Kale, collards, mustard and turnip greens--all these leafy greens are delicious fresh, lightly steamed, or cooked. Because all of these are members of the cabbage family they also make wonderful kraut. But there are a few things to consider:
They are much more pungent than cabbage and therefore can produce a strongly-flavored kraut.
Different leafy greens have a different texture to them. Collard greens tend to be thicker and tougher, while mustard greens can be lighter. This can cause varying textures and rates of fermentation.
A mix of sharper greens like kale and peppery ones like mustard can produce a well-rounded leafy green kraut.
Goitrogens are still present in fermented greens so do be sensible about how much leafy green kraut you are consuming if this is a concern for you.
2 or more bunches shredded leafy greens, enough to fill a quart jar