First I'd like to point out that Seattle falls well within USDA 8. After that, you might want to pass on the silk tree because it has a consistent branch die-back problem in Seattle. Given a suitable location, it certainly grows, but a possible majority of local examples bristle with those infernal dead branches.

Here is a link that might be useful: Article - Trees that merit planting in Seattle by Arthur Lee Jacobson

Almost every Silk Tree I've seen growing in all sorts of situations have the branch die-back that Bboy mentions. Not a good choice in my opinion.
An upright Japanese Maple might work. You could prune it to the density you want for shadows in your house.
Mike

Mt Ash has it's problems also. They pop up here and there in my garden as sort of a weed, like English Holly. Not a problem, but a mild nuisance.
Deer chew on the small ones and turn them into bushes. This year, some kind of a worm ate the leaves while I was on vacation, leaving only the ribs.
I have an 8 footer potted up you can have. Just come and get it or it's compost.
Mike....in Maple Valley

My neighbor has mature Mimosa (Albizia julibrissin) that is really the star of the neighborhood. It is beautiful, but besides being delicate in this climate as described, and hard to get started, they are also wider & shorter like you mentioned (not invasive here at all).
How about a real ash? I have a 'Rayburn' ash that doesn't grow quite as tall as the big ones. It has delicate lacey leaves, slightly reminiscent of your Mimosa, and really amazing burgundy fall color, with a more upright habit.
- Tom

You're liable to be describing Fraxinus angustifolia 'Raywood' - which was being seen over 70' tall by 1982 - and has been seen to be prone to breakage.

I wonder if the OP is thinking of an acacia when she says Mimosa, and potentially invasive. Acacias are too tender here, and they are invasive where they don't freeze. Albizias freeze here in Olympia but if you get lucky and have a string of mild winters after planting, you can get them established. Seattle is milder but apparently has its own problems, with the branch die back mentioned above.

I'd second the recommendation for a japanese maple, or a vine maple if it doesn't get hot reflected sun. Or perhaps a honeylocust, they have the open structure and fine textured compound leaves similar to an albizia. They leaf out late in spring and drop early in fall. No flowers. I've only seen one around here though, at the BofA across from Capitol Mall, they don't seem very popular. It's a golden leaved cultivar.

Bboy, yes that is the cultivar. Breakage is more of a concern in very high wind areas or with very heavy snow loads with this one, and neither is a big concern here, especially for a heavily branched trees like these that can easily afford to loose a few. Also, most all sources list this one as smaller than the average full size ashes, despite the outlier you mentioned on this cultivar, which has been around for over a century.
Here and here are some nice photos at OSU. I also like the easy to mow no-rake leaves. Great tree for this area. :)
- Tom

Due to the history of failures here

A street originally planted with 'Flame' will usually have replacement trees of 'Raywood', and even more recent replacements of White ash cultivars. The replacements are, in each case, an effort to improve on the previous choice

--A.L. Jacobson, Trees of Seattle - Second Edition (2006)

who goes on to list 4 specific locations where narrowleaf ashes at least 70' tall could be seen in Seattle at the time of publication. The tallest of these was a 'Raywood' measuring 78 1/2'.

Honeylocust is also a large-growing tree, again known recently in Seattle over 70' tall in more than one instance. And too commonly seen here to produce the level of interest something less familiar.

Bboy, Jacobson is saying that White Ash cultivars have been more common replacements now, but not that they are replacing Raywoods. First, Raywoods replaced Flame, and now White Ash are also replacing Flame. There is nothing there about Raywood failures. :)
- Tom

The problem is that the narrowleaf ashes break up.

I like Albizia; I think they're worth the trouble. Not a lot of other that trees that will survive here at all have such a distinctive tropical appearance. Branch dieback seems related to frost damage and branch age. I have seen a few of them die off. I think they need to be watered well for a couple years to start off then kept on the dry side for best results. I think planting them small also helps. It does not reseed itself this far north but will sucker if the roots are disturbed. There are (or were) a handful of big ones in Olympia including a couple on the lower west side and a giant one on Bethel Ave NE well away from the water.

It's supposed to be a pathogenic problem. The dead branches on trees of some size are usually inside or beneath the main umbrella, with the damage to branch ends associated with cold damage not being usual. It's as if shaded portions are more susceptible and become infested.

Probably searching the web brings it up, maybe on the Oregon State University plant disease web pages for instance.

Came across this today:

The first problem to appear is one common to most ashes: shallow, aggressive roots. Next came the breakage of main limbs due [to] the genetic characteristics of this tree to produce a crowded cluster of limbs, all emerging from one point on the trunk. Recently, a more deadly problem has appeared: the infection of main branches by an air-borne fungal disease (Botryosphaeria ribis). As a result, 'Raywood' ash should no longer be planted

--Pacific Horticulture, Volume 72, Number 4, p. 6

I'm with Ian - I like Albizias also and despite all the nay saying apparent here, you can find numerous mature healthy ones in various Seattle locations. I concede they may be a bit tricky to get established but once achieved, a handsome looking tree. 30" maybe a bit optimistic but should provide adequate dappled shade. Again, not the slightest bit invasive in this climate. I also like true ashes, the honeylocust, Robinia 'Frisia' or 'Purple Robe' and goldenrain tree, Koelreuteria paniculata. All offer compound leaves and will provide a dappled canopy eventually.

