Specifically, coastal, north facing, hillsides. That's not too specific, is it? This list is for Juli's native garden so it's not intended to be comprehensive - for example you'll notice that there's no trees on the list since I'm not sure that there's enough space. The most tree-like is Toyon. Although I thought I'd have few choices of plant materials, it turns out that there's lots of possibilities. Since even heavy soil can qualify as well-drained on a hillside, I haven't paid much attention to soil requirements other than trying to avoid those plants that need rocky soils, dune sand, or the like.
There is a lot of overlap between these plants and plants that might grow well under an oak tree.
I've already planted the first 6 on this list: V. carperiodes in 1 gallon containers and several packets of seeds of the following five annuals which were the contents of the Theodore Payne Foundation "Shady Mixture".
Venegasia carpesiodes (Canyon sunflower), drought tolerant to moderate. to 6'.
Clarkia unguiculata (Elegant Clarkia)
Clarkia amoena (Farewell to Spring)
Collinsia heterphylla (Chinese Houses)
Nemophilia maculata (Five Spot)
Nemophilia mensiesii (Baby Blue Eyes)
Symphoricarpos mollis (Creeping Snowberry), drought tolerant to occasional. CNPG recommends planting "with woodland strawberry, foothill sedge, and coastal wood fern to create a pleasing woodland" in sites with dry shade. Snowberry, Symphoricarpos albus var. laevigatus, has a more erect growth habit and is found on north slopes in large open colonies.
Toyon (Heteromeles arbutifolia), sun to part shade, drought tolerant to occasional. large shrub/small tree.
Lepechinia spp. (Pitcher sage), sun to part shade, drought tolerant to occasional,
Dryopteris arguta (Coastal Wood Fern), partial shade to shade, drought tolerant to occasional.
Rhus trilobata (Basket Bush, Skunkbush, Squawbush), drought tolerant to moderate, full sun to part shade, 3-5' high
Rhamnus californica (Coffee Berry), sun to shade, drought tolerant to occasional. Especially useful in areas where the exposure can change from deep shade to full sun. 'Leatherleaf' grows to 5 ft high - the smallest of the well-known cultivars.
Ribes speciosum (Fuchsia Flowered Gooseberry), sun to shade, drought tolerant to occasional, summer dormancy period can be visually mitigated by interplanting with an evergreen species. Sharp spines.
Mimulus aurantiacus (Sticky Monkey Flower) sun to part shade, drought tolerant to moderate. Mimulus aurantiacus can bloom almost year round on un-irrigated north-facing hillsides.
Satureja douglasii (Yerba Buena), partial shade to shade, infrequent to moderate
Viola pedunculata (Johnny jump-up), shade to partial sun . Santa Cruz Island habitats: grassy north-facing hillsides, rocky slopes, coastal scrub and grasslands. TPF says that it "likes some water" but doesn't specify beyond that.
Calochortus albus (Fairy lantern or Globe Lilly), sun to partial shade, drought tolerant to moderate, bulb, Santa Cruz Island habitats: mostly on north-facing slopes in chaparral, coastal scrub, oak woodland and pine forest.
Ranunculus californicus (California Buttercup), sun to partial shade, drought tolerant to occasional. Summer deciduous. Avoid summer water.
Lilium humboldtii s. ocellatum (Humboldt's Lily), shade, Summer dry! Santa Cruz Island habitats: coastal bluffs, shaded north-facing slopes, creek banks, chaparral, costal scrub, oak woodland and pine forest.
Pholistoma auritum (v. auritum) (Blue Fiesta Flower, Fiesta Flower) is in the Waterleaf family. Santa Cruz Island habitats: canyon bottoms, coastal bluffs, shaded north-facing slopes, grassy hillsides, chaparral, coastal scrub, grasslands and oak woodland. Claflora states that it grows well on hillsides.
Fremontodendron californicum (Fremontia, Flannel Bush) sun to partial shade, drought tolerant. Soil should be well-drained. Las Pilitas identifies decumbens as a preferred species, while CNPG extolls decumbens hybrids 'El Dorado Gold' as the most garden tolerant (4-6' high and twice as wide) and 'Ken Taylor' for it's graceful cascading growth habit. It is said to be particularly effective on top of retaining walls. These plants are also known for their irritating hairs.
Heuchera (Alumroot, Island Alumroot), part shade to shade, occasional to moderate. CNPG suggests that the best-sited plants will have winter sun and summer shade - for example under deciduous trees. CNPG also states that Heuchera maxima will grow and flower in dense shade whereas its cutlivars will not. Las Pilitas favors Heuchera micrantha which has the same site requirements.
Salvia spathacea (Hummingbird Sage), sun to partial shade , drought tolerant to occasional.
Near seeps or a neighbor's sprinkler these might be more appropriate:
Asarum caudatum (Wild Ginger), partial shade to shade, occasional to regular.
Mimulus cardinalis is a perennial that grows in damp soils in much of the west; it attracts hummingbirds and reseeds freely. Give it full shade in the hottest inland areas; otherwise plant it in sun or shade
California Native Plants for the Garden, Bornstein, Fross, O'Brien. (CNPG)
Theodore Payne Foundation
Las Pilitas Nursery
other less well-known web sites