The herb and flowers of weld (Reseda luteola). Weld is a very old dyeing plant – it was identified on textile fragments from the Iron Age and possibly it was used even in the Neolithic period. It was one of the favourites dyestuff in Anglo-Saxon England, known also by the Vikings, still widely used in medieval Europe and even later, up to the introducing modern chemical dyes.
With alum mordant, it provides bright yellow, and with iron mordant you will get olive green. Check out the photo for examples of received colours (all of them were received with the batch of dyestuff currently in the sale!)