Most wildflower seeds are best planted or scattered in fall. Some might do okay later, but germination is always dependant on a lot of things, including weather. I've had difficulty during drought years especially here, since my area is a bit arid compared with the eastern states. Some species are partly parasitic and need to be planted near certain other species. Prairie Paintbrush is one I'd like to introduce on my place, but don't know if I have enough compatible species. I've seen them in another area and they are so beautiful!
