phoebe:

The best long-term answer is to resolve the drainage issue. That can be done in at least two ways:

1. Break through the hardpan. If you're lucky, the hardpan is only 1-4 feet deep. Rent a machine auger (or hire a contractor who can drill big holes) and pop 4-6"-wide holes through the hardpan in various places. Fill the holes with rock/gravel and perforated piping for some drainage chmineys. If the hardpan is deeper/thicker, you'll at least have created some cisterns that will collect a good deal of the excess water.

2. Install French drains. Essentially, they're trenches filled with piping/gravel and are graded/sloped to channel the water off the site. Details available upon request.

There are a few plants that do "okay" on shallow soils with poor drainage. Knowing where you live (or Sunset Western Garden Book zone) would help all posters in providing you with a worthwhile list.

Joe