the trees are the reason. Shade and falling organic matter make algae grow back faster on those driveways than on an open, sunny one. Many homes there also have stone walls and trim, and stone holds algae deeper, which is why it can look black rather than just dirty.

The good news is that organic growth is the easiest of the common driveway stains to remove. It sits on the surface instead of soaking in the way oil does. A surface cleaner lifts it evenly across the whole slab, and a cleaning solution given time to soak into the stone and the edges deals with what is left in the pores. What we do not do is blast stone or the edges of the concrete at high pressure. That etches the surface and leaves it rougher, and rough concrete holds the next round of growth even better.

It will come back, because the tree is not going anywhere. A few habits slow it down. Blow or sweep leaves off the driveway instead of letting them sit through a rain. Check that the sprinklers are watering the lawn and not the concrete, since overspray keeps the slab wet every day. And if one low branch is the only thing shading the driveway, trimming it lets the sun do some of the work. A shaded driveway will still need cleaning more often than a sunny one. How often depends on how much shade it gets, and we would rather tell you honestly than sell you a schedule.

Driveway cleaning starts at $80, the sidewalk can be added in the same visit, and we offer a senior discount. Send a photo of your driveway through the quote form and we will tell you what we are looking at. Free quotes, seven days a week.