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Pigweeds: Know The Enemy
There are nine types of pigweeds in the U.S. Midwest. As very young plants, they look similar. Palmer amaranth is different than the others in many ways. In immature plants there are no hairs; stems and leaves are smooth (which differentiate Palmer from redroot pigweed, smooth pigweed, and Powell amaranth). Petioles are as long or longer than leaf blades. The plant has a poinsettia-like appearance and leaves sometimes have a V-shape variegation. In mature plants, the flowering structure is 1- to 2-feet long, thick, and mostly nonbranched. Each plant is either male or female. Male flowering structures feel soft and shed pollen, while female flowering structures feel prickly and contain seed.
Here are quick identification tips on the other pigweed species:
- Redroot pigweed: Has small fine hairs on the plant; first leaves are rounded; leaf and stem surfaces are rough.
- Smooth pigweed: Similar traits as redroot pigweed when young. As mature plants, they have many branches, and branches of the flowering structure are compact and usually more than 1.5 inches long.
- Powell amaranth: Similar to redroot and smooth when immature, although first leaves are more tapered and slightly pinched at the end. When mature, branches are usually 4 to 8 inches long and thicker than a pencil.
- Spiny amaranth: In immature plants, sharp spines are present at points of leaf attachments (no nodes). Leaves have a V-shape variegation. Stems are smooth with no hairs. As mature plants, they have sharp spines and flowering clusters. Leaves are V-shape variegation; stems are smooth and don’t have hairs.
- Tumble pigweed: As small plants, leaves are egg-shape with wavy edges; plant is often an olive-green color. Leaves are small, less than 1.5 inches long. Mature plants are 2 to 3 feet tall and round-shape. Flowers are located at points of leaf attachment to the stem. Plant may break off at the ground and roll when mature.
- Prostrate pigweed: As immature plants, it is low growing and prostrate. The leaves are small and spatulate. As mature plants, they are low growing, and they spread. Leaves are waxy, spatulate, and narrow toward the base.
- Waterhemp (common and tall): As immature plants, they have long, narrow leaves that appear waxy (although leaf shape is variable). There are no hairs on the plant. As mature plants, the leaves are long and narrow. Each plant is either male or female (males shed pollen, females produce seed). Flowering structures are open and near the top of the plant, and at the tips of the branches. Stems and leaves are smooth and hairless.
For an illustrated guide, check out Pigweed Identification, from K-State Research and Extension. http://www.bookstore.ksre.ksu.edu/pubs/s80.pdf