Weld - Reseda luteola
The plant is a native of Europe, and also grows in the US. It has been used as a dye plant for hundreds of years. Reputedly it was used to dye the robes of the vestal virgins in roman times, and apparently was only used for womens clothes.
The plant is a biennial like Woad and also grows in a rosette in the first year. The leaves are much smaller, quite thin and long. The second year, when it flowers, the flower spike can reach 150 cm.
I had no trouble growing this plant. It germinated very well and also grew quite well in my vegetable garden, even though that is a little wet. The plant likes chalky and dry soil, so I might find a better place for it next year. The yellow it produced was spectacular. I have never seen such strong clear yellow from plants. It was almost too much. I have a small skein of Alpaca wool that I dyed and am waiting to overdye it with Indigo or Weld. Some say that the dried plant material produces slightly less brilliant colours. I will find out since I dried some leaves to use later this winter. But when the plant was used commercially in large quantities it was always used in the dried state.
The unmordanted wool didn't really take any colour and the Rhubarb mordant didn't turn yellow, just a beige. But the Iron and Copper really produced pretty lime to olive green that I could see using. Acid practically removed the colour, while alkaline just made it a tad stronger and slightly more orange in tone.