Why are my plants separating in the middle?

Some of my plants in the garden (Heucheras and Campanulas) seem to be separating in the middle - does anyone know why? They are both really healthy (well till now!) so I don't want to lose them if I can help it. I've attached a couple of pics - any advice would be appreciated - thanks in advance.
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I live in west central Scotland - not where that photo is...