As I understand it, the "perennial" nature of brassicas is a little different than, say, jerusalem artichoke,
apple, or other rooty or woody plants that stay alive for many seasons.
The main difference is that tree collards just don't get
enough dry heat to set seed ("bolt") in most of our temperate growth contexts, and because they never get the signal from their flowers and seeds to shot down, and they are cold hardy, they just keep going.
Other kale or collards will usually overwinter, and if you can keep them cool through the summer, often won't produce seed for a while, and will therefore keep making leaves. But they eventually produce flowers and seed, and that gives chemical signals to the rest of the plant that it isn't needed.
The thing I'm not certain about is where those chemical signals. I believe that they are produced by the tissues in the flower stalk/seed heads, but I may be wrong. If this is the case, pinching out the flower stalks of kale
should help keep it producing (I've never tried this).
If instead, those signals originate in the
roots, then grafting should have some effect.
But grafting on it's own isn't a broadly generalizable way to "combine properties" of different plants. We would need to know more about which plant tissues are sending which chemical signals to the rest of the plant.
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