Meadowsweet was, according to some sources, an herb held sacred by the Druids, along with vervain and water-mint; the source for this assertion appears to be Maud Grieve's A Modern Herbal (linked below). However, most studies of the Druids place a far higher priority on trees, especially the oak, and mistletoe as sacred plants (vervain, on the other hand, is widely attested as a protective and counter-magic herb). Northern European pagan cultures seem to have used meadowsweet primarily for medicine, and as a perfume and odor-fighter, rather than for religious ritual. However, the plant does play a small role in the Mabinogion, a collection of Welsh myth and folklore: meadowsweet is one of the plants, along with broom and flowers of the oak, used by the wizards Math and Gwydion to create the woman Blodeuwedd. In addition, according to Roy Vickery's Dictionary of Plant-Lore, there are a few vague references in English folklore to meadowsweet as an unlucky plant to have indoors, but these are far outweighed by the many references to its popularity as a strewing herb. Contrary to the episode, meadowsweet is neither rare nor expensive: it is a common weed in North America, and is usually sold at a price similar to common culinary herbs.
It is also the plant from which salicylic acid (aspirin) was first isolated. Source