Spiny Amaranth

What is spiny amaranth?

Spiny amaranth is a summer annual in the pigweed family, and it is the one pigweed that arms itself. Reliable spiny amaranth identification comes down to stiff spines at the nodes where each leaf stalk joins the stem. Tennessee Extension notes those spines alone separate it from its relatives, which otherwise share the same green flower spikes, oval leaves and reddish stems.

It causes trouble in two ways. It is a seed factory, and fresh seedlings appear after every summer rain. And it claims ground people want to use: livestock avoid grazing near the spines, so it spreads unchecked across worn pasture and can eventually dominate a field. In gardens it can outgrow and shade vegetables, and the spines make weeding painful.

Spiny Amaranth identification: what to look for

Check a leaf joint first, then the stem surface, then the flower arrangement. Spines plus a hairless stem is the giveaway.

• Spines: Rigid, sharp spines sit at the node where each leaf stalk attaches, roughly 1/5 to 2/5 inch long. No other common pigweed has them.
• Stems: Smooth and hairless; green, red, or green with red stripes. Stems stand upright.
• Leaves: Alternate, egg- to diamond-shaped, hairless, about 1 to 2 1/2 inches long, on fairly long stalks. Some show a pale or purplish watermark.
• Flowers: Tiny green flowers in a narrow spike at each stem tip (mostly male) and in round clusters in the leaf joints (mostly female), all on the same plant.
• Roots: A taproot, sometimes red, with fibrous side roots. Seedlings pull easily; older plants hold on harder.

Where it grows in the United States

USDA PLANTS lists spiny amaranth as native to the lower 48 states and Puerto Rico, and Tennessee Extension traces it to tropical America. Extension services across a wide area deal with it: Florida calls it very common statewide, Tennessee finds it throughout the state, a Louisiana AgCenter agent reports it is the pigweed he sees most in home gardens, and Maryland includes it in its pigweed guide.

Soil condition matters more than region. It thrives on bare, compacted, heavily used ground where grass is thin: around feeding pens and water troughs, where hay was fed over winter, and in overstocked pastures. Tennessee Extension notes it is generally not a problem in hay fields, where dense forage keeps the soil shaded. In gardens, freshly worked, sunny beds suit it well.

Spiny Amaranth look-alikes

A few plants get mixed up with it:

• Smooth pigweed (Amaranthus hybridus): Hairy stems and no spines at the leaf bases. Its leaves often have wavy edges, and its flower spikes are thinner and looser than redroot pigweed's.
• Redroot pigweed (Amaranthus retroflexus): Also hairy-stemmed, sometimes with hairy leaf undersides, and its seed heads are thick and dense. No spines.
• Palmer amaranth (Amaranthus palmeri): Shares the hairless stem but has no leaf-base spines. Older leaf stalks are as long as or longer than the blades, male and female flowers grow on separate plants, and female plants carry very long seed spikes (often 2 to 3 feet) with sharp bracts. Sharp parts on the seed head mean Palmer; at the leaf joints, spiny amaranth.

How to get rid of spiny amaranth

Pulling, digging and mowing: Pull or hoe plants while young, before the taproot toughens, the spines harden and seed forms. Wear thick leather gloves for anything past the seedling stage. Carry pulled plants out of the garden rather than leaving them on the soil, since some pigweeds can re-root from pieces touching moist ground. In pastures, remove or spot-treat the first plants you see before they seed the area for years.

Cultural control matters most, because the right conditions keep it from coming back.

• Keep pastures thick; dense forage shades the soil so seedlings struggle to establish.
• Move hay feeders through the winter to avoid one big patch of bare, churned soil.
• Ease stocking pressure around pens and troughs, where thin grass gives this weed its best foothold.
• Mulch vegetable gardens with plastic or pine straw to block light from seeds.
• Scout after summer rains, since each soaking can bring up a new flush.

Herbicides: For pastures, Tennessee Extension lists 2,4-D, 2,4-D plus dicamba, and aminopyralid plus 2,4-D, and says treatments work best on seedlings, with late-spring and summer passes helping stop seed spread. University of Florida trials found chlorsulfuron and aminopyralid gave excellent control at one month, but new seedlings returned by three months, so plan on two or three treatments a season; chlorsulfuron alone also misses many other pasture weeds. In vegetable gardens, a preemergence product containing trifluralin can keep seeds from sprouting, and glyphosate can be carefully wiped or spot-applied on emerged weeds, though it kills any vegetable it touches. Use only products labeled for your site (pasture, garden or specific crop); the label is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Spiny amaranth is an annual, so stopping seed for a few years starves the problem. Pull, hoe or treat early, and don't let plants mature in out-of-the-way corners.

Long term, deny it bare ground: keep forage dense, rotate feeding sites, repair overgrazed spots and mulch garden beds.

Questions we get about spiny amaranth

Similar weeds

Smooth PigweedAmaranthus hybridusRedroot PigweedAmaranthus retroflexusPalmer AmaranthAmaranthus palmeri