Like in prior springs, we often see a trend in the weed identification questions in certain parts of the state. This spring, a small mustard species with finely-divided (I like to say frilly or lacy) leaves is the most frequent plant requiring identification by our clients. There are several species we may run into in the spring with these lacy or finely-divided leaves. I’ve outlined the species in question, Descurainia species, and a few others that could look similar.
Tansy mustard and flixweed
Two very similar mustard species exist in the state of Iowa – tansy mustard (Descurainia pinnata) and flixweed (Descurainia sophia). From our perspective, identification to the species is not particularly important as they both usually act as winter annuals and management options would be similar for the two. Like other winter annuals, they flower and produce seed early in the spring, often allowing them to escape other management tactics and persist in disturbed areas. These are fairly common but mostly unimportant agronomic weeds in our current crop production system. We aren’t aware of these building up to economic levels in any crop fields.
You’ll notice these weeds as rosettes (circular arrangement of leaves) that will bolt (produce a flowering stem) and grow to about 1-2.5 feet tall. Leaves are very finely-divided, and the entire plant is covered in fine hair. Both species have four-petaled yellow flowers and produce small, orange seed in seed pods called siliques. For those really interested in careful identification, the Illinois Wildflowers website reports that flixweed vegetation usually has a more blue appearance and produces seed pods that are thinner (~1 mm) and longer (1/2 inch in length) than tansy mustard.
Pineapple weed (Matricaria discoidea) is a very early annual species with ‘fern-like’ leaves that rarely shows up in crop fields but is common in residential landscapes and waste areas. The plants may be 3-12 inches tall and look like small bushes. The leaves and stems are hairless and may appear succulent. The plants produce small, inconspicuous, greenish-yellow flowers arranged in a dome shape. What this weed lacks in showy flowers, it makes up for in a smell that appropriately reflects its name – pineapple.
Wild carrot (Queen Anne’s Lace)
Wild carrot (Daucus carota) is a biennial species that has a rosette shape similar to tansy mustard and flixweed, but matures to a larger, 2-5 foot tall plant with white flowers. It is common in pastures, hayfields, and non-crop perennial habitats. The rosette may look similar to tansy mustard or flixweed, with fine divisions in the leaves and a rosette up to 12 inches in diameter. Leaves and stems are usually covered in rough hair but may be glabrous or appear hairless as the plant ages. The white flowers are in an umbel (like an umbrella) shape with many short flower stalks originating from the same point, and the umbel is often flat across the top. Wild carrot is known for often having a single flower in the center of the umbel with purple petals. See more images of wild carrot here.
Poison hemlock (Conium maculatum) is another biennial species that has finely-divided leaves, though this plant may already be bolting and is the largest of the species discussed here. Poison hemlock rosettes may be up to 2 feet in diameter and flowering plants are rarely less than 6 feet tall. Poison hemlock is usually a weed of perennial habitats like pastures, hayfields, or non-crop areas. Leaves of this species are much larger than the counterparts in this article; they usually have a wide base (more triangular-shape) and shiny appearance. The plant is hairless. As the plant matures, the leaf petioles and flowering stalk often have a waxy appearance with purple blotches. White flowers in a compound umbel shape are similar to wild carrot. See more images of poison hemlock here.
The final frilly-leaved weed is common ragweed (Ambrosia artemisiifolia), a summer annual species that germinates early in crop fields and other disturbed areas. It grows to a mature height of about 3 feet tall and will often branch to look like a small, herbaceous bush. Common ragweed seedlings should be easily identified by the presence of round cotyledons at the base of the plant. Subsequent leaves are finely-divided and oppositely arranged during early growth. These plants do not produce a rosette like all other species noted in this article, but rather grow in an upright manner from emergence to maturity.
Have you seen either of the winter annual mustards with finely-divided leaves or their lacy look-a-likes out this spring? Check out the resources below for more information: