You can eat what most people call a weed. Dandelion.
Here is another foraging video we did on how to harvest and cook dandelion.
Wikipedia has some good info on dandelion, though I don't agree you have to always cook it down before eating. There are plenty of us who like it raw. If you like mustard greens, you'll like dandelion.
You can grow it in your garden, it will last longer into the summer than most any other green. I think the horticultural version is milder, and I prefer the wild version. I was walking around the barn this weekend looking for dandelion, there is still a bunch of it around depsite the record heat wave. Dandelion starts growing early in the spring, flowers and quickly goes to seed. Dandelion then pops up again in the fall, so keep an eye out for it all through the growing season. You can even grow it in a cold frame or hoop house. Click these to see our how to build a hoop house and cold frame videos
Don't harvest it from roadside areas or where your dogs like to hang out. 10-4?
if you want to learn more about foraging, here is a how-to book to get you started. I like this book, and learned a bunch about acorns, which you can make a flour with. who knew?