While all weeds are problematic, some are worse than others. The four worst? Waterhemp, marestail, palmer amaranth, giant ragweed and Italian ryegrass, because they are resistant to multiple herbicide groups, makes these yield-robbers difficult to control.
And, in addition to being resistant to many herbicide, these four notorious plants have a natural ability to evade herbicide control methods.
What it is: Summer annual in the eastern and central U.S.
What it looks like: Grows up to 9' tall; stems are bright red or green, oar-shaped cotyledon leaves; first true leaves are oval with a notch at the tip; true leaves are alternate, oval, hairless and waxy with green to dark pink flowers with spikes.
What it's resistant to: ALS (Group 2), T1R1 Auxin Receptors (Group 4), Photosystem II inhibitors (Group 5), PPO Inhibitors (Group 14), EPSP synthase inhibitors (Group 9, glyphosate), HPPD Inhibitors (Group 27).
What it is: Winter or summer annual all over U.S.
What it looks like: Grows up to 6.5’ tall; hairy; tiny, oval cotyledons; egg-shaped and hairy young leaves with toothed edges; 4” narrow and toothed true leaves that crowd the step and get smaller near the top with small white or yellow flowers.
What it's resistant to: PSI Electron Diverter (Group 22), EPSP synthase inhibitor (Group 9, glyphosate), ALS inhibitors (Group 2), Photosystem II inhibitors (Group 5), PSII inhibitors (Group 7).
3. Palmer Amaranth
What it is: Summer annual in southern U.S. moving north; up to 6.5’ tall; green to red cotyledons with red and hairless hypocotyls; 2” to 8” long lance- or egg-shaped hairless true leaves with white veins on lower side and small, green, spiked flower clusters long a 6” to 18” panicle.
What it's resistant to: Microtubule inhibitors (Group 3), ALS inhibitors (Group 2), Photosystem II inhibitors (Group 5), EPSP synthase inhibitors (Group 9, glyphosate), HPPD inhibitors (Group 27), PPO inhibitors (Group 14).
4. Giant Ragweed
What it is: Summer annual throughout U.S. (with the exception of the Pacific coast, parts of Southwest, Florida and Maine).
What it looks like: Grows up to 16’ tall. hairy; round, thick and large cotyledons with a hypocotyl that’s usually purple; true leaves are opposite, hairy, three- or five-lobed with toothed edges and 4" to 8" wide by 6" long with small and green flowers at the end of branches or bases of upper leaves.
What it's resistant to: ALS inhibitors (Group 2), EPSP synthase inhibitors (Group 9, glyphosate).
5. Italian Ryegrass
What it is: Winter annual or biennial (depending on conditions) throughout the U.S.
What it looks like: About 3' tall; stems can be single or in clumps and are round to slightly flat; ligules are membranous and up to 1⁄10"; auricles are up to 1⁄12" or not present and the flowerhead is 3" to 12" with stalkless spikelets that alternate along the main flowering stem.
What it's resistant to: ACCase inhibitors (Group 1), ALS inhibitors (Group 2), EPSP synthase inhibitors (Group 9, glyphosate), Long chain fatty acid inhibitors (Group 15), Glutamine synthase inhibitor (Group 10, glufosinate).
“Weed resistance is something you ought to respect, it’s going to keep happening if we do the same thing over and over,” says Kevin Bradley, University of Missouri Extension weed scientist. “Figure out what’s your most problematic weed and target some of its weaknesses.”
What can you do if you find these weeds in your fields? Here are a few ideas:
Five Steps to Proactive Weed Control
Blasting Weeds into Oblivion