Ok I was slightly wrong, it's the giant purple thistle that is used for clabbering milk for cheese. It usually curdles milk overnight. This is directly from the Stocking Up(which I strongly recommend) book:
The giant purple thistle is a very tall-growing species of thistle, possessing all over very cruel prickles, so that no animals can eat it, and only the bees visit it and the goldfinches carry off the downtopped seeds for their nests and for food. The stems and foliage are grayish, and the flowerheads a rich purple, and of typical thistle shape. The part used are the flowerets when the thistlehead and the flowerets of which it consists have turned brown. When the thistledown begins to appear it is getting too late for the gathering, and the flowerets are less strong and soon will be carried away over the countryside by the wind. The flowerets should be air dried, either in shallow baskets or perforated brown paper bags, and then stored in jars to last until the next summer.
The herb, in carefully controlled quantity, has to be prepared for adding to the milk. It should be pulverized with a mortar and pestle. The herb is well pounded, then a little warm water(or whey), merely enough to cover it, is added, then left to soak for 5 minutes, pounded again for 5 minutes, soaked again, repounded, usually 3 times, at least until dark brown-colored liquid forms. The herb is then strained and then a heaping teaspoon of the herbal liquid is added to every quart of warm milk. If too much herb is used, it tastes strongly in the cheese, and it will cause indigestion, being a very potent herb. Therefore, be careful not to add too much.
(Instead of making a liquid from the herb, you can also dip the whole herb in the milk to make it adhesive, press the flowerets together, and bind them with coarse white cotton thread into a sort of rough plait, leaving the long end of the cotton hanging out of the crock, so you can pull out the herb when the milk curdles, which is approximately overnight. Do not ad the herb loose into the milk, as it is then like having hairs in one's mouth when consuming the delicious soft curds and whey.)