Winter Annual Weeds: A Comprehensive Guide

Are you wondering why those small, rosette-shaped weeds that emerged last fall are still alive and thriving in your field in spring? If so, you’re likely dealing with winter annual weeds. Growers around the country are facing a similar issue as several of these aggressive species have developed resistance to ALS-inhibiting herbicides and glyphosate. Preventing a winter weed invasion, or controlling it before next season, depends on your ability to do three pivotal things: 

1. Identify the species and scope of the winter weed problem. 

1. Plan for a strong fall ​​burndown with carefully selected herbicides. 

1. Monitor threats in the surrounding landscape to plan ahead. Many winter weed seeds are dispersed by the wind and can travel between fields. 

We’re going to focus on the first key skill you’ll need: scouting. When you understand the species you’re dealing with, you’ll have a better chance of more accurately and efficiently addressing the issue. 

📸:@Susan Vineyard via Canva/Featuring flowering henbit as found in fields. 

Scouting winter weeds 

Remember that winter annual weeds generally have two phases and, therefore, two sets of identifiable features.  

1. Rosette: If you’re scouting in late summer or fall, you’ll be looking for the rosette stage of these weeds. These small, badge-shaped clusters sit near the soil line, storing nutrients and developing a taproot or dispersing smaller root systems. 

1. Mature: If you’re scouting in late spring or early summer, you’re more likely to encounter mature winter weeds. Flowering winter annual plants come in a wider variety of shapes and sizes, making them easier to locate and distinguish from each other. Unfortunately, at this stage, they are also typically harder to manage. 

📸: @ivanastar via Canva/Featuring a side-by-side view of a marestail rosette (Left) and mature plant (Right). 

Marestail (horseweed)  

1. Characteristics: Marestail (Conyza canadensis) has two visual profiles that you may encounter: rosettes and elongated stems.  

1. Marestail Rosettes: Rosettes sit close to the soil line. They are most likely to emerge at soil depths of less than ½”, but they can emerge at depths of up to 1”. Rosettes look like a circular fan of club-shaped leaves covered in small white hairs. Rosettes can be encountered at almost any time of year. 

1. Elongated Stems: The second version of marestail is an elongated stem of densely grouped leaves that will eventually produce cone-shaped seed heads. One plant can produce an average of up to 200,000 seeds that disperse on the wind. Mature plants can reach heights of 6 feet, forming dense collections of weeds that crowd out all other vegetation. Marestail can commonly be found in no-till fields as well as in cultivated areas, pastures, and on roadsides. From here, it has the ability to spread through various means. 

1. Flowering period: Stalks produce flowers in July or August. 

1. Life cycle: Up to 86% of seeds germinate shortly after shedding off the seedhead. The rest lie dormant in coarse, well-drained, and fertile loam soils. Rosettes typically emerge in October and survive the winter. In mid-March to early April, the rosette starts to elongate into an upright growth. Seed longevity has not been established. However, one report found viable seeds in the seedbank of a pasture that had been abandoned for 20 years.²  

1. Treatment and control: Marestail became a problematic weed when growers started to no-till soybeans in the late 1970s. It was initially well controlled with ALS-inhibiting herbicides until it became resistant. In the early to mid-2000s, midwestern marestail populations also became resistant to glyphosate. Solutions like Sharpen® and Clarity® resulted in significant reductions of marestail when applied in the rosette stage. 

📸: @jay Struner via Flickr and @Matt Lavin via Flickr/Featuring rosette (Left) and mature field pennycress (Right). 

 Field pennycress  

1. Characteristics: Field pennycress (Thlaspi arvense) comes from Eurasia and is now widespread throughout North America. It belongs to the mustard family and prefers cultivated areas with full sun, moderate moisture, and loam or clay soils. Its rosette looks similar to marestail, with more elongated leaves and without the white hairs. The plants have a shallow root system with a central taproot and are suppressed by arid conditions. They germinate in fall and overwinter until late spring. Crushed leaves emit a garlicky odor. 

