Rhizomes for dinner! Stachys affinis (Chinese Artichoke) spreads by rhizomes i.e. underground stems that put out roots and shoots. The tubers are a thickened part of a rhizome that swells to store nutrients. The potato is another such plant. Like the potato S. affinis has edible tubers, though much smaller!
 I have yet to try one but it is said they can be eaten raw or used in stir frys. Chinese Artichokes do indeed originate in China but they're not an artichoke being of the Lamiaceae (mint/deadnettle) family. The flowers resemble those of Stachys sylvatica (Hedge Woundwort) and Stachys officinalis (Betony), in fact the plant has sometimes been called Artichoke Betony. A further common name is Crosne, the area of France where they were grown as a vegetable in the late 1800s, possibly the first such cultivation in Europe.
 I bought the pot shown above from Edulis, the specialist plant nursery that propagates many interesting and unusual edibles and plants in general. Indeed it started out as a grower of plants suitable for forest gardening and permaculture.
  I saw that the "Crosnes" had already begun to multiply so I teased out some of the loose tubers and planted them with the main clump on the allotment. I noted the spot with a square of canes because there's nothing to see as yet.
 NB to the left of the photo are some Leeks and towards the right above the square Allium ampeloprasum var babinigtonii is coming up. These are Babington's Leek aka the Wild Leek which is the ancestor of our cultivated Leek: