Not sure how much height you really need, how fast. One thing to consider is that when you plant to the south, you may not get as much shade as you expect during the actual hot months, when the sun is to the north; and you may get more than you want in early spring and late fall, when the sun is to the south and you want your patio to warm up. Trees to the west-northwest are probably best for shading a patio in most of New England.

Take a look at heptacodium - it's quite narrow and in fact is adaptable as far as the number of main trunks. A lovely and unusual tree, and because it flowers late, and has long-lasting, colorful calyxes, it looks good for a long time after the flowers fall. The exfoliating bark is interesting, and a well-grown specimen can look good even when the leaves are gone. It can be found relatively inexpensively because it's not especially hard to propagate and grows fairly fast when it's young.

If you don't want messy flower fall, I'd stay away from cherry. Of all the cherries, I'd maybe recommend prunus mume, but it's not reliably hardy in your zone as far as I know.

Stewartia is a lovely smaller tree, but prefers some shade. In your zone, it might be fine in sun - I have some in full sun that do well, but it's cooler here on the cape in summer, so you might want to check with your arborist or a local nursery. It can be quite pricey for larger specimens, in my opinion, but it's a great tree. S. pseudocamellia is the most common (and, most people think, the most beautiful) but some other varieties are narrower and might be better in your situation.

There are some smaller varieties of katsura (cercidiphyllum) - one of the trees on my "must have" list. One, Heronswood Globe, only reaches 15' - not sure if that's tall enough for you. That's one of the most beautiful trees - no flowers, but really great foliage, lovely color and shape.

lovesummer,

The problem is that you want both fast growing and small. That's really kind of an oxymoron, unfortunately. Are you planning on buying decent sized trees? They are a lot more expensive, but if you can do that it will be a big help.

I agree with DTD's suggestions. I have Cercidiphyllum (katsura) 'Heronswood Globe'. It's gorgeous, but I have to say it is pretty slow growing, or maybe just slow to establish. I bought a tiny starter tree and planted it last year and so far have gotten, at max, 12 inches of growth. Maybe it will spring up this year, we'll see. But I think it could easily take you 8 years to get a screening height from it.

Another option is Styrax Japonicus. There are varieties that are more columnar than others. It has pretty bell shaped flowers, so it is well situated next to a deck or walkway where people can see the flowers from underneath. In my experience, it's a pretty fast grower.

Amalanchier laevis (Serviceberry) might be a good option. I think it might be faster growing than my Katsura, but I'm not sure. It does get berries, but I don't think they are very big, and most people enjoy eating them. Birds flying overhead may be a problem, though.

How about Parrotia? I'm not sure about it's growth rate, but it has interesting bark like paperbark maple.

Is the area wet or dry or in between? What kind of soil? If the soil is fairly wet, you could do a Magnolia virginiana. (Sweetbay magnolia). The variety 'Moonglow' is a fastigiate fast grower.

If you want to browse various options, the broken arrow nursery catalog has pretty good descriptions of various trees and their cultivars. See the link below. Good luck!

Here is a link that might be useful: Broken Arrow Trees & Shrubs catalog

I think you probably have too many things on your "wish" list to be completely satisfied with any one kind of tree. You want year round interest, but not evergreen. That likely means you will have something that will "drop"...petals, berries, fruit, etc. And of courese, any deciduous tree will drop its leaves. You want fast growing, but a limited ultimate height. However, as paige points out fast growing usually equals tall and, I would add, weak, therefore susceptible to damage.

I would probably choose what two characteristics are most important and go from there. Trees are neither fences nor umbrellas. You can possibly find something that combines several of your "wish list" items in a grafted shrub or tree, that literally combines more than one plant.

Thanks for all the comments and ideas, and the link to Broken Arrow. Yeah, I realize I'm asking for a lot! We're going to put in fairly good-sized trees, so I'd rather get hardy than fast-growing. The area is pretty average soil, I think, probably more dry than damp. I'd prefer something with trunk(s) you can walk under rather than something more shrubby, though many of those look beautiful.

Another couple of ideas came up in my browsing. Any opinions on Ginkgo biloba 'Princeton Sentry', or on Swedish columnar aspen?

Thanks!

I think there are plenty of small trees that will fit the bill, especially since you're willing to buy larger sized plants to begin with. I have a fastigate Ginkgo, and I love it. Like others here, I tend to buy small trees, often from Forestfarm, so it takes a long time to get screening from them. If you want to be able to walk under (or sit under) branches, though, the Ginkgo might be a little too narrow. Mine's only about 6' tall now, though, so I could be wrong about its ultimate shape. Interestingly, I think this tree drops all its leaves at once, so that would make cleanup a snap.

I'm not familiar with the aspen, but that might give a really nice effect, with the way the leaves shimmer in the breeze. Does it have a strong enough shape, or interesting bark, to provide interest when it's bare? That's one thing I like about the Stewartia - it looks somewhat sculptural without its leaves.

The tree forum folks are really helpful - if sometimes strongly opinionated (!) - and between them they know an awful lot about trees. You might want to ask them for more ideas, or for comments on the aspen.

If it was here I'd say lilacs and service berry. The advantage to these types of plants is that they have low bushiness instead of just trunks to provide screen.