dyer's-weed

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various plants yielding coloring matter used as dye.

from The Century Dictionary.

• noun The woad, weld, or yellow-weed, Reseda luteola, affording a yellow dye, and cultivated in Europe on that account.
• noun The woad, Isatis tinctoria.
• noun Either of the two American goldenrods, Solidago nemoralis and S. rugosa, somewhat used in dyeing. See Solidago.