1. Flowering period: Late spring through midsummer. 

1. Life cycle: ​​Plants begin to emerge in fall once conditions are cool and wet enough. They form rosettes and gather nutrients to survive the winter. Then, in late spring or midsummer, they grow into an erect flowering stem with wings or flanges along the ribs. As pennycress blooms, it produces many small, white flowers with four petals about ⅛” wide when open. The flowers lead to large, flattened, oval seed pods that contain three to eight seeds that mature within a week of flowering. Each plant can produce 1,000 to 15,000 seeds. The seeds contain a poisonous ​glucoside that taints the milk of animals that feed on them. 

1. Treatment and control: Delaying planting of corn and soy in spring and fall fields allows pennycress to germinate before tillage. Field pennycress is cold-hardy and can survive 7 F with little damage. Seeds can persist for 17 to 30 years in undisturbed soil.³ In 2001, field pennycress developed resistance to ALS-inhibiting herbicides.⁴ This weed drops seeds exceptionally early in spring, so it must be controlled in fall. 

📸: @JJ Gouin via Istock & @igaguri_1 via Canva/Featuring examples of a henbit rosette (Left) and mature plant (Right). 

Henbit  

1. Characteristics: Henbit (Lamium amplexicaule) is a plant from the mint family. It has been listed as one of the 10 most troublesome weed species in the U.S. Native to Europe, western Asia, and northern Africa, it is now widespread throughout the country. It prefers open areas, waste ground, pastures, cultivated fields, lawns, and unmanaged areas. Henbit is a fast-growing, spring-germinating annual. Young henbit plants have round, heart-shaped leaves with scalloped edges. They grow from a shallow taproot and have a number of weak stems. 

1. Flowering period: A month or two into spring and sometimes in the fall. 

1. Life cycle: As henbit grows from a seedling into a young plant in the fall, it produces many weak stems that cover a wide area along the soil. Then, in the spring, the stems arise from the base to be erect, or they can lie horizontally along the ground. Plants may be 4” to 15” tall, and they produce delicate purple flowers. The flowers result in 40 to 200 seeds per plant. Henbit also threatens corn and soybean acres by acting as an overwintering host for corn earworm, soybean cyst nematode, and two-spotted spider mites. 

1. Treatment and control: Beginning in the 2000s, henbit began to exhibit signs of resistance to ALS-inhibiting herbicides. Apply pre-emergence herbicides in mid-September or post-emerge herbicides in late October or early November. 

📸: @Matt Lavin via Flickr/Featuring a seedling (Left) and mature downy brome cluster (Right). 

Downy brome 

1. Characteristics: Downy brome (Bromus tectorum), also known as drooping brome or cheatgrass, is a winter annual grass native to Europe, Asia, and Africa. Plants are less than 2 feet tall and are made up of clumps of grassy leaf sheaths and nodes covered in soft hairs. The leaves usually twist clockwise. It is typically found in cultivated areas like fields, pastures, and roadsides. In its early stages, downy brome could function as cattle feed, but as it grows, its longer awns (bristles) may injure grazing animals and cause tetanus. 

1. Flowering period: Late April to May 

1. Life cycle: Downy brome seedlings are light green and begin as clumps of short leaf blades. As they mature, the stems elongate to an average of 24”, ending in a fluffy or hairy-looking seedhead. These soft, dense, purple-tinged tips produce yellowish-brown seeds with long awns. Downy brome creates around 300 seeds per plant and disperses them in the wind or by attaching to animal fur. 

1. Treatment and Control: In 2022, a study in Canada confirmed the first report of glyphosate-resistant downy brome.¹ Apply a fall burndown to control weeds before they produce seeds. 

📸: @Adam Grubb and Annie Raser-Rowland via Flickr & @Liudmyla via Canva/Featuring a prickly lettuce rosette (Left) and mature plant (Right). 

Prickly lettuce 

1. Characteristics: Prickly lettuce (Lactuca serriola), also called milk thistle, compass plant, or scarole, is an annual or biennial weed native to Europe, Asia, and Africa. As the name suggests, these weeds are made up of bluish-green leaves with prickly edges. It is extremely common in the Great Plains and prefers disturbed habitats like gardens, pastures and cultivated fields. 

