The line between whether a plant is a "tree" or a "shrub" is sometimes very thin and quite blurry! We use the following criteria to decide which type applies to a certain plant:
Asimina triloba, Paw Paw
Calycanthus x raulstonii 'Hartlage Wine'
Chaenomeles × superba 'Cameo'
Cornus florida 'Suwannee Squat'
Diospyros rhombifolia, Diamond-leaf Persimmon
Magnolia macrophylla var asheii, PICK UP ONLY
Viburnum plicatum 'Nanum'