Got one word for you, David - sawzall. You could cut out some sections of the old plant in the clay container without hurting the container. Fill the gaps with some new soil and I'll bet it would be fine for another few years at least.
Mine's been in the same container for at least 6 years, and blooms each year. I've just added some fresh potting mix and alfalfa pellets along with a little fert in the springtime, watered it in and the plant devours it all. The soil/root level never goes up. I think these plants are cannibals, living off each other's roots or something . ..
This last year, though, I only got two blooms, so the plant is declining now. Gotta take care of it this year. Hmm, maybe treat myself to a new variety, too? There are some neat new colors, dark purple, light blue, and white is always nice. I tried the jazzy one with the black stems "Back in Black" but it just petered out and died on me.
