April is an excellent time to treat pastures and other non-tilled areas for biennial weeds. The biennial thistles (musk, bull, tall, etc.) are all classified as noxious weeds by the Iowa Noxious Weed Law, but musk thistle is by far the most invasive and troublesome of this group. In recent years, several biennials in the Apiacae (carrot) family have become much more prevalent across Iowa, including wild carrot, wild parsnip, and poison hemlock. Wild carrot and poison hemlock are on the Iowa noxious weed list. Biennials have a two-year life cycle: in the first year a basal rosette (circular cluster of prostrate leaves) is produced, in the second year a central flowering stalk elongates, and the plant dies after seed maturation.
Hartzler, Bob, "Now’s the time to control biennial weeds" (2006). Integrated Crop Management News. 2346.