Generally I think that foraging between the months of Febuary and the end of October is best. Thats not to say that that there isn't anything out there over the Winter months, milder and later Autumn and Winters considered you can still pick fruits leaves etc right up until November. Indeed Chickweed seems to be impervious even to the harshest of weather. But the Winter months allow the plants to replenish energies and rest before the growing season, so foraging over these months for me is not appropriate.
Perhaps Winter months can be spent familiarizing yourself with your local wilderness. Exploring the edges of the park, down the side of the canal, overgrown and disused lots. What trees are growing near you. Knowing it now in its denuded form and watching the changes over the coming months is a wonderful experience that connects you to your local.