Few hebes are very low growing :-) If they survive long term, most get to be quite large shrubs.

I'd consider sunrose, Helianthemum nummularia. A low, spreading, evergreen shrublet that produces small, tissue paper-like flowers in a wide range of bright colors in late spring to early summer. Very drought tolerant once established.

Other very common choices for this sort of application would be heaths and heathers or candytuft, Iberis sempervirens.

Logician - the lewisia plants are lovely! The wall is a bit hard to see but the “rocks” underneath are not very attractive concrete blocks. Is there a way to disguise that with the lewisia (and maybe another plant)? The pics I saw of the lewisia showed them tucked in between very attractive stones. I am trying to beautify and disguise also...

Gardengal - the sunrose is quite pretty too! Regarding the idea of hebes - I am looking for some type of a low hedge or deliniation at the edge of the grass and the top of the retaining wall - like boxwoods but not as fickle. Are you thinking the sunrose could serve that purpose?

In our climate, hebes are probably more fickle than box and not nearly as hardy :-) Those that are will get big.

The sunroses will be low....usually not more than 8-10" tall at most. You might consider Vaccinium moupinense. Or the heaths and heathers. Once established, these tend to be quite drought tolerant.

Another thought would be one of the dwarf variegated abelias, like 'Kaleidoscope' or 'Confetti' or 'Lemon Zest' or any of the Anniversary series. Most will stay right around 2' tall and wide but may need some light pruning of longer, wayward branching

Thank you! One more question: for the “front” of the retaining wall, would you try to take out what’s there to plant new? Or just prune back dramatically and plant new among what’s there? The plants have been there awhile and my concern is that the roots might have woven their way into to concrete blocks of the wall and I don’t want to compromise the integrity of the wall.

I was considering the cleared area at the top of the wall for Lewisia; some Lewisia plants will slowly form a mat via the hen-and-chick process, other Lewisia plants may remain a single rosette of leaves. If you want to put them in a more vertical part of the wall, you would need a quite wide pocket or crevice to transplant the size of plants typically found for sale. If you can get seeds, those could be started in quite narrow crevices; the seedlings often bloom in year 3. Lewisias would take a very long time to disguise large rocks. If planted on a horizontal surface, place a shallow layer of small gravel just below the leaves; otherwise soil, weeds or grass might contribute to the plants rotting out.

Iberis sempervirens would work.