The bark of buckthorn (Frangula alnus). Buckthorn is one of the dyestuffs traditionally used in Europe for hundreds of years. It is very lightfast and versatile dyeing plants.
With alum mordant, it provides mustard yellows, and with the addition of alkali, you get orange shades. Check out the photo for examples of received colours (all of them were received with the batch of dyestuff currently in the sale!)