FWIW, with few notable exceptions, you are going to encounter pros and cons with all manner of tree choices and certainly generate some strong opinions from various GW participants. But unless the tree is a listed invasive (and mountain ash/Sorbus aucuparia is getting pretty darn close) or extremely disease/pest prone, there are few, if any, overwhelming reasons NOT to pick whatever you like. It is afterall going to be YOUR tree :-)

The partial die-back of silk tree is quite consistent in this area. One would have to be regular about cutting these whole dead branches out to maintain a clean appearance.

Or not care that the tree has a bunch of dead branches.

If deciding to try one be sure to look for a named form with more pink flowers, as ordinary paler pink examples are not nearly as effective in bloom.

Thanks all for your contributions. I don't think I want another mountain ash. Aside from the foliage there isn't another attribute that I like about it. I'll look at the suggested varieties, though, but I'm thinking the height on many is much too large. 30ish is what I'm aiming for. Potentially 40. The tree I was interested in is albizia, aka "mimosa." I want something where light filters through the habit and that has compound leaves or separation btwn venation. Maples are lovely, but I think we have enough on our lot. If there are any more thoughts/suggestions, pls send them my way! Cheers, Sara

Jacobson's site is great, I just wish he posted images within his txt.

Quick follow-up to bboy: I did print out Jacobson's list and there are three contenders: Chitalpa tashkentensis (though the form and color of the flowers does not appeal to me) and Embothrium coccineum (but the habit seems willy-nilly). Koelreuteria paniculata is almost perfect.

Koelreuteria paniculata, despite glowing descriptions, seems a bit underwhelming to me. Fall color being the biggest disappointment. More ugly brown rather than shiny yellow even though planted in almost full sun.
I had 10 or 12 raised from seed, and finally cut them down. They sprouted right back up. I've been halfheartedly trying to get rid of them ever since. They are located in a out of the way part of the garden or they would be gone by now.
I have no experience in transplanting them. I imagine them to be container grown for sale, rather than bare root because of their spreading root system.
This is just my opinion for use in my garden. I'm sure others have a different view.
Mike

There's a large range of different Sorbus, varying from small creeping shrubs to fairly tall trees. The Washington Park Arboretum in Seattle has an entire collection accessible right off of Arboretum Drive.

Instead of fall leaf color the golden rain tree has startling colorful (pinkish) new growth in spring.

I completely forgot about that.
That pink growth does get attention for a short while.
Mike

The Goldenrain tree has always been one of my favorites, especially when they get bigger, although they can look a little coarse when younger.
Another one of my very favorite trees is Sourwood, Oxydendrum arboreum. It has a more upright habit and is really drop-dead gorgeous IMHO. It can get taller, but it's very slow growing and easily kept to size. There are also some very nice Crabapple cultivars that are disease resistant in our area and are sure beautiful in bloom.
Bboy your info on 'Raywood' Ash is taken way out of context. Those are California authors/experts from a California organization (Saratoga) published in a journal with many Cali-centric articles. Ash diseases are different for them, where they are trying to grow these just too warm. Raywoods do great here and in actual practice most have that stereotypical near-perfect ash shape. They're on many of the recommended lists for Seattle trees by local experts. [1 2 3 4]

Locust8, while Albizia is also one of my very favorite trees, I just want to make sure you know that they are almost always considerably wider than taller.
- Tom

All anyone has to do is visit old enough local street tree plantings - such as the familiar one on 35th - of Caucasian ash cultivars to see

1) These are not small-growing trees

2) These tend to split apart

Without the new disease issue you still have the problem of the aggressive roots and the poor branching structure. So even if the disease is not found up here, so what? The tree is still a stinker.

I have to add my two cents. I love my Albezia. It's only 6 years old and it is roof height and quite wide. A lovely shape, flowers from July until frost, a real stunner. It sure is messy though, don't plant it anywhere near patios or walkways. And, yes, there has been a little die back of small twiggy branches but nothing that affects the look of the tree.
btw, I also have a Chitalpa but it is unremarkable. flowers are insipid.

My Albezia was a beautiful 11 year old specimen tree in one of the front flower beds, causing much attention. Although late to leaf out, and leading to its demise, very late to drop its leaves. But the profusion of flowers and the dainty leaves from July to October more than compensated for the minor die back and bareness until early summer. But last year's early snowfall in November while we were in New Zealand sealed its fate. No one was around to gently tease the snow off its branches and they split so badly that we had to remove the tree when we returned. I am still considering replacing it. But I have another tree which has a more subdued beauty, is pest and disease free and has leaves that appear to be paint splattered during the spring and summer and is flaming orange in the fall that I would recommend if you can find a nursery that carries it. It is a Nyssa sinsis or Chinese gum tree. In our area it is virtually impossible to find these days.