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:
Athyrium niponicum var. pictum
Athyrium niponicum, Ghost Fern
Dryopteris erythrosora 'Brilliance'
Hydrangea luteovenosa 'Aureovariegata'
Rhododendron stenopetalum 'Linearifolium'