It looks like they need more consistent feeding, possibly with a higher nitrogen food, if you are using a"bloom buster" type plant food.

Thanks. I never feed them. I'll be sure to do it next year.

They often do this contained in a pot, its never much of a problem in the ground. Is it possible they're potbound, there's a lot of stems in that pot?

Thank you. I repotted them into fresh compost in the spring and kept them in a cold frame. Maybe they need bigger pots and to be outside. I don't put them in the ground because they get eaten by slugs.