Veronique, Any pots of seed without any seedlings showing, may still germinate. I would advise sowing seeds in pots that then get sunk in your garden (without any special protection) for the winter. I did this with extra seed I had of Trillium undulatum, and now I have a new challenge on my hands, how to separate a "Lawn" of Trillium seedlings (photo was from last year).
It is even more crowded this year, so I will have to try and give the plants some space.