Great Mullein (Verbascum thapsus)
Aaron's Rod appears to be an American name for this species.
It is a hairy biennial plant that can grow to 2 metres tall or more. Its small yellow flowers are densely grouped on a tall stem, which bolts from a large rosette of leaves. It grows in a wide variety of habitats, but prefers well-lit disturbed soils, where it can appear soon after the ground receives light, from long-lived seeds that persist in the soil seed bank. It is a common weedy plant that spreads by prolifically producing seeds, but rarely becomes aggressively invasive, since its seed require open ground to germinate. It is a very minor problem for most agricultural crops, since it is not a very competitive species, being intolerant of shade from other plants and unable to survive tilling. It also hosts many insects, some of which can be harmful to other plants. Although individuals are easy to remove by hand, populations are difficult to eliminate permanently.