If growing your own plants from seed indoors they need to be stratified for six weeks. Crowfoot and Downy Yellow Violets are more tricky. They seem to prefer cooler germination temperatures and they also have a slower growth rate after germination occurs. I had good success with trays outside in early May when it was colder than normal. If seedling doesn't work you can always transplant any seedlings that have sprung up in the garden in either spring or fall.
Violets are so versatile they can enhance any garden, sunny or shady, wet or dry.
Shirley Froehlich runs Prairie Originals, one of Manitoba's native wildflower nurseries. She offers a wide range of plants for sale, including native prairie wildflowers and grasses, as well as some species from forest and wetland settings.