Here are three tough perennials that thrive in the USDA Zones noted, in poor soil, full sun, and with very little water or other attention:
Yarrow (Achillea sp.), Zones 3 to 9, This flowering perennial grows to about three feet and is hardy in a wide range zones, depending on cultivar. Many cultivars have yellow blooms; some are red to pink, or white; foliage is either green or gray-green. They like good drainage but will thrive even in clay.
Dwarf plumbago or leadwort (Ceratostigma plumbaginoides), Zones 5 to 9- This short (about 6-8 inches tall) ground cover isn't picky about soil, water or sun. Dark green foliage sports true-blue flowers in summer, and turns to bronze-red in Autumn with tiny, burr-like seedheads. This plant spreads like crazy, so give it some room beside the road.
Artemisia Powis Castle, Zones 5 to 8- A shrubby, woody perennial, ?Powis Castle? features silvery-green, lacy foliage that grows to about two feet high by three feet wide. This plant gets raggedy-looking in cold winters; all you have to do is cut it back to a few inches above the ground in Spring to rejuvenate it.
Most plants, no matter how hardy, need regular watering in their first growing season to establish themselves. Once established, this trio will take care of themselves. Plant them together for a real traffic stopper. Stephanie Hainsfurther