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:
Acorus graminus 'Ogon'
Acorus graminus 'Variegatus'
Epimedium pinnatum ssp. colchicum, Fairy Wings
Polygonatum multiflorum, Solomon's Seal
Polygonatum odoratum 'Variegata'
Rohdea japonica, Japanese Sacred Lily