Family: Pigweed, Amaranthaceae.
Habitat: Crop fields, pastures, and roadsides.
Life cycle: Summer annual.
Growth habit: Erect up to 6 ft. high.
Leaves: Prominent white veins on the undersurface unlike redroot pigweed, not pubescent, alternate, without hairs (glabrous), and lance or egg-shaped. Leaves are 2 to 8 inches long and 1/2 to 2 1/2 inches wide with prominent white veins on the undersurface. Leaves occur on relatively long petioles.
Flower: Small, green, inconspicuous flowers are produced in dense, compact, terminal panicles that are from 1/2 to 1 1/2 feet long. Smaller lateral flowers also occur between the stem and the leaf petioles (leaf axils). Male and female flowers occur on separate plants. Each terminal panicle contains many densely packed branched spikes that have bracts that are 3 to 6 mm long; can produce 500 thousand to 1 million seeds per plant.
Roots: Taproot that is often, but not always, reddish in color
Stem: One central stem occurs from which several lateral branches arise.
Similar Plants: Loosely resembles many other pigweed species. Palmer’s petioles are as long or longer than the actual leaf. This plant is hairless and has elongated seed heads. Leaves are typically more diamond shaped than other pigweed species, and occasionally has one hair at the tip of the leaf.
The Problem is……..Palmer amaranth is one of the most difficult weeds to control in agricultural crops. It developed a major glyphosate resistance problem in the southern US from 2006-2010, and has been spreading in the midwestern US since, causing crop loss and increases in weed management costs. Characteristics that make it a successful annual weed include: rapid growth rate; wide window of emergence (early May through late summer); prolific seed production (upwards of 500,000 seeds/plant); tendency to develop herbicide resistance; and tolerance to many post-emergence herbicides when more than 3 inches tall.
Currently there are 21 weeds on the Ohio Prohibited Noxious Weed List:
- Shattercane (Sorghum bicolor) – February 8
- Russian thistle (Salsola Kali var. tenuifolia) – February 22
- Johnsongrass (Sorghum halepense L. (Pers.)) May 24
- Wild parsnip (Pastinaca sativa) – May 21
- Wild carrot (Queen Anne’s lace) (Daucus carota L.) – June 5
- Oxeye daisy (Chrysanthermum leucanthemum var. pinnatifidum) – May 3
- Wild mustard (Brassica kaber var. pinnatifida) – June 12
- Grapevines: when growing in groups of one hundred or more and not pruned, sprayed,cultivated, or otherwise maintained for two consecutive years. – February 15
- Canada thistle (Cirsium arvense L. (Scop.)) – March 29
- Poison hemlock (Conium maculatum) – May 31
- Cressleaf groundsel (Senecio glabellus) – June 21
- Musk thistle (Carduus nutans) – March 26
- Purple loosestrife (Lythrum salicaria) – April 17
- Mile-A-Minute Weed (Polygonum perfoliatum) – March 7
- Giant Hogweed (Heracleum mantegazzianum) – March 14
- Apple of Peru (Nicandra physalodes) – February 28
- Marestail (Conyza canadensis) July 5
- Kochia (Bassia scoparia) – May 1
- Palmer amaranth (Amaranthus palmeri) July 14
- Kudzu (Pueraria montana var. lobata) – April 11
- Japanese Knotweed (Polygonum cuspidatum) – April 2