There are two different species of Wax Myrtle (Northern Wax Myrtle and Southern Wax Myrtle). The Northern is semi-evergreen to deciduous; Southern one is deciduous. Northern one would probably is a better choice for you. Regarding the Crape Myrtles, many are fine to zone 6b/7a. Most all of mine survived...even after the Winter from he// (deep snow probably helped with that btw). The thing is, you need to consider heat requirements (as well as cold minimums). If a zone does not experience enough Summer heat, the wood may not properly ripen. So ironically, there are many semi-tropicals that will grow robustly in the Mid-Atlantic zone 7ish climates that do less well in Pacific NW (Zone 8+ climates). Crapes have a relatively high, Summer heat requirement. It's July, see any in bloom around you. Finally, you specifically wanted a 'tree'. Crape may be a viable option for you as a shrub (with some degree if annual dieback), but not sure if a tree is possible. Give it a try and good luck!

Pacific wax myrtle is locally native. You should have no problems with either cold or drought. Yes, Tacoma is colder in winter than Seattle, but that's not saying much, is it. Wax myrtle grows wild in the Aberdeen/Hoquiam/Westport/Long Beach areas and they're all colder than Seattle. They get more rain too, but still get none in summer, and it grows in sandy and rocky soils that do not hold water anyway. Pacific wax myrtle makes a great low maintenance hedge. I am always highly in favor of replacing Photinia because of its disease issues with our wet climate. Won't be a problem with wax myrtle. Ceanothus are too wide growing for your hedge. Oregon grape, Mahonia aquifolium, is another native evergreen shrub to consider, but it doesn't normally grow in quite so much sun, and only about 4-6ft tall. I do see it planted in full sun but it's not its preference. Plus, it's very expensive and slow growing, but it does bloom in March, and people won't push through it because it's so prickly. Incense cedar is a tree. You don't want a tree. You could not keep it trimmed to a hedge. There are also hollies and yews that might do what you want. I see Yew HM Eddie planted as a hedge, but I think it's slow growing and I don't know how tall it's supposed to get, probably not 12ft. I've seen hedges of small-leafed hollies but I don't know what species or cultivars they are - I. meservae maybe, or Blue Boy and Blue Girl. Then there's Portuguese laurel, one of my favorites. Evergreen, wants to be a tree but lots of people keep it trimmed as a hedge. Smaller leaved and less of a monster than its cousin the English laurel, which you do NOT want. I like the purple-red stems of the Port. laurel, contrasting with the very dark green leaves. Full sun, no irrigation. And strawberry bush, Arbutus unedo and Arbutus 'Marina'. Very pretty, cousins of the native madrone, but more hedge like. A. unedo has white flowers in fall and red fruits in spring if winter was kind to the flowers. Red bark, small dark evergreen leaves. A. 'Marina' is more tree like, I don't know about keeping it as a hedge, but it's very pretty and does not get any of the leaf spots that plague both A. unedo and the madrone, although you should have no disease problems with the A. unedo in full sun with no overhead watering. I see hedges of A. unedo all over the place. I had an A. unedo 'Compacta' that got to be easily 6ft tall and wide. I pruned it once a year to keep it in its place. Full sun, no irrigation. Oh, it takes years and years to keep all the plants straight! and there's new ones every year, too. No one ever knows them all, that's why there's so many reference books. People will specialize in one group or another, like Conifers or Natives or Australian plants or Perennials.

Thanks Sherlock! Unfortunately they are not webworms. The cocoons are real small and they are right on the tips of the new growth. There are small brownish orangish colored little specks inside the small white webbing. I usually spray water on them and get it out with my fingers. I have a camera but it is a cheap model and does not do closeup very well. I've seen brown grasshoppers hanging out in the shrubs. It gets my goat because the shrubs are trying to grow yet their new leaves are being munched short. :-(

Generally speaking, you can usually take off about a quarter to a third of the top growth of most bushes without too much lasting damage although it will take a while for the plant to recover. The problem here is that you didn't provide a picture of how big the plants are now or their present shape so its not easy to say yes or no to your proposed trimming plan. Certainly, you can always selectively trim errant stems, trunks, or branches that are not doing what you'd like them to do but major pruning should always be done with an eye toward minimal cutting at any one time rather than major pruning in one fell swoop. Hope this helps. Steve