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:
Aesculus flava, Yellow Buckeye
Aesculus pavia, Red Buckeye
Aesculus x splendens
Asimina triloba, Paw Paw
Callicarpa americana 'Welch's Pink'
Callicarpa japonica 'Heavy Berry'
Calycanthus x raulstonii 'Hartlage Wine'
Caryopteris divaricata 'Snow Fairy'
Chaenomeles x 'Bonita'