Help Map Herbicide-Resistant Weeds in Maine
Hi everyone,
We’re currently collaborating with experts across the Northeast to study the status and management of herbicide-resistant weeds. This is a major priority for Maine, as we haven’t had a comprehensive look at herbicide resistance in our state for over two decades.
How You Can Help
If you spot any of the following weeds on your farm, in your nursery, or even around your home, please let me know:
• Waterhemp – Seeds mid Aug-Sept in Maine
• Horseweed (Marestail) – Seeds Aug-Oct in Maine
• Palmer Amaranth – Aug-Sept
• Pigweed – Aug-Oct
Note: You don’t need to know if they are resistant or not—we will handle the testing and analysis as part of the project. You may have resistant weeds even if you are an organic farmer, weed seeds’ transport don’t recognize farm boundaries!
What to do next
If you ever see these plants, or have a history of occurrence of these weeds, send me an email as soon as possible. We want to track these specific populations until they reach the seeding stage. Once they are ready, either a county representative or I will come out to collect the seeds for our study. (**Please remember to take note of the location and co-ordinates from where the sample is collected**)
Please reach out to us: Manjot Sidhu (manjot.sidhu@maine.edu) or Nick Rowley (nicholas.rowley@maine.edu)
Horseweed (Mare’s tail):
• Seedlings: Starts as a basal rosette of egg-shaped leaves with toothed margins.
• Mature Stems: Grows straight upward, highly covered with coarse, bristly hairs.
• Leaves: Alternate, crowded, and lanceolate. They often appear wilted or droopy in hot weather.
• Flowers: Numerous, small flower heads (about 1/4-inch wide) form a large cluster at the very top of the plant.
• Roots: Possesses a shallow taproot, making it relatively easy to pull when small
• Horseweed is incredibly common throughout Maine. It thrives in open, disturbed soils and does not tolerate heavily shaded or consistently mowed areas.
Palmer Amaranth, Waterhemp and Pigweed:
| Feature | Palmer Amaranth | Waterhemp | Common/Smooth Pigweed |
| Stem | Hairless, smooth texture. | Hairless, smooth texture. | Covered in fine hairs (rough to the touch). |
| Petiole Length | Extremely long; petioles are as long as or longer than the actual leaf blade. | Short; petioles are typically much shorter than the leaf blade. | Short; petioles are noticeably shorter than the leaf blade. |
| Leaf Shape | Egg- to diamond-shaped, sometimes with a slight V-shaped watermark. | Long, narrow, and lance-shaped; often glossy and yellowish-green. | Oval- to egg-shaped, dull green with wavy margins. |
| Leaf Tip | Often has a single, small, soft hair in the notch at the leaf tip. | Smooth, lacking any distinct hair or notch at the tip. | Pointy, without a single hair in a notch. |
| Growth Habit | Symmetrical, often grows taller in a bushy “poinsettia” shape. | Can grow up to 10 feet tall but appears less symmetrical/bushy. | Reddish taproot and a stockier, less aggressive vertical spread. |
| Flower Heads | Long, stout, and thick terminal spikes (1–2 feet long). | Long, slender, and less compact terminal spikes. | Short, thick, and stubby terminal spikes. |
| Where in Maine? | Rare instances in New England | Scattered, emerging locations in Central and Southern Maine agricultural zones. | Every county of Maine |
[1] https://www.cropscience.bayer.us
[4] https://syngentathrive.com
[5] https://realgreenservices.com