Bill,
It is recommended that you only plant grass above your drainfield to
prevent roots from clogging and damaging your drainfield. Given the
cost of replacing a drainfield, I wouldn't risk putting the shrubs
above it. Maybe some of those ornamental grasses could be a good
compromise. I am an Engineer for our Water and Wastewater Utility and
get questions about these systems quite a bit. A good homeowner's
guide for septic system information can be found at:
http://www.epa.gov/owm/septic/pubs/h...guide_long.pdf

The general EPA onsite site is:
http://cfpub.epa.gov/owm/septic/home.cfm?program_id=70

Please contact me if you have any additional questions.

James