Last year I grew curly kale (starts from the farmer’s market) and Red Russian kale (replanted annually in my own garden since about 2012). They overwintered, flowered, and are now falling over under the weight of their seed pods.
I plan to harvest the seeds and wondered, will the resulting plants be a cross between the varieties, given that the plants are flowering just a couple of feet apart from each other? It appears they won’t, because the two kales are actually different species. According to the Comox Valley Growers and Seed Savers (2015):
All the curly kales and the lacinato belong to the brassica oleracea species. They will cross with each other and with many other crucifers – cabbage, broccoli, brussels sprouts, cauliflower and collard greens.
The Red Russian and Siberian kales belong to brassica napus species and will cross with each other but not with the other kales. They will also cross with rutabagas, rape and canola. It seems that the napus variety can self pollinate without suffering from inbreeding depression and also it does not have a self incompatibility mechanism which so many plants do.
So the two varieties of kale should stay true in their seeds. We’ll see next year!
Update in June 2022: indeed, I harvested about a cup of seeds from the Red Russian plants in 2020 and they have stayed true to the cultivar as I’ve grown them since.