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:
Ardisia crenata, Coralberry
Buxus microphylla var japonica 'Borderline'
Buxus sempervirens 'Unraveled'
Clethra barbinervis 'Takeda Nishiki'
Gardenia jasminoides 'Kaleidoscope'
Hydrangea luteovenosa 'Aureovariegata'
Osmanthus heterophyllus 'Goshiki'