Peonies fall into two principle classes: herbaceous peonies and tree peonies. Herbaceous peonies have large, deep green, divided leaves with large mid to late spring, single, semi-double or double blooms, up to 10 inches across. Tree peonies are shrubs that grow up to 6 feet tall, with very large, single to double blooms, up to 1 foot in width, held erect above the foliage in early to mid-spring. Both types require ample root space to obtain full growth.
Herbaceous peonies grow from thickened, tuberous roots. Well grown clumps may reach up to 2 to 4 feet tall and spread to an equal width. Plant them in the fall, in well prepared soil at least 1 ½ feet deep. Tubers should be placed no more than 2 inches deep, allowing 2 to 4 feet for root clumps and top growth to spread.
Plant tree peonies in fall, or very early spring, in deep, rich soil, well away from competing tree roots. Plant roots deep, with the union of the rootstock and the scion at least 5 to 6 inches below ground level, and allow 4 to 5 feet for roots and top growth to develop and spread. Plant in container plants any time of year.
Herbaceous peonies require a period of pronounced winter chill. They require afternoon shade and ample water in areas with spring days that are hot and dry, but in colder climates, they thrive in full sun. Tree peonia blooms are fragile, so plant where they will have shelter from strong winds.
“Sunset Western Garden Book”. Sunset Books and Sunset Magazine, eds. 1998. p. 398
Cornell Cooperative Extension – Suffolk County: “The Culture of Herbaceous and Tree Peonies”: http://ccesuffolk.org/assets/Horticulture-Leaflets/The-Culture-Of-Herbaceous-And-Tree-Peonies.pdf