Mullein (Verbascum thapsus)
A large plant growing up to two metres tall with tiny seeds that can remain dormant in the soil for decades. Lots of small bright yellow flowers with five petals. Its a biennial which even if it makes good growth in its first year does not go onto flower, but needs a period of cold weather before it flowers in its second year. Has a long history as a medicinal herb, and has also been used as a dye. Needs an open sunny position.