What are the large trees?

Do you have room for fairly large evergreens like western red cedar underneath?

They take quite a bit of shade.

Maybe you can reduce the shade by pruning the trees. Even dead twig removal can increase filtered light.

Clumping bamboos (Fargesia).

Coast redwoods like shade. Not sure how much they want. They grow fast but need a lot of water. They don't like really cold weather. Some ppl prune them into tall hedges.

Thanks for the quick responses. I'm not quite sure about the species -- some kind of fir, maybe? Green pines year-round, wide enough at the base that a grown man can't hug around the trunk and towering about 3-4 times taller than our 2-story house. We have 13 of these in our back yard -- almost like a miniature forest -- which is really nice except it make pruning etc. almost impossible due to the sheer height and size.

How quick do clumping bamboos grow?

I thought coast redwoods depended on more or less constant morning fog. Used to live on N. California coast and we'd get fog almost all year.

Difficult to achieve anything truly "fast growing" in an area where there would seem to be a lot of competition from established tree roots. A broadleaved evergreen would offer some welcome contrast to the needled evergreens - Japanese aucuba will grow in pretty much full shade and will get to be of considerable height in time. Variegated forms will brighten up what must be a rather dark area as well.

Regardless of what you select, be prepared to be patient and offer some TLC for a few years until it becomes established and a bit more self-sufficient among the conifers.

Coast redwoods prefer fog. They water themselves with fog moisture. But there are 2 enormous ones growing down the street from us, so it's possible for them to survive. Of course these were planted decades ago before the climate changed and the sun became so much harsher. We're having trouble keeping our little ones alive because the summers are just too hot here. We spray them down with a mist from hose every day. I think once they're established, with some shade, they'll be OK. But if no fog, then watering, a lot. Of course they love it when it rains!

I have a Japanese variegated aucuba, 2 actually, that definitely prefer shade and drier soil. A hedge would be pretty!

Bamboo is going to grow much faster than any conifer or broadleaved evergreen.

If it were me, I'd plant different plants for more interest. Rather than a row of one thing.

Amen to that.

Ok, that's good input. Thanks. The reason I'm thinking a straight "wall" is that I'm going to pitch the idea as a joint effort with two neighbors who are in the same boat. Just me doing it will be an improvement, but you can still catch the warehouse corners; by coming with a packaged solution, "let's buy 40 each of these for X dollars and solve this once and for all!", I hope to make it easy to say "Yes".

Speaking of which, are any of the options mentioned above subject to restrictions in Snohomish cty/Washington? I know some spieces are off-limits to certain states and counties, and some even require zoning permission.

Speaking of which, are any of the options mentioned above subject to restrictions in Snohomish cty/Washington? I know some spieces are off-limits to certain states and counties, and some even require zoning permission.

No. All are non-invasive, commonly sold and widely planted. Most zoning or permitable concerns relate to the removal of existing trees and their suitable replacements or the placement of the tree in the first place (primarily as they relate to powerlines).

If the building is right on your property line, can you grow a vine up it? If so, I'm sure there are some that would like the shade and look quite pretty on the building