You'll often hear this being talked about by gardeners, especially at this time of year: but what is it, and why do we do it? There are various theories about…
Upright stems may need staking, particularly in a shady site.
By seed. Clumps take a long time to establish so only more mature plants can be successfully divided. Divide in early spring or autumn.
Pests, diseases or disorders
Fungal stem rot, Verticillium wilt and aphids may be a problem.