I don't see why not. I used to work at a nursery, and one spring we did get a shipment of several topiary forsythias. These were "Northern Gold" which is the only variety that flowers fairly reliably in my zone 2b climate. It will grow almost as large as a lilac, and with similar leaves and growth habit, if not pruned. The topiaries we reieved were about 5 feet tall, a single stem with a small ball (maybe 2 feet across) of branches & leaves on top. They bloomed beautifully a few days after we got them and customers snapped them up immediately. I suspect you will get numerous shoots wanting to come from the base, but keeping these cut off shouldn't be a terrible job. Because of our climate there are very few truly small trees that do well here, and I've seem standard topiaries made of many large shrubs like Diabolo ninebark, red elderberry, dwarf Korean lilac, etc. They seem to do just fine as long as you keep on top of the basal shoots. corrie