Cooking Dark Leafy Greens

In general, chard and spinach cook quickly. Collards, esp. big old leaves, cook slowest, and turnips, kale and mustard cook rather slowly. You can cook turnips, kale, mustard and collards basically all you want, 15 - 20 minutes or more is sometimes needed. Southerners have cooked collards for hours. Older tougher leaves take longest.

It helps to chop them rather finely before cooking, they will cook faster this way.

The more you cook mustard greens, the less spicey they are. Cooking turnips and collards can remove bitterness. Cooking with vinegar helps remove bitterness too.
By Richard Geller. Posted to the PaleoRecipe Mailing List, Feb. 2001

Other greens worth trying are Lambs Quarters, Stinging Nettles or Dandelion leaves. Stinging nettles become soft after cooking and no longer *sting*.
By Trish Tipton. Posted to the PaleoRecipe Mailing List, March 2001

Oh, speaking of other greens, I said I would get back to y'all on the carrot tops in the broth. NICE!! Gives it a different flavor. They're a bit bitter eaten raw but a "clean" taste to them.
By Oliva. Posted to the PaleoRecipe Mailing List, March 2001

You can use carrot tops like parsley. They are strong tasting, similarly to parsley. Just snip them up into main-dish salads or soups.
By Lynnet Bannion. Posted to the PaleoRecipe Mailing List, March 2001