Well, if you're hitting trees now, add a foot or two and you'll really be in trouble. A couple of things you might consider.
First, the higher you mount the antenna the better the performance all else being equal. While I easily get by with an antenna whose tip is 16 feet up, you apparently can not. DX Engineering (http://www.dxengineering.com
) makes tophats for Hustlers and most screwdriver antennas. Properly installed, the antenna will perform as well as one with a whip twice as long. Put another way, you trade length for additional windloading and complexity.
Lastly, you could mount the antenna lower and lose some efficiency. If you do, the most important item to remember is to keep the antenna as far away from metal as you can, especially the coil. This is nearly impossible on some screwdrivers as their masts are rather short. You might think about mounting it on the front of the vehicle instead of the rear.