Answer: Prostrate abelia will grow in part shade/part sun but won't perform will in a mostly shaded area (such as you describe). Instead, you might consider winter bergenia (Bergenia crassifolia) which has big cabbage-like leaves. The leaves will get a hint of red if they are drought stressed or cold. Then for a bonus Winter Blooming Bergenia blooms when most everything else is still languishing under the effects of winter. It grows best in light shade and can tolerate very little water. I have seen it perform admirably with no additional water in the shade of oak trees. The leaves grow to 6" and the flowers to 8-10". It slowly spreads by surface runners.
Or, consider Leonotis leonurus (Lion's Tail). It is one of the most consistently colorful plants in the dry garden. Once established they require little extra irrigation in the coastal area except in containers or extremely well drained soils. Lion's Tail will grow in full sun to dark shade. Leonotis leonurus plants will slowly spread by root suckers. It looks its best if the dead flowers are trimmed, but will still put on new growth and blooms without the extra grooming. There is a white sport of Leonotis that can add a little interest and spice.
A final suggestion is Plectranthus argentatus (Silver Spurflower). The Plectranthus argentatus blooms are nice but really are not the main attraction. The large velvety gray succulent leaves make the show for me. Silver Spurflower gets to 3 feet tall by 6' across. It really prefers some shade and can even take dark shade. Not too much water is required. I use Plectranthus argentatus under oaks because of its water thrifty ways. Plectranthus argentatus brings a great light to dark spaces.
Best wishes with your landscape!
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