Collards? Why not? I love collards. And I see that Kitchen Butterfly uses the very same method I would. Be sure to trim out the stems first.

You can freeze Kale but I'd suggest blanching it and chopping it then freezing in zippies, with allt he air squeezed out. In the Netherlands where I live, the Dutch LOVE kale (Boerenkool its called - famer's cabbage!) and we buy it frozen almost all year round.

Pierino, does the same freezing process apply to collard greens?

I guess you could if you really had to. But you would need to steam it cooked first and then squeeze out as much moisture as possible before wrapping it for freezing. It would come out something like frozen spinach. And after that should be ready for use in soups or something.