Amaranthus blitoides (Prostrate Pigweed)

Plant Info
Also known as: Mat Amaranth, Spreading Amaranth
Genus:Amaranthus
Family:Amaranthaceae (Amaranth)
Life cycle:annual
Origin:native
Status:
  • Weedy
Habitat:part shade, sun; dry disturbed sandy or gravelly soil; roadsides, railroads, gravel pits, waste places, farm fields, gardens
Bloom season:June - September
Plant height:2 to 16 inches
Wetland Indicator Status:GP: FAC MW: FACU NCNE: FACU
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: indistinct Cluster type: spike

[photo of flower clusters] Tiny flowers are tightly packed in small clusters (glomerules) in a spike-like arrangement at the tip of branching stems and arising from leaf axils, with a few to several flowers in a cluster. Flowers are either male (staminate) or female (pistillate), both on the same plant (monoecious) and usually mixed within the same glomerule.

[photo of of bracts and tepals] Male flowers have 3 yellow stamens and 3 or 4 sepals, female have a 3-parted style at the tip of an oval green ovary and 4 or 5 sepals. Both lack petals. Sepals are up to 3 mm long, slightly unequal in size and have a green midrib that extends to a pale spine-like tip. At the base of each flower is a bract that is similar to the sepals and about as long, or slightly longer. The sepals, bracts and stalks are all smooth and hairless.

Leaves and stems: Leaf attachment: alternate Leaf type: simple

[photo of leaves] Leaves are alternate, 3/8 to ¾ inch long, up to 3/8 inch wide, elliptic to spatula-shaped, widest at or above the middle, toothless, wedge-shaped at the base tapering to a stalk about half as long as the blade. Leaves are usually flat but edges are sometimes slightly wavy and are often whitish. Surfaces are hairless, dull or shiny. The tip is mostly rounded, sometimes indented, and has a small spine-like extension at the apex. Stems are green to red, hairless, usually prostrate, sometimes ascending, and much branched, the branches spreading in all directions. A prostrate stem may grow as long as 3 feet but only rises a few inches off the ground.

Fruit: Fruit type: capsule/pod Fruit type: seed_without_plume

[photo of developing fruit] Fruit is a dry seed enclosed in the persistent ovary shell (pericarp) that has a smooth texture and a seam around the middle, the top coming off like a cap to expose the seed. Mature fruit is about as long as the sepals. Seeds are lens-shaped, 1.3 to 1.7 mm long, smooth, dull black.

Notes:

Prostrate Pigweed is found in gravelly or sandy disturbed soils such as roadsides, railroads, cultivated fields, gravel pits, construction sites and vacant lots. It was once considered an agricultural pest in Minnesota but Round-up ready crops took care of that. While generally considered native to the central US and Canada, including Minnesota, the DNR calls it introduced here. It is distinguished by the (usually) sprawling growth, small elliptic to spatula-shaped leaves with a small spine at the tip, separate male and female flowers on the same plant, flowers with 3 to 5 sepals about as long as the floral bracts, and dull, black lens-shaped seeds over 1 mm in diameter. A similar species is Tumbleweed Amaranth (Amaranthus albus), which usually has a bushy, erect form, leaves that are often crinkled along the edges, 3 sepals, bracts twice or more as long as the sepals, and shiny seeds .6 to 1 mm diameter. Somewhat similar is Purslane (Portulaca oleracea), another prostrate, weedy species, which has 5-petaled yellow flowers, leaves lack the spine at the tip and have minute stalks, and its capsules contain many seeds.

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More photos

Photos by Peter M. Dziuk taken in Ramsey and St. Louis counties.

Comments

Have you seen this plant in Minnesota, or have any other comments about it?

Posted by: Tara Jenson - Little Fork
on: 2019-07-22 23:14:20

My folks have a small-plot farm just off of Hwy 217 outside of Little Fork. For the first time they're wrestling a weed that they just cannot stay on top of so they sent me a photo. Digging around, looks like prostate pigweed is a dead ringer for it - maybe not so noxious as the Palmer amaranth, but plenty troublesome for my folks' gardens.

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