This summer I took a class from Linda K. Shelhamer called Natural Dyeing Yarn: From the Yard, Indigo, and Cochineal. You can find out more about her at www.montanamadetradingpost.com/blog/.
I was so inspired I picked about 5 pounds of Black Eyed Susan flowers form the roadside. First I made a dye with HEAT. I flick carded some Cormo and spun it up into a 2-ply. Then I used TIN (with Tartaric Acid as Linda suggested) as a mordant and this is the color I got after only 20 minutes in the dye bath.