About now, but you need to get some root with the suckers. It's said this plant is shallow rooted, and usually only produces suckers when there's some damage to its roots - not sure if that's true, but I've never had one produce suckers of the three I've grown.

If you want new plants, I have found that cuttings root readily in late summer, early autumn. Just prune off a 6 inch stem, remove the bottom leaves and stick in sandy soil in a sheltered spot.