Ones to Watch

March 25, 2011 11:39 PM

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 Today they’ve been replaced by a new crop of troublemakers that are increasingly resistant to some of our favorite controls.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.
Resistance: Site of action groups 2, 5, 9, 14; 11 states; multiple resistance in Delaware, Ohio

Giant ragweedGiant Ragweed

Scientific name: Ambrosia trifida
Family: Sunflower
Alternate name: Buffalo-weed, great ragweed, horseweed
Origin: Native
Features: Dicot. Erect summer annual herb. Large spoon-shaped cotyledons; green underneath. Leaves serrated with three to five deep lobes; leaves and stems have rough surface with stiff hairs. Mature plants can reach up to 17' tall. Management complicated by extended emergence period and rapid growth rate. Monecious; male flowers found at top of plants; female flowers cluster at axils below male flowers.
Resistance: Site of action groups 2, 9; 10 states; multiple resistance in Ohio, Minnesota

Horseweed young plantHorseweed

Scientific name: Conyza canadensis
Family: Sunflower
Alternate Name: Marestail, fleabane
Origin: Native
Features: Dicot. Erect, coarse annual herb. First leaves appear in rosette with toothed margins. Stem leaves hairy; attached to stem without petioles. Stems erect and stout, unbranched; grows 1' to 6' tall. University of Tennessee research found horseweed will germinate 10 months out of the year. Favorite host for tarnished plant bugs, a major pest of cotton. Contains oils and acids that may cause irritation in livestock and humans. One of the few native weeds "given" to Europe.
Resistance: Site of action groups 2, 5, 7, 9, 22; 15 states; multiple resistance in Michigan,
Mississippi, Ohio

italian ryegrassItalian Ryegrass

Scientific name: Lolium perenne L. ssp. multiflorum
Family: Grass
Alternate name: Annual ryegrass
Origin: Europe
Features: Monocot. Tufted winter annual or biennial, occasionally a short-lived perennial in some climates. Shiny seedling leaves. Mature plants grow to about 3'. Stems grow singly or in clumps; leaf blades flat, glossy, generally hairless; leaves are rolled in the bud. Reproduces by seed; open-pollinated species; cross-pollinates freely with perennial ryegrass. Grows rapidly; extended emergence window.
Resistance: Site of action groups 1, 2, 9 and 15; 12 states; multiple resistance in Arkansas, Georgia, Idaho


Scientific name: Sorghum halepense
Family: Grass
Alternate name: Egyptian millet
Origin: Asia/India
Features: Monocot. Coarse, upright perennial grass originally introduced as a forage crop. Often grows 6' to 8'; wide leaves with thickened whitish midribs; panicles (seed heads) are open; many branches support thousands of spikelets from which seeds are readily scattered. Prolific seed that shatters easily and can remain dormant for years. Plant rhizomes aggressive; seedling plants can initiate rhizomes as few as 19 days following emergence. Grows well along creek and river beds, where seed can be carried by water.
Resistance: Site of action groups 1, 2, 3, 9; 8 states

Kochia seedling 4Kochia

Scientific name: Kochia scoparia
Family: Goosefoot
Alternate name: Burning bush, Mexican fireweed, summer cypress
Origin: Southern and eastern Russia, originally introduced as an ornamental hedge
Features: Dicot. Vigorous summer annual; mature plants break off at soil line and tumble in the wind. Leaves alternate, simple, hairy, without petioles. Stems erect, usually lacking hairs, grooved. Grows from 2' to 5'; usually branched from base. Mature plants may turn red. Extremely water-efficient; thrives in warm, low-rainfall environments such as cereal-growing regions and rangeland.
Resistance: Site of action groups 2, 4, 5 and 9; multiple resistance in Illinois, Indiana

morning gloryMorningglory

Scientific name: Ipomoea spp.
Family: Morningglory
Alternate name: Common morningglory
Origin: Native to tropical America
Features: Dicot. Summer annual twining, climbing broadleaf vines that may reach as much as 10' in length. Cotyledons are butterfly-shaped, with a two-lobed tip. First true leaves typically heart-shaped, sometimes three-lobed. Distinctive, funnel-shaped purple to blue showy flowers can vary in color. Many troublesome species depending on region (tall, ivyleaf, pitted, field bindweed). Morningglory vines can pull down crops and create vine mats that cause harvest problems. Often cultivated as ornamentals.
Resistance: None confirmed

Young palmerPalmer Amaranth

Scientific name: Amaranthus palmeri
Family: Pigweed
Alternate name: Palmer pigweed, carelessweed
Origin: Native
Features: Dicot. Erect, branched summer annual herb. Hairless leaves are diamond-shaped with long petioles and white veins; older leaves may or may not have watermarks. Dioecious; female plants have prickly bracts; male flowers are soft and may release pollen if shaken. Fast-growing C4 weed that can reach 10'. Lateral branches arise from the main stem; vigorous root system. Most aggressive pigweed species.
Resistance: Site of action groups 2, 3, 5, 9; 14 states; multiple resistance in Georgia, Mississippi


Scientific name: Abutilon theophrasti
Family: Mallow
Alternate name: Butterprint, buttonweed, Indian mallow, piemarker, wild cotton
Origin: Asia
Features: Erect, sparingly branched annual broadleaf initially introduced as a potential fiber crop. Cotyledons may differ slightly in shape, but covered with soft, tiny hairs. Velvety heart-shaped leaves have distinct odor when crushed. Germinates throughout summer; self-fertilizing; yellow flowers appear from July through August where leaf stalk meets stem. Fruit is a capsule or button. Seed can remain viable for 50 years. Tall growth can severely reduce light penetration.
Resistance: Site of action group 5; 4 states

Waterhemp seedling 1Waterhemp

Scientific name: Amaranthus tuberculatus
Family: Pigweed
Alternate name: Common waterhemp, tall waterhemp, roughfruit amaranth
Origin: Native
Features: Dicot. Erect summer annual herb; male and female plants; highly variable plant shape; leaves long and slender; lack of hair on stems and leaves gives plants a glossy appearance. Prolific seed production; prolonged emergence; also spread by wind pollination, generating large amount of genetic diversity.
Resistance: Site of action groups 2, 5, 9, 14, 27; 14 states; multiple resistance in Illinois, Iowa, Kansas, Missouri

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