The Polygonaceae, the buckwheat- or knotweed family contains about 1200 mostly perennial herbaceous species in about 50 genera, with a few trees, vines and shrubs as exception to the rule. Center of diversity is the temperate zone of the northern hemisphere. The family includes crops like buckwheat (Fagopyrum), sorrel (Rumex) and rhubarb (Rheum), but also some of the worst known invasive weeds like some species of Persicaria, Polygonum, Rumex and the most infamous Japanese knotweed (now Reynoutria japonica, former Fallopia).
We have a page for Rheum on the PBS wiki.