I have been looking for this plant all summer and I can't find it in my area! I am going to look even harder in August because that is when it blooms and it will be easier to spot. This plant is called Thistle and it will make rennet that you can use to make cheese! You can see how to do that here.
I have learned a lot about Thistle this year. One of the disappointing things I have learned is that it only works with goat's milk or sheep's milk. If you use cow's milk, the cheese will turn bitter.
In addition, I now know that there are many different kinds of Thistle. The variety you need to make cheese is called Bull Thistle. It is also know as Scots or Scottish Thistle, Spear Thistle or common Thistle. Here is an article from Wikipedia about it. (The picture above also came from Wikipedia.) The kinds of Thistle you see in your back yard may also be Milk Thistle, Musk Thistle, Sow Thistle, Star Thistle, Golden Thistle or one of many more varieties! The problem for me is, I am not experienced enough to know what I am looking at. The flowers of all of them look the same to me (in the pictures I looked at). Even when I do find a real plant, I am not sure I will know what variety it is!
I have also learned that you can make rennet from the sap of fig trees (our fig tree died from the extreme cold last winter - I have not replaced it), artichoke flowers, Stinging Nettle or the fronds of yucca plants. Stinging Nettle and the flowers of the artichoke can be used with cow's milk.
I am going to add looking for Stinging Nettle to my search for Bull Thistle this month. Planting artichokes in my garden may be in my future as well! I will need to do more research on artichokes to see if one variety is better than another for making cheese.
Making cheese from thistle rennet was a goal of mine this year. While I make not achieve that goal, I sure have learned a lot about many different plants that can be used in the cheese making process!
Eventually, I will have success in making cheese from a sustainable rennet source!