Nettle grows wild in Europe and in parts of North America. This interesting plant has a stinging affect when touched so they need to be harvested with gloves. The sting is caused by formic acid on the tips of the leaves and cooking removes the acid. Cook nettle and use it as you would spinach or any other green.
Nettles grows here in California so I have some first-hand experience. I moved to a cottage with a large yard and a lot of weeds. I'm not good at putting on gloves before I start weeding so I learned quickly about the nettle plant. Those suckers sting! You will learn to recognize them quickly.
If you don't have nettle in your yard they can be purchased a farmer's markets in some areas. Besides using it as a cooked green, it can be made into a nice pesto, added to a simple risotto, or a delicious nettle soup.
If you find a recipe that uses nettle and you don't have any, then substitute equal amounts of one of these options: