bistort Polygonum bistorta, is a knotweed. A knotweed is so called because its roots are knotted or twisted; bistort means twice twisted.
Bistort, the best-known European member of a populous genus, is found from Britain to Siberia and bears attractive pink flower spikes in the summer. It is sometimes called patience-dock or passion-dock, the former name by confusion with the latter and the latter because associated by Christians with the Passion and eaten at Passiontide.
In the same family, Polygonaceae, there is another plant, Rumex patientia, which is the plant with the strongest claim to the name patience-dock; see dock. It too is associated with Easter, perhaps because of an understandable confusion between patience and Passion.