I have always been intrigued by natural dyeing of yarns, though the only experience I'd had with dyeing was with Kool-Aid. I decided I had enough dandelions in my yarn that I should try dyeing with them.
I have a bunch of off-white wool I recycled from a sweater a long time ago that I like to use for experimental projects like this, so I grabbed a ball, made it back into a hank and got started with my natural dye experiment.
I started more or less following the instructions for dyeing with goldenrod found in Eva Lambert and Tracy Kendall's The Complete Guide to Natural Dyeing. The book suggests using a mordant, or substance used to fix the dye and make it more water and light resistant.
The easiest one for me to get my hands on was alum, which you can find on the spice aisle at many grocery stores. The book suggests using alum in a quantity equal to 8 percent of the weight of the fiber. In my case I had 30 grams of yarn so I needed about 2.5 grams of alum. That's less than a teaspoon.
To mordant the yarn, fill a pot with water, alum and the yarn. Bring to a boil slowly so as not to felt the yarn, the allow to simmer for 30-45 minutes.
Allow the yarn to cool in the pot and either dye immediately or store in a plastic bag so it will stay wet until you dye with it.
Dye Bath Preparation
To make the dye bath, either use the same water you used for the mordanting process or fresh. Add your dandelions -- this same method could be used for a variety of natural dyestuffs, I'm sure -- bring to a boil and let boil for 2-3 hours. Allow to cool completely (the book suggests overnight, which I ended up following) and strain before dyeing.
How much dyestuff do you need? For goldenrod, the book suggested 100 percent of the weight of the yarn in fresh flowers or 50 percent in dried. I had picked flowers over the course of a week or so and kept them in the refrigerator so they were semi-fresh, and I went ahead and used almost all that I had -- 70 grams, or more than 200 percent of the weight of the yarn. The yarn did not exhaust the dye, however, so I clearly could have gotten away with less.
The point is that there's a lot you can predict about dyeing with natural materials, and a lot that you can't, so there's probably a wide range in which the amount of dyestuff you have will dye your yarn more or less vividly.