Vine weevils are partial to them and they eat all the roots away.
In the soil you might see their larvae - they're like white grubs and they eat the roots of plants. They are also partial to Primroses.

That is typical vine weevil damage I am afraid.

All may not be lost. When this happens to mine I usually check the remaining stems for a grub inside the stem. Then I plant them into a gritty compost. Most will root and then I have extra.

Same here, I stopped growing them for that reason. If you are growing in containers then you can treat the compost with Vine Weevil Killer but if you are growing in open ground then you can use nematodes. They will feed on the weevils but will also die out when the weevils are finished. Can be expensive to buy; need to be used in warmer temperatures and will be an on-going problem.

Thank you for your responses. I had a look at what was left of the plants and there were little grubs in there, I washed them out and replanted them, fingers crossed. My Enchinacea and dianthus have also suffered the same fate.
Thanks for the info.

You did use fresh compost didn't you? If not there will probably still be grubs in there. I usually spread the old compost on the soil. But rereading I gather they were in the ground, lightly turn the soil to expose other grubs to the birds. Put the heucheras into pots of a gritty compost to encourage them to root.

Never seen vine weevil grubs on Dianthus before. That could be a blow, we have hundreds of them along the edge of the Drive.

OB, I think you're going to be OK with your Dianthus. They have had a go at my Heucheras in my back bed but haven't touched my Dianthus.

Yes, all fresh compost used, but i didn't think about the birds eating the grubs :( my plants are all in pots and troughs. As for the Dianthus, they were in 2 troughs, planted up at the same time but only 1 trough was attacked, the other 1 is going strong :)
Enjoy the changing colour that are happening now.

If you are growing heucheras in pots, mulch them with grit as this deters vine weevils. I've only lost two in 15 years using this method