photo: 'Diabolo' ninebark trained as a small tree
I saw this idea several years ago at an Ohio nursery. Common ninebark (Physocarpus opulifolius) is basically a utility shrub, inexpensive to purchase in large quantities. This midwest native grows in almost any soil or landscape environ except shade and bog areas. Over the past decade ninebark has gone to "beauty school", and a number of new cultivars, including purple leaved 'Diabolo', are currently available.
Ninebark is a vigorous grower and possesses a unique exfoliating bark trait. In the hands of a capable pruner, ninebark can be easily shaped into a small 10-12 foot single stemmed tree. The purple 'Diabolo' foliage plus its inner light tan-colored ninebark adds up to a winning landscape combination.
I suspect that suckering at the crown will likely continue, necessitating sucker removal by a hand pruner (do not use a weed wacker) every few months. 'Diabolo' ninebark purple leaf color holds the longest under full sun and bleaches out to green by mid-summer. 'Diabolo' offers pinkish white flowers from mid-May into June, followed by red fruit clusters (seed capsules) through the summer months.