Most goldenrod (Solidago sp.) species are native to North America. The European goldenrod was known as "woundwort" in England, and prized for its medicinal and magical properties. In period, the medicinal uses included making a tea of goldenrod as a remedy for phlemy coughs, and as a kind of mouthwash. As far as more arcane purposes, it was believed that if you w\held goldenrod in your hand, it would divine hidden riches, and the plant itself was believed to grow where secret springs lay beneath. As far as I know goldenrod was not used in Europe as a dye plant in period, weld being the most popular yellow dye. Weld is easy to grow in the garden, but around here, goldenrod grows wild, and its fine clear yellows are just as nice as weld's.

Fibers to be dyed with natural dyes usually need to be mordanted first. Mordants are metal salts to help fix the dyes to the fibers. Some of these mordants are very poisonous (like copper or chrome salts) so I don't dare use them around my young children, but alum (potassium aluminum sulfate) is pretty safe, even sometimes used in pickling recipes. Since our well water, like everybody else in West Virginia, is high in calcium and magnesium salts, I use water collected in a rain barrel for dyeing purposes.

Also, though these dyes and mordants are considered non-toxic, once I use a pot (and strainer) for dyeing, I use them only for dyeing and soap-making thereafter. Just to be safe.

Goldenrod:
Collect fresh goldenrod tops. The more fully in bloom they area, the brighter the yellows you will get. Weigh the tops.

Put the goldenrod in a dyepot, and crush them with a potato masher. Cover blossoms with rainwater, bring to a boil, then immediately bring the temperature down and gently simmer for 1-2 hours.

Strain out the plant materials, then add wet fiber to the byebath (dye two parts flowers: one part fiber). Simmer gently for an hour, then let cool entirely in the dyebath. Rinse fibers, and hang skeins or spread out wool to dry.
Goldenrod makes an especially nice dyebath as it has beautiful color and smells sweetly fragrant.

Black Walnut (a bit more involved):

Day 1- Pry the hulls off the nuts with gloved fingers (wearing gloves is a good idea, both against the staining and the nasty surprises you sometimes find in decaying walnut hulls). Weight the hull fragments, then let them soak in a dyepot of rainwater overnight. (Dye three parts hulls: one part fiber) I used over-mature, blackened hulls; I've read that the green hulls give a lighter brown, but have not tried it.

Day 2 - Simmer the walnut hull solution for an hour, then let set for several hours more. Strain out hulls.

Simmer fiber in dyebath for a hour (or until desired color is reached), then let cool entirely in the dyebath. Rinse fibers, and hang skeins or spread out wool to dry.

References:
J. Jepson. Goldenrod. In The Herb Companion, Volume 5(6), August/September 1993.
Dyes from Nature. Ed Rita Buchanan. Brooklyn Botanic Garden Inc., 1990.