I have just picked up a leaflet showing various events around Dinan and i was surprised to see there is a plant exchange (Troc aux plantes) on 17th April from 2 to 6pm in St Carné. It must be their first one.
It will take place in the Salle des Fêtes.
St Carné is 5 min South of Dinan.
On the same day you will also find a Vide Grenier in Plesder, see my other post about it. Plesder is only a short drive away from St Carné.