Nasturtiums?

Look at the 'Poor soil plants' on the Goypeadia page I think you might be pleasantly surprised? My suggestion is Dianthus and here is a list of some others: You have not mentioned shade?

Perennials that will grow in poor soil:

Achillea serrata (Millfoil or Yarrow)
Ajuga genevensis (Geneva Bugle)
Alyssum saxatile (Goldentuft)
Arabis albida (Wallcress)
Cerastium tomentosum (Snow-in-Summer)
Dianthus deltoides (Maiden Pink)
Dianthus plumarius (Grass Pink)
Eryngium campestre (Hundred Thistle)
Euphorbia Myrsinites (Myrsinites-like Spurge)
Geranium sanguineum (Blood Red Cranesbill)
Geranium maculatum (Spotted Cranesbill)
Gypsophila paniculata (Baby's-breath)
Helianthemum vulgare (Rock or Sun Rose)
Iberis sempervirens (Evergreen Candytuft)
Linaria vulgaris (Toadflax)
Nasteriums
Phlox subulata (Moss Phlox)
Potentilla tridentata (Wineleaf Cinquefoil)
Salvia azurea (Azure Sage)
Saxifraga pennsylvanica (Penn. or Swamp Saxifrage)
Sedum acre (Goldmoss)
Sedum stoloniferum (Running Sedum)
Sempervivum (Roof Houseleek)
Sempervivum arvernense (Auvergne Houseleek)
Verbascum Thapsus
Veronica rupestris (Creeping Speedwell)
Viola cucullata (Blue Marsh Violet)

You haven't said whether the area is sunny or shady - the list Drc gives covers well for sun and part sun - if shady and dry, Lamium maculatum varieties.

Thank you for the answers they are very helpful. The area is sunny, the soil is clay, so gets very dry, but even with continued watering nothing much seems to grow.