Another problem, no solution

I've been writing about little problems that I've been fixing lately. Today I have another problem to show you, but I admit I don't have a solution to this one.

It's plumbago (or leadwort), Ceratostigma plumbaginoides. This is one of the first perennials I planted back in 2002, and it's been a care-free performer for me. There's a problem though...

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...this plant spreads underground via rhizomes. It has been a relatively slow spreading process in this bed, but after 10 years, well, it's taken over:

At least half of the bed is now full of this plant.

That wasn't my goal when I planted it, but I don't think there's anything I can do about it now. There are some other perennials mixed in here, mainly Echinacea purpurea. I have to dig a hole out of the plumbago mat whenever I plant a new one.

I love the blue flowers in summer on this plant...

August 2011

...and the dried seed heads are attractive too:

Seed is not the problem here though -- at least I don't think it is. The rhizomes are everywhere in this bed, and although I like rhizomatous plants this is too much.

I've taken divisions and planted them in challenging locations, where they spread very slowly -- no problems there.

So the question is: do I let it continue to spread throughout the entire bed? If not, what do I do to stop it? It seems like it's too late, and that my only recourse is to remove the above-ground growth in areas that I don't want this plant to be.

Any suggestions?

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