In one place, there's a broken limb that has growth buds that died.
Lift the plant carefully and separate the root tangle so that each division has a strong eye or growth bud.
This shouldn't be done until the new growth buds start to appear.
In Zone 5 or colder, winter often kills crucial growth buds even when it does not kill stems.
Once the extent of loss is clear, cut back to strong growth buds.
Make sure each division has five to seven of the pinkish growth buds, or eyes.
Cut it into sections with at least three growth buds.
Replant with growth buds no more than two inches below the surface.
Taller plants should be cut right above a closely placed pair of leaves with growth buds in the axils.
Shorten each stem to two or three feet, cutting right above a pair of healthy-looking growth buds.