some that come to mind are:
Hydrangea 'PG'
Burning bush
Lilac (will be more grove than clump-like)
Kolkwitzia
several Viburnums (in your size range such as snowball)
Some Jap Maple
Smokebush (Cotinus) ... nice, but check local performance.
Spicebush (Lindera)
Sweet Mockorange (Philadelphus)

I grow most of my shrubs in more or less that format. They may not all perform in your area but they are leatherleaf viburnum (V. rhytidophyllum), Hydrangea aspera, Philadelphus, elderberry (Sambucus nigra 'Guincho Purple'), Disanthus cercidiphyllum, and one that naturally grows in this shape, Leycesteria formosana. And, for that matter, some of my rhododendrons.

The primary criterion for success with this form, it seems to me, is that the shrub has to not sprout too aggressively from the base once it has relatively mature stems. This rules out grafted shrubs, or at least, it does if you don't always want to be pruning suckers from the base. In fact my contorted filbert (Corylus avellana 'Harry Lauder's Walking Stick') would also qualify, and Hamamelis cultivars such as 'Diane' could do so, but there are always suckers. And even on others, you have to ruthlessly remove new growth from the base unless you want one to grow into a replacement stem - and I do prune out old stems from time to time.

I also failed to achieve this form with Vitex, because mine sprouted aggressively and very laterally from the base - but that could have been because we cut it back severely at one point. I'm just working on converting it to a single stem tree form.

The Vitex's history actually brings me to another criterion for success, which is that the reason we cut it back hard was that we were moving it AWAY from the house wall where it WOULD NOT grow upright - it leaned so far out that the adjacent patio became useless. It, and all my deciduous shrubs, now are planted away from structures and fences so that they get light from all sides, and they grow nice and straight )although if shaded by each other or other trees they also grow their bulk toward the sun). Next to a structure or wall, any deciduous shrub will lean away. If the garage is fairly low and the plant can get light once it grows above it, you can train it upright until it gets there. If the garage is taller and you can determinedly tie to the garage even as the plant grows to its full height, then of course you can grow whatever you want there - just don't expect it to happen naturally.

I finally had to ask on another forum why conifers don't seem to have this problem... turns out conifers will grow upright even next to walls because they grow against gravity rather than toward light. Deciduous shrubs, on the other hand, grow toward light. Geotropic vs. phototropic, I think it was. Broadleaf evergreen shrubs, like rhodos and the Viburnum, seem to have some mix of the two properties, but don't quote me on that. I base this on noting how they grow in one-sided light conditions.

Karin L

PS another good candidate might be Hibiscus syriacus.

Karin, I have a 'Shoal Creek' vitex that doesn't produce any seedlings. Mine has the lateral growth. I actually prefer that to a single trunk. I think it gives the tree more interest.

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"The primary criterion for success with this form, it seems to me, is that the shrub has to not sprout too aggressively from the base once it has relatively mature stems."

Karin, that is the excellent point to make on this subject and one which I frequently forget to mention, or don't take the time to do it. Keeping the trunk structure "clean" of foliage requires a cooperative, not too suckery shrub. Beyond that, a great deal has to do with developing a canopy that produces good shade and discourages new growth from below. Once the canopy and trunk are established, I'd be inclined to zap any new growth (before it's allowed to develop at all, with Roundup (to REALLY discourage it.) But there is only so much one can fight Mother Nature without losing, so FIRST choose a shrub that does not sucker Endlessly.

If one does choose a forever-suckering shrub for the creation of a small tree, the reasonable form is just to allow foliage, sheared to and as part of the trunk form... where one cannot view through the trunk structure. But this is a different animal altogether.

The plant selections were very helpful. Since trimming/sculpting shrubs is new to me, these give me a much better selection to choose from. For us this plant will be seen more in the late spring to early fall (and by the car in the winter). We've had viburnums & rose of sharon in the past but never thought to trim them into a multi stemmed tree. I read somewhere that to have berries on the viburnum that another viburnum must be nearby (male & female?) to produce. Can anyone explain exactly what I would need to do. I'm starting to run out of area but perhaps could put another viburnum on the corner of the property.

The reasoning behind the degree of suckering was very helpful. My DH likes to trim but as many plantings I will am planning on, I don't want it to be an outright war.

Natal, I heard flowers sell and yours is definitely proof of that. I read that vitex is only good to zone 6 but would grow from the ground up in zone 5. We're on a lake and we've seen we're typically about 4 degrees warmer in the winter when the lake hasn't frozen over. The blooms might be delayed some, which would be alright with us. I didn't find out how long the bloom period was, can you elaborate on that? I think it may be worth a try at least.

A tree is but a monster shrub that someone or the forces of nature decided to separate from it's lower branches and foliage. There is no reason not to take advantage of right-sized shrubs that lend themselves to being made into trees by the same method... any more than there is in not using an Oak or Maple this way. If we can enjoy multi-trunked Birch or Japanese Maple as trees, there's no reason not to enjoy multi-trunked Burning Bush or PG Hydrangea "trees" in the same way. Some of the most outstanding plants I've ever seen have been this diminutive tree form. In the yard, it is frequently a smaller "tree" which fits where height needs to be limited.

Aloha, I'm linking a thread from the cottage gardening forum. Someone there bought one a couple years ago, but it didn't survive. She also lives in zone 5.

Here is a link that might be useful: Vitex ... Chaste Tree