Vegetable Rennet. Yes, you can make this from various plants. It never acts as fast as regular rennet. At best it will curdle milk overnight. You can make veggie rennet from yellow (lady's) bedstraw, nettle, lemon or common sorrel, fumitory, unripe fig sap, or the giant purple thistle. Nettle and thistle are the best. All species of Compositae thistles have milk-curdling magic. It's best in giant, thorny kinds. Regular purple thistle will work. Globe artichoke is a Compositae thistle variety, and can be used also. Cardoon is another option.
To harvest thistle for rennet, gather the thistle flowers when they have turned brown. If you see thistledown, the plant is over-mature. Get it right after the end of bloom and before the stage where down blows away. Air dry the flowers. You can store them in jars to wait until needed for cheese-making.
To use your veggie rennet, a quick way is just to tie a bundle of thistle flowers together with string and leave it in the milk until it clabbers. But the more professional way is to pound and extract. You take out enough - 5 heaping t. of pounded dry herb will be needed per 1 gal. milk to be curdled. Pound in a mortar with your pestle until quite crushed. Then pour just a little warm water or whey over, just enough to cover. Let soak 5 minutes. Pound 5 minutes more. Repeat the soaking, and repeat pounding until you've pounded at least 4 times total. You should be seeing a dark (brown) fluid. Strain. Add the fluid to your milk. Be careful not to add too much of any veggie rennet herb because excess can, at best, be unpleasant-tasting for the cheese-eater at the end of the line, and at worst, actually cause indigestion.