Now that they have bloomed, can you cut them back to the ground.?
Ideally, you should have dead-headed them already, cut the finished flowers off so the plant doesn't expend energy on making seeds, but rather gives it to the tuber. If there are seed pods, cut them off unless you plan on getting the seeds later after they dry.

Leaving the green leaves now allows the plant to gain energy and make a stronger tuber. I'd wait on cutting them back until the tops show brown.