Purple weeds and other early-season weeds in untilled fields

I have been observing, and also receiving phone calls and e-mails about, purple weeds in untilled fields. If you observe such an area, the weeds are probably either purple deadnettle (Lamium purpureum) or henbit (Lamium amplexicaule) or a mixture of the two. They are very similar in appearance and growth habit, so you need to “get up close and dirty” to differentiate between them. Rebecca Vittetoe, my counterpart to the northwest, wrote an excellent article about identification and management in 2016; you can find it here.

I am also observing some members of the mustard family, like shepherd’s purse (Capsella bursa-pastoris), field pennycress (Thlapsi arvense), and yellow rocket (Barbarea vulgaris) beginning to flower, more in my southern counties than my northern counties and more on south-facing slopes. These areas are warmer and allow the plants to be farther along in growth and development.  The preponderance of what I see are still rosettes, which makes them fairly easy to kill with herbicides before they can produce seeds.

Marestail/horseweed (Conyza Canadensis (L.) Cronq.) is a summer or, usually, a winter annual, which can be easily found in some fields. Likewise, I am seeing the biennial species wild carrot/Queen Anne’s Lace (Daucus carota) growing in warmer, long-term undisturbed areas, such as ditches, fence lines, CRP, waste areas, and pastures.

The only method of reproduction for all of these weeds is seed production. Thus, keeping the weeds from going to seed is the key to management.

Rebecca noted that, with the purple deadnettle and henbit, it may be difficult to prevent seed production of these weeds at this time. Killing those weeds in fall or early spring, either chemically or with tillage, will result in better prevention of seed production.

The situation with other winter annuals is similar. Achieving good control chemically is difficult when weeds are either flowering or beginning to bolt, where the stems elongate. Meaghan Anderson wrote an excellent article that goes into further detail in 2024.

Because the mustards have a taller growth habit, mowing them off at flowering before seeds become viable is also an option; this may be attractive in pastures where legumes are present and desired.

Wild carrot can also be managed chemically or by mowing at early flowering before seeds become viable. Jim Isleib (Michigan State University) has an excellent article about this topic, which can be found here.

Within the next two months, many will be observing other purple flowers in pastures, ditches, CRP, and waste areas – thistles. I was in a hay field yesterday and observed several biennial thistle rosettes. All thistles except Canada thistle are biennials and preventing seed production, either chemically (in year one or prior to bolting in year 2) or by cutting them off at flowering before seeds become viable, is the key to management. See Bob Hartzler’s article for identification and management details. For Canada thistle (Cirsium arvense (L.) Scop.), a perennial, Meaghan Anderson’s article provides excellent identification characteristics. Merrill Ross (Purdue) has a great article on identification and management also listed below. The use of a chemical that will kill the root system is almost required for Canada thistle. A far less attractive alternative is frequent short mowing of Canada thistle for a long period of time to drain energy from the root system causing it to collapse, which is the only alternative for organic production.

With all of these problem weeds, proper identification is crucial before correct management decisions can be made. Prevention of the production of viable seeds is the key to managing all of these weeds except Canada thistle. If chemical management is used, always read and follow label directions for your safety and for good efficacy.

List of screen reader accessible links in this article:

• Purple weeds by Vittetoe: https://crops.extension.iastate.edu/blog/rebecca-vittetoe/whats-purple-weed#:~:text=It%20is%20probably%20either%20purple,confused%20because%20they%20look%20similar
• Shepherd’s purse (Capsella bursa-pastoris): https://crops.extension.iastate.edu/encyclopedia/shepherdspurse
• Field pennycress (Thlapsi arvense): https://crops.extension.iastate.edu/encyclopedia/field-pennycress
• Yellow rocket (Barbarea vulgaris): https://www.canr.msu.edu/resources/yellow-rocket-barbarea-vulgaris
• Marestail/horseweed (Conyza Canadensis (L.) Cronq.): https://crops.extension.iastate.edu/encyclopedia/horseweed-marestail
• Wild carrot / Queen Anne’s Lace (Daucus carota): https://crops.extension.iastate.edu/encyclopedia/wild-carrot
• Chemical Control of winter annuals by Anderson: https://crops.extension.iastate.edu/cropnews/2024/03/weeds-took-advantage-mild-winter
• Managing wild carrot by Isleib: https://www.canr.msu.edu/news/controlling_wild_carrot_in_hay_fields_and_pastures
• Identification and management of biennial thistles by Hartzler: https://crops.extension.iastate.edu/encyclopedia/biennial-thistles-iowa
• Identification of Canada thistle by Anderson: https://crops.extension.iastate.edu/encyclopedia/canada-thistle
• Identification and management of Canada thistle by Ross: https://www.btny.purdue.edu/Pubs/WS/CanadaThistle/CanadaThistle.html