I see your Bradford Pears, and raise you the virtually useless Cotton Poplar. 
Thanks, elelle. I don’t know if the Carolina Sapphire or Blue Ice would be able to get through the winter in my zone (I’m on a 5/6 border). I love the look of them both, though. As an accent plant, I’d try one or two, but I don’t think I could use them for a long privacy screen.
The Yoshino, though, could be an option. Perhaps as a plant to intersperse with the Green Giants. I like the idea of “Pacific woods looking”. Again, I’m concerned about the zone. I think the Green Giants are leading the pack, but I’ve also stumbled onto a Thuja “Green Splendor”, but I can’t find out much about it.
CM
Generally, Arizona cypress is listed as Zone 7 and warmer, that’s it’s native range, but it can take higher elevations, so is probably more cold-hardy. Here’s a site indicating that it does well in Colorado, doing fine in zone 5.
For Cryptomeria ‘Yoshino’, in a scoot thru various hort oriented message boards, some in zone 5 have have grown fine, some, in harsher winter wind conditions, perish. If you aren’t using it as a true windscreen (the only trees in a cleared area), I’d give it a try. I couldn’t find a photo on the web that does it justice, but it is a stunning tree
Thuja plicata ‘Green Splendor’ (for whatever reason, botanical cultivars are indicated by single quote marks) was first known as Thuja ‘Hoyt’, after the Hoyt Arboretum, in Portland, Oregon. It has the same attributes as Thuja ‘Green Giant’, but is more columnar, ie, a narrower profile than Th GG.
Thuja ‘Green Giant’ is a hybrid of the native Th plicata and Th standishii, more readily available, so you should be able to get good sized trees for a reasonable price. As I said before, get a 4’ tree rather than a 1’ one, it’s worth the extra money.
Thuja ‘Steeplechase’ is a sport of ‘Green Giant’ (a noteworthy branch from a GG tree that exhibited different traits, cuttings taken and propagated). It is more feathery in texture than GG, and is relatively new, being patented by Wayside Gardens. Pricier, and less available. It would be harder to find larger trees to plant.
There ya go. Go out and plant them trees, and remember to give them some extra nurturing the first couple of years for best growth.
Thanks again for all of the opinions, especially elelle.
Before I buy, I’ve stumbled onto two other potentials (damn internet!)
Siberian Elm (ulmus pumila)
Sub-Zero Hardy Canadian Hemlock (tsuga canadensis)
Anyone have any experience with these? They appear to form a much nicer wall than the green giants, but I have no idea on their growth rate.
I promise not to throw any other curve balls. I need to make a decision and place an order.
Well, Consty, I’m not going to abandon you at this point…
Siberian Elm is quick growing, but can have brittle wood and suffer from storm damage. There’s also the issue of invasiveness. From the above site:
That means it’s pretty seedy. At the least, that means you’ll have to mow or pull hundreds of seedlings in your own yard, which will be a PITA. Also, it may be bad for the environment, displacing native species.
From Dr. Michael Dirr, the reigning tree and shrub master, author of the hefty reference book used by most horticulturalists: Siberian elm is “one of, if not the, world’s worst trees…a poor ornamental that does not deserve to be planted anywhere”. And he really tries to give a tree a break in promotion.
Canadian Hemlock is a beautiful tree. Not especially fast, but worthy as a screen tree. In the same screen row with the Cryptomeria ‘Yoshino’ previously mentioned, the next tree is a Canadian hemlock. Both were topped by a bad ice storm 5 years ago, and both recovered equally well. Here’s the NC State University page on Tsuga can. They are a nice reference for many plants as to garden worthiness, especially trees.
I’ve seen both the elm and hemlock used planted closely together and sheared to make a hedge, but that does require some maintenance. Might cut down on the seedy and breakage issues with the elm, though.
Umm… hemlock?
Quartz, I’ve waited a bit to see if you would say what your question was exactly about Hemlock, but, I suppose you are just confusing the legendary hemlock poison that sucked Socrates[url+http://www.botanical.com/botanical/mgmh/h/hemloc18.html]last breathe, with Tsuga canadensis. They are not at all the same plant. Tsuga c. is a big ol’ tree, native to North America, and the last gasp of Socrates is, by all accounts,
Well, then, hit the wrong button…see if I can recoup a bit…
Tsuga canadensis is a tree, native to North America. Socrates poison was Conicum maculatum, a large plant that looks like parsely or Queen Anne’s lace that is very deadly. I remember it well, as it’s the only plant my well-taught herb teacher yelled at me as being stupid to even touch it. That’s why you find a good teacher.
So, why’s it called Hemlock, then? I’ve been educated here about the idiocy of American laws on poisonous flora, and you can bet your boots that some lawyer’s going to ask, “And why exactly did you plant a tree called Hemlock?”