Use the leaves for a while (by cutting back the stems). After a month or two, it will become less productive and will produce flower buds almost immediately after cutting. Then let it flower and produce seeds.

It is getting on in the season, so I will let one stalk grow and flower as I continue to harvest the rest. Just to be sure that I do get flowers and seed. Then I let the flower stalks just dry out right on the plant over winter. It is one of the few things that I do not clear out of the garden. Come spring, seeds are still clinging to the stalk so I can plant them where I want them.