Research the narrow growing yews. Last year during an extended drought period, I planted a row of two yews and a tree form crape myrtle with that drifting group replanted in an alternating pattern over a narrow 60 foot long area which runs in the space between a chain link fence and a driveway. I do not remember the cultivar name of the Yews, but they are suppossed to grow much more narrow than even the Hick's or the Plum yews grow. We kept them watered regularly and they are still surviving. These yews are evergreens, which are supposed to grow 10 to 12 feet tall, but can be pruned shorter and spread only about a foot or so. Some are in shade and some are in more sun. The Crape Myrtles that are planted amid them will eventually provide for the yews almost constant shade during the Crape myrtle's growing season. The Yews will also receive quite a bit of part-sun in the winter when the tree form Crape myrtles are dormant. The bed is currently mulched with shredded cedar mulch, and is gradually being fill in with flowering ground cover shrubs that should grow at the feet of the Yews and Cm's along with filling in the area of the currently mulched bed. at the end of the chain link fence and still along the driveway I planted an ornamental and narrow growing, pink blooming, Corinthian Peach tree. At its feet I planted some wave patunias on the north side, a dwarf spirea on the south and so for two Dwarf Yaupon holly on the east side of the the tree. Later on I will extend the bed along the driveway and plant a Black Tupelo tree about 20 feet south of the ornamental peach. Since that tree will be in the front yard area I will be planting the tree on the property line along that narrow side of the driveway, and as it grows and spreads, I plan to keep it pruned up to provide enough space under the canopy for pedestrians and traffic flow on the street which is about 15 feet further south running along the front yard curbing. As of now I have not planned anything further to be planted on that side of the driveway between the ornamental peach and the Black gum, Tupelo tree. Over time though I might change my mind.

I wonder if you asked the head gardener/groundsman at a botanical garden, if he/she might know of a nursery that will carry the size you are looking for? They sometimes have to fill in empty places, and also may have a 'stash' of large container plants in their back-up nursery. I wonder also, if one might plant a rapid-growing "temporary" tree to fill the light-blocking needs and act as a nursery-tree for slower growing natives? The 'temp' could be cut down after the native had reached a suitable height. Does that sound workable? Here is a link that might be useful: CA Botanical Gardens

If it looks at all like a giant Queen Anne's Lace, it might be poison hemlock. The leaves are lacy, but not as fine as fennel or dill, and the flowers are a collection of tiny flowers in a circular, flat arrangement, as in Queen Anne's Lace. It likes to grow along shady margins, in fairly dampish soil. The ones around here in better soil easily reach 6 or 7 feet.

I'd recommend summersweet clethra. It comes in white or pink, grows to about 5 or 6 feet and has fragrant flowers from about mid July to mid August. This plant has a very clean appearance even without pruning