Although it produces semi-woody stems during the growing season, Gaura is considered an herbaceous perennial and will typically dieback to the root crown or a basal rosette of foliage in winter. It is very particular about drainage and heavy or very damp soils in winter can result in it not reappearing again in spring. Otherwise, it is moderately cold hardy - zone 5.

I think what I thought was milkweed syriaca (sp) is actually dogbane. I have a growing stand of it way in the back that I've let go for the past two years. It showed up on its own, and if it is dogbane, I'm going to yank it, dig it, destroy it this year. I'm trying to remember what the seed heads looked like, but I'm drawing a blank. I do remember getting the tent caterpillars on them last year, and I've read the dogbanes will get those. So, in answer to your question, that's the only experience I've had with it, and I don't want it to continue. LOL.

Resembles Spring Phlox to me. What do the flowers look like? Are yours spreading or are you just reading about Dame's rockets and thinking that that is what you have? Phlox does spread easily but no where as much as DRs.

Thank you Hoovb! After a research I also thought it was very similar to 'Dick Koster'. But the hardiness zone 6b makes me doubt. Montreal's area is technically a zone 5. However winter can be regularly without a good snow coverage acting as a protection. Plants with zone 5 limit hardly live through winter. Thus it's more a zone 4. 'Margo Koster' seems to be hardier than 'Dick Koster' but it's not the right color.