Scouting for weeds: prostrate knotweed, prostrate pigweed, prostrate spurge and common purslane

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Landscape professionals and homeowners may encounter some common weeds in landscape beds, gardens and driveways with a prostrate, mat-forming growth. Four of the most common low-growing, summer annual weeds include prostrate knotweed, prostrate pigweed, prostrate spurge and common purslane. While similar in habit, these plants have specific characteristics that aid in their identification (see below). Just as with any other pest, correct weed identification is the first step to creating an integrated weed management program.

Editor’s note
:  The identification information is from the new field guide An IPM Pocket Guide for Weed Identification in Nurseries and Landscapes. For ordering information, call 517-353-6740 or visit: http://www.ipm.msu.edu/pdf/pocketGuidesLandsc07.pdf         

Prostrate knotweed: Polygonum aviculare L. (view photo)

Life cycle: Prostrate,  mat-forming summer annual.

Leaves:
Alternate, narrow oval to oblong leaves with pointed tips, smooth margins and short petioles.

Stems:
Prostrate, tough, wiry stems with distinct nodes are highly branched and mat-forming. Plants perform well in compacted areas. A membranous sheath (ocrea) surrounds the stem at the base of each petiole

Flowers and fruit:
Small, inconspicuous flowers are white to pinkish and clustered in the leaf axils. The seed is enclosed in a single-seeded, dark reddish brown, three-sided fruit.

Reproduction:
Seeds.

Prostrate pigweed: Amaranthus blitoides S. Wats. (view photo)

Life cycle: Prostrate, mat-forming summer annual.

Leaves:
Alternate and pale green to shiny, dark green leaves are oblong, oval to egg-shaped with pointed, rounded to slightly indented leaf tips. Leaf margins are smooth and usually whitish. Leaves are numerous at the stem ends.

Stems:
Prostrate and nearly smooth, light green to red-dish stems form thick, circular mats. Stems may be 1 to 3 feet long and mostly erect at the tip.

Flowers and fruit:
Small, greenish flowers are found in dense clusters in the leaf axils. Flowers have bracts as long as the sepals and yield small, round, shiny black seeds.

Reproduction:
Seeds.

Prostrate spurge: Chamaesyce humistrata (Engelm. ex Gray) Small (view photo)

Life cycle: Prostrate summer annual.

Leaves:
Cotyledons are oval with maroon undersides and short petioles. Opposite, pale green, egg-shaped leaves with rounded tips have small, irregular teeth along the margins and short petioles. The upper leaf surface often has a maroon watermark.

Stems:
Prostrate, densely hairy stems are highly branched from the base, forming circular mats up to 16 inches in diameter. Stems may root at the lower nodes and exude a milky sap when damaged.

Flowers and fruit:
Inconspicuous flowers are formed in the leaf axils. Fruit are hairy, three-lobed capsules that contain three wrinkled, three-sided, pale brown seeds.

Reproduction:
Seeds.

Similar weeds:
Spotted spurge [Chamaesyce maculata (L.) Small] Differs by having stems that do not root at the nodes, narrow oval to oblong leaves, and ridged, not wrinkled, seeds.

Common purslane: Portulaca oleracea L. (view photo)

Life cycle: Prostrate,  mat-forming summer annual.

Leaves:
Cotyledons are oblong, hairless and succulent. Cotyledons and young leaves are maroon-tinted on the undersides. Thick, fleshy, succulent leaves are hairless, spatula-shaped and about 1 inch long or less. Leaves are alternate but appear opposite and are often crowded near the stem tips.

Stems:
Prostrate, purplish red or green stems have multiple branches that form circular mats. Stems are thick, fleshy, succulent and hairless and up to 20 inches long.

Flowers and fruit:
Small, yellow, star-shaped flowers with five petals are produced in the leaf axils, opening only on sunny mornings. Fruit are egg-shaped capsules that split around the middle, scattering many tiny, black, shiny seeds.

Reproduction:
Seeds.

Prostrate knotweed seedling
Prostrate knotweed seedling.

Prostrate stem
Prostrate stem and shiny green foliage
of prostrate pigweed.

Prostrate spurge plant
Prostrate spurge plant.

Common purslane plant
Common purslane plant.

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