I think they might rot at the base - that's the only thing that strikes me at the moment.

A lot of the hardier pinks do better with a mulch of fine gravel, if you have to mulch them--many are native to tor tops and scree slopes!

Yes, exactly, Tug - I should have qualified what I said - to me, a mulch is a layer of compost, so I'd say that damp compost around a Dianthus would probably kill it, wouldn't you?

Yep. Okay with something that dries out fast, but anything that holds water is stressful for them.