Wie kann man kürzen, ohne Schaden zu nehmen?

Dig it up and divide it from the roots and put a half back. Just make sure you go wide to get as much root system as possible without damage.

Give the divisions to friends, family, neighbours or share on the seed/plant swap sites.

Might be a bit of a bugger to dig it up, but cutting back foliage won’t be a remedy for long

Edit. I’d wait till march when the ground is probably a bit easier to work with