1. Flowering period: July to September 

1. Life cycle: Prickly lettuce seedlings are low-lying rosettes resembling a burst of spiky leaves. Leaves alternate and are lined with toothlike jags. The middle to lower leaves have stiff bristles along the bottom mid-rib, and stems contain a milky white latex substance. As the plant matures, it produces as many as 25 pale yellow flowers that resemble small dandelions. 

1. Treatment and Control: Prickly lettuce is extremely tolerant of glyphosate and resistant to Group II herbicides in the northwestern United States. In some regions of the Pacific Northwest, prickly lettuce has shown resistance to synthetic auxins (2,4-D, dicamba, and MCPA).² Glyphosate application in fall still typically results in satisfactory control. 

📸: @Luis nunes alberto & @Evelyn Simak/Featuring a shepherd’s purse rosette (Left) and mature form (Right). 

Shepherd’s purse 

1. Characteristics: Shepherd’s purse (Capsella bursa-pastoris) gets its name from the purse-like shape of its triangular, flat fruits. It is native to eastern Europe and Asia. It prefers cultivated areas like fields, lawns, and gardens. Shepherd’s purse needs cold temperatures to break dormancy and germinate when soil temperatures get below 60 F. Notably, scientists have found evidence that this weed is a “protocarnivore” due to its ability to attract and kill nematodes, enriching the soil around it.³ 

1. Flowering period: March to November 

1. Life cycle: Shepherd’s purse seeds are extremely long-lived. Evidence suggests they can last for 35 years in undisturbed soil. Seedlings are round or spatulate in shape and may be slightly indented at the tips. Leaves become more deeply lobed as they mature. Fully grown plants can be 6 to 18” tall. In early spring, they produce small white flowers and flat, heart-shaped seed capsules that persist. 

1. Treatment and Control: In the United States, shepherd’s purse is tolerant of both cold and drought. Dense planting, as well as a strong fall burndown program, help control this weed. 

📸: @Andrey Zharkikh via Flickr & @Jared Quentin via Canva/Featuring a tansy mustard rosette (Left) and mature plant (Right). 

Tansy mustard 

1. Characteristics: Tansy mustard (Descurainia sophia), also known as herb sophia or flixweed, is an annual weed found in rangeland, roadsides, and fields. It prefers dark brown prairie or black prairie soils. Plants typically look like small clusters of dark green, narrow leaves or needles. It contains toxic levels of nitrate and is harmful for cattle to consume. 

1. Flowering period: March to August 

1. Life cycle: Seedlings are light green, short, and sparsely hairy. They develop into low-lying rosettes that wither during flowering. In spring, the plant stem bolts up to heights of up to 32”. The mature plant has a hairy stem topped with leaves branching into star-shaped hairs. Flowers are green-yellow, upward-facing, and hold four petals. The stem continuously lengthens as new flowers develop. The fruit of tansy mustard are long, skinny pods divided into two chambers that turn brown and release thumb-shaped seeds in rows of 10 or 20. Seeds are sticky when wet. 

1. Treatment and Control: There have been reports of ALS-inhibitor resistance in tansy mustard across Kansas, but in general, early application (when plants are smaller than 2 or 3” across) controls tansy mustard well. 

📸: Public domain image & @Elijah Hiett via unsplash/Featuring a dandelion rosette (Left) and iconic seeds (Right). 

Dandelion 

1. Characteristics: Dandelion (Taraxacum officinale), also known as lions-tooth, blowball, cankerwort, and faceclock, is a perennial weed you’ve almost certainly seen if you live near lawns or gardens. It is aggressive, invasive, and easily recognizable by its bright yellow flowers and globular seedheads.  

1. Flowering period: April to October 

1. Life cycle: Dandelions emerge as seedlings with alternating, hairless, spatula-shaped leaves with wavy edges. Later-emerging leaves are widely toothed and deeply lobed, with tips that point toward the center of the rosette. As the plant matures, it produces a stem that exudes a milky white sap when cut. The stem flowers into the aforementioned bright yellow head, consisting of 100 to 300 petals. Eventually, this flower gives way to an orb-shaped seed head covered in feathery white florets. Seeds disperse on the wind and can travel long distances. 

