Today I threw the leaves and flowers into a 5L glass jar and poured hot water over the leaves. I'll let this steep and stew overnight and tomorrow during the workshop we'll boil up the leaves with our fibers.
With so much fennel growing everywhere, this makes sense as a wonderful dye for our area. But, always be sure to ask the landowner before you go out with your clippers. Another very important point is that this plant, though introduced and invasive, is a very important habitat species. It is the host plant for the Anise Swallowtail butterfly. After checking in with the land-owner also be sure to avoid harvesting caterpillar eggs along with your dyestuff.