Heat has little to no effect on oxalic acid.
Spinach, chard, beet greens, and cabbage are fast-cooking greens and only need to be boiled/sauteed for a few minutes. Kale, collards and turnip greens are more fibrous and will take a little longer--around 10 or 15 minutes. I like cooking my greens in a pot of broth with a ham hock, chopped onion, and some spices thrown in. Simmer, covered, for about 20 minutes.
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