Tt's very dry and rooty near a yew hedge, and if I understand your description, we're talking about a northern exposure for the perennials. You might consider Geranium maccrorhizum (Big Root Geranium). This is a groundcover plant, about 8 inches high, with highly fragrant leaves and is semi evergreen. Most varieties bloom fuschia in late spring. I believe there is a white flowering type and perhaps a variety with variegated leaves (which might be slower to establish). It does well in very dry soil and shade, though not heavy shade. A handsome plant for difficult areas and is also highly deer resistant.

Shade isn't a concern for mine. I have some in full sun and some shaded by my cherry tree between where they are in shade from 1pm on.....all bloom and grow equally well. I would go for a short sunflower.....I'm trying some this year that grow to 3' but I received them in trade with no name. Even Moulin Rouge would be nice. Mine grew to about 4.5' last year, but very open form and many blooms per plant. I'll have some of those which have self-sown themselves very near to my Lavender as well as Amaranthus 'Intense Purple. It should be pretty interesting with all those hues of purple :D

Which lavenders are you planning to grow? angustifolia? stoechas? other? And, if you're planning to harvest then consider your ease of access. A wider path is always more comfortable than a sidle track. Rosa rugosa is very drought tolerant, has attractive flowers and interesting hips. Myrtle has aromatic foliage and tolerates clipping. Santolina or Artemisia would also be aromatic as well as silvery-leaved. Spathufolium could be an interesting edging. The silvery one with the purplish cast to the leaves. (Capo Blanco?) They each have yellow flowers but they could be shorn to prevent flowering if so desired.

My neighbor who lives across the street of me planted white standard iceberg roses with a hedge of french lavenders in the front yard. It matches perfectly with the white picket fences. They use blue stone /bricks to hedge the garden bed. It creates this really nice comforty, cottage feeling. My neighbor sticks to the white, green, blue, and silver color scheme. Almost a white garden....They grew this climbing SOLANUM JASMINOIDES - that produces floriferous white flowers throughout the season, along the front porch. Points to consider: What color is this retaining wall? and would you intend to plant other flowers beside these roses and lavenders? I'm pretty sure that bright red and yellow colours would destroy this cottage feeling if blended together with those lavenders and white roses. I've always see my neighbor pruning their roses and lavenders deeply...so if you're after a low maintenence planting scheme - I don't think it's a right choice. The roses would loose their leaves in winter etc. In my honest opinion - that kind of planting look nice in the front yard along the white picket fence - prove me wrong. Have you consider a row of camilleas? Or maybe a hedge of dawf size apple trees....or maybe a hedge of star jasmine?