I wonder if anyone has an idea about this... A couple of weeks ago, I made Cook's Illustrated (the "Soup & Stews" special edition) Caldo Verde recipe, and it worked out great even if I changed it a bit. It used only the leaf part of the kale, and since it was the first time I cooked with the green, I thought it was a shame to spoil all the stems, so I chopped rather finely the stems (maybe 1/4 inch) and put it in the stew at least half an hour before the leaves. I loved it, the stems had the crunchy texture of cooked celery.
I did the recipe again yesterday, but this time I wasn't as lucky. Any idea why? I have noticed that the bunch of kale I used yesterday consisted of branches of leaves attached together rather than separated leaves like the last time. Could that have to do with it?
There are several varieties of kale, and some require different cooking prep and times.
I cook kale in my regular rotation of greens and never had a problem with stringyness.... using the stems as well as the leaves. Mostly I braise it though.
Perhaps the second bunch you cooked was old???