A: That’s par for St. Johnsworts. They usually hold onto their leaves throughout winter even though they’ve completely browned.
St. Johnsworts also often die back to the ground in winter. That means last year’s stems won’t resprout, but new growth will emerge later in spring from around the base.
One way to tell the status of yours is to cut or snap a branch or two to see if there’s any green in it. If there is and you want to keep the plants as tall as possible, hold off on pruning. You can strip the brown leaves off with your fingers if you want to neaten the plants in the meantime.
Another option is simply waiting until you see new growth emerge to determine what’s dead and what’s just dormant. St. Johnsworts are late wakers, so it may be early May until you see any activity at all.
My guess is that all or most of the top growth has died and you’ll end up pruning off that growth to make way for a new set of shoots this year.
I have three St. Johnsworts (a different cultivar), and they always die back. In late winter, I cut them all back to stubs. By May, they’re up and growing fresh.