That makes it likely that it is the wet that is getting to them. You could try something that like a bit of moisture in the soil, such as some of the Persicarias or Astilbe/Filipendula, which would give you a choice of size and colour. If they do alright you can try other bog-loving plants, maybe Ligularia, grassy Hakonacloa or some Primulas such as P. Florindae and candelabra types for some early colour. You might even find yourself including a small pond in your re-vamp
The first two will show their appreciation by growing and spreading apace, but they are not difficult to keep under control as long as you don't tale your eyes off them for too long!
It does sound like drainage. Lots of muck and grit will help and you could plant tougher types in the wetter soil and those that prefer dry conditions higher up. I'm amazed, honestly, that you have plants 'starting to finish early' . Almost everything in my garden frizzled up and died back weeks ago. This summer has been utterly extraordinary, so do allow for that in your assessment!
Your garden looks lovely! I too have a problem border, but different problems from yours as mine is dry sandy soil, made even worse by this summer - we've had no rain here for weeks and weeks. I intend to dig everything up, improve the soil with whatever is in my compost bin and soil conditioner and replant - maybe moving some of the things, like the persicaria and putting in more drought tolerant things.
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I'm amazed, honestly, that you have plants 'starting to finish early' . Almost everything in my garden frizzled up and died back weeks ago. This summer has been utterly extraordinary, so do allow for that in your assessment!