Just let it die back, no need to trim it. They do naturally enlarge themselves and can be divided easily to make more plants, just divide up with a spade or old kitchen knife depending on how big /tough it is so you have growing shoots on each section then plant up and water well.

The plant will grow into lovely big clumps so you might want to wait a couple of years before dividing it.

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Always aim for the best result possible not the best possible result

Peanut aka Joanna