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Ones to Watch

March 26, 2011

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Remember Jimsonweed? Foxtail? Sicklepod? Those weeds probably topped your list of tough-to-control weeds a few years ago. Today they’ve been replaced by a new crop of troublemakers that are increasingly resistant to some of our favorite controls.

A March 2011 poll of Farm Journal readers shows that waterhemp currently tops the list of worst weed nightmares. Giant ragweed, common ragweed, Palmer amaranth, horseweed (marestail) and velvetleaf rank as the yield-robbing runner-ups.

Regionally, other weeds rise to the top of the charts. Pigweed may be the perfect summer annual weed, but the open-pollinated Italian ryegrass takes the honors for winter annuals in places such as Mississippi.

In the western U.S., Phil Westra, Colorado State University weed scientist, has clocked kochia tumbleweeds blowing across the landscape at 40 mph. "Weeds like kochia, Russian thistle or tumble mustard leave a long trail of progeny as seeds dislodge from plants," he says.

University of Missouri weed scientist Kevin Bradley notes that volunteer corn may well be the most under-reported weed on the landscape today. "As we move forward with stacked-trait technology, volunteer corn promises to get more complex," he says.

The cornerstone of any weed control program is proper identification. In the spring, Bradley likes to see farmers scouting seven to 14 days after crop emergence to determine weed species and plant density. This is also the time to assess performance of preplant and pre-emergence herbicides and the need for supplemental postemergence strategies.

Keep scouting throughout the season. Weeds such as waterhemp and Palmer amaranth can have flush after flush. Some weeds don’t emerge until well into the cropping season. Keep in mind that weeds that survive despite repeated herbicide applications are a good indication of a possible herbicide-resistant population.

Below are 10 common weed troublemakers. More weed identification references are available from your state Extension weed specialist.

common ragweedCommon Ragweed

Scientific name: Ambrosia artemisiifolia

Family: Sunflower

Alternate name: Annual ragweed, bitterweed, hogweed

Origin: Native

Features: Dicot. Erect to branching summer annual herb found throughout North America but prevalent in northern latitudes. Coytledons have deep purple underside; leaves pinnately lobed; strong odor when crushed. Grows 3' to 6' tall. Seeds require winter dormant period before germinating in late April or May. Hairy stems are green to pinkish red. Abundant pollen; contributor to hay fever.

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FEATURED IN: Farm Journal - Early Spring 2011

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