In sun or shade is the first question, plus I'd say they're not deep enough for small trees, other than dwarf ones.

Dwarf pieris - variegated leaf colour, evergreen, flowers? Generally hardy but need an ericaceous/tree compost with good handful of grit to give free drainage. Pernettya is a pretty evergreen slightly ericaceous plant with either white, pink or deep pink berries in winter, best grown together in a group. For a hardy perrenial you cannot beat the agapanthus or hammerocallis varieties - very drought tolerant in any free draining soil in sun for most of the day.