Ask Mr. Smarty Plants

Our advice is, don't do it. To have hope of being successful planting a tree, it would have to be quite small and still could have transplant shock. The site you describe, bounded by a driveway and a curb, is also risky. Any tree's roots are going to extend as much as 3 times the drip or shade line of the tree. Either the driveway, curbing and the street beyond it is going to hurt the roots, or the roots are going to push up and crack the concrete. They just don't co-exist well. You might have to wait 10 years for a tree to get up tall enough to shade the 2 cars, by which time you could have well moved on to somewhere with a 2-car garage or even existing trees. But, to us, the most compelling argument against deliberately planting trees over a parking area is the mess. It doesn't much matter which tree it is, or what time of year, something is always dropping. Surely you have seen the cars with what looks like green fur on them driving around Austin? And where have they been? Why, parking under an oak which has been dropping its lovely allergenic green blossoms. And how about aphids getting in the trees? They you have that charming aphid by-product, honeydew, making a sticky mess on your car. If you plant a fast-growing tree that doesn't take the time to develop a very strong structure, a windstorm or snow (which we now know can happen in Austin) could drop a branch on the "protected" car. Leaves, seeds, acorns, bugs, twigs and branches-we love trees, too, but they make lousy carports.

Since you asked, we will suggest some locally native trees that would be nice. As for trees with no low branches, the only way you avoid low branches is to prune them up as the tree grows. And you can't trim them up until the upper parts of the tree have developed enough leaves to nourish the tree. We chose these four trees because they are native in and around Travis County, are considered good shade trees, and have moderate to fast growth rates. 

Possible Shade Trees Native to the Austin Area: 

Carya illinoinensis (pecan) - deciduous, takes 15 to 20 years before it starts bearing pecans, best production is on trees 75 to 225 years old. From Floridata: Carya illinoiensis

Fraxinus texensis (Texas ash) deciduous, 30 to 45 ft. tall, moderate to slow growing

Platanus occidentalis (American sycamore) - deciduous, 75 to 100 ft tall, fast growing, can reach 80 ft. in 20 years or less Floridata American Sycamore

Taxodium distichum (bald cypress) - deciduous, grows to 50 to 75 ft. or taller, moderately fast-growing, reaching 40 to 50 ft. in 15 to 25 years. USDA Forest Service Baldcypress

From our Native Plant Image Gallery:

Carya illinoinensis

Carya illinoinensis

Fraxinus texensis

Fraxinus texensis

Platanus occidentalis

Platanus occidentalis

Taxodium distichum

Taxodium distichum