Mounting fixtures to pipe is always a path to destruction.

Try using Rab PVC Mighty-Posts. Durable, easy to fix if busted, and corrosion-proof. To alleviate most landscaper damage, you can also set them in a 12" sono-tube base. They are also really easy to install and inexpensive.

http://www.rabweb.com/product_line_detail.php?prodline=POST

It's becoming the common mounting system for most landscape light lighting out here. I wouldn't use them for any heavy fixtures, but for up-lights and pagoda type fixtures, you can't go wrong.

Good Luck!