1. Treatment and Control: Dandelion is generally susceptible to herbicide treatment. The most effective time to treat is in the fall. 

Why are winter weeds hard to control? 

Winter annual weeds are incredibly varied, but the thing they all have in common is their ability to overwinter in your fields. It’s easy to underestimate the scope of the problem in the fall when rosettes germinate. Many growers don’t notice winter weeds until spring, when the mature plants begin to shoot up, growing stems and seed pods. By then, the window of greatest opportunity for their control had already passed. 

📸: @thomas_Zsebok_images via Canva/Featuring winter weeds overwintering in a snowy field. 

Fall burndown guide for winter weeds 

Fall is the optimal time of year to reduce winter annual weed populations with herbicide. For effective control, target early October (although timing can vary). If possible, apply after a frost. If you’re unsure how to develop an appropriate burndown program for the weed you’ve scouted, contact one of our experts. They can help you evaluate your options, including residual herbicides for continued control. 

If the opportunity for fall burndown has already passed, effective control will likely be challenging and require postemergence herbicides and residual control. Due to the higher likelihood of varying temperatures and moisture, expect reduced effectiveness and the potential need for additional control in the fall. 

BASF solutions for winter annual weeds 

 📸: @aleksandrkondratov via Canva/Featuring a sprayer applying a fall burndown treatment. 

1. Sharpen® Herbicide: Sharpen® Herbicide drives burndown three to five times faster than 2, 4-D, or glyphosate. It provides rapid burndown of broadleaf weeds, including difficult-to-control weeds like glyphosate-resistant marestail. Depending on usage rate, Sharpen Herbicide® can increase residual control of broadleaf species, including early emerging, large-seeded broadleaves. 

1. Clarity® Herbicide¹: Clarity® Herbicide is a proven, highly effective tool for fall control of annual, winter annual, and perennial broadleaf weeds. Manage winter annuals, including marestail in the fall when they are small and easy to control. Any crop can be planted 4 months after application.

1. Zidua® SC Herbicide: Control tenacious winter weeds with Zidua® SC Herbicide. It offers broad-spectrum control that lasts up to two weeks longer than other comparable herbicides. In addition, it offers better soil stability and a lower use rate. Try Zidua SC herbicide this fall to see how it helps protect your yield and helps prevent loss next season. 

Keep winter weeds where they belong 

Refer back to this guide whenever you have suspicions about new vegetation sprouting in the fall. The earlier you’re able to identify and control these winter annual weeds, the better your chances are of keeping costs low and yields high. Don’t forget that fall is your best opportunity to make an impact, and don’t hesitate to speak with a BASF representative as soon as you have questions. 

Learn more at Grow Smart® Live: 

See more examples of ways to combat winter annual weeds: 

1. Marestail Burndown: Zidua PRO herbicide vs. Authority First® herbicide 
2. Targeting weed control based on weed lifecycle 
3. Tillage and herbicide strategies to control winter annuals 

📸: @aleksandrkondratov via Canva/Featuring a grower manually weeding a field with a hoe.

Sharpen, Clarity, and Zidua are registered trademarks of BASF. All other trademarks are the property of their respective owners and use of any such trademark does not imply any affiliation with or endorsement by its owner.

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Endnotes 

1. “First report of glyphosate-resistant downy brome (Bromus tectorum L.) in Canada” Geddes, C.M., Pittman, M.M. 2022. https://doi.org/10.1038/s41598-022-21942-6 
2. “Prickly Lettuce” WSU Dryland Cropping Systems Team, Washington University, 2023. https://smallgrains.wsu.edu/weed-resources/common-weed-list/prickly-lettuce/ 
3. “Evidence for facultative protocarnivory in Capsella bursa-pastoris seeds,” Roberts, H.R., Warren, J.M. & Provan, J. Scientific Reports, 2018. https://www.nature.com/articles/s41598-018-28564-x 
4. “List of Herbicide Resistant Weeds by Weed Species,” WeedScience.org, 2023. https://weedscience.org/Pages/Species.aspx