Colorado has three species of invasive chamomile; Corn, Mayweed, and Scentless. Mayweed and Scentless are List B noxious weeds.
Generally, the invasive chamomiles have white daisy-like flowers that grow at the end of branched stems. The leaves are alternate, 1-3 times pinnately divided. All are annual but Scentless may sometimes also be a short-lived perennial. All are natives of Europe/Eurasia. They can be found along roadsides, in fields, in disturbed areas, and cropland.
The leaves of Mayweed chamomile leaves are similar to corn chamomile but are more oblong and not as hairy. Mayweed chamomile’s stems are ridged and sometimes reddish. Mayweed chamomile, also known as dog fennel, has a very unpleasant smell. Corn chamomile is very limited in Colorado.
Scentless chamomile is the more common of the three. It has thread-like leaves. The single stems branch about halfway up and the flowers tend to be all at the same height, forming a flattish inflorescence.
For comparison of the floristic features for the three species, please refer to this chart
Flora of North America
Wikimedia Commons – Corn Chamomile
Invasive.org – Mayweed Chamomile
Invasive Species Compendium – Mayweed Chamomile
Manitoba – Scentless Chamomile
TechLine – Scentless Chamomile
USDA Weed Risk Assessment – Scentless chamomile
WASHINGTON STATE Noxious Weed Control Board – Scentless Chamomile