What is redroot pigweed?

Redroot pigweed identification starts at ground level. Pull one up, and the pink-to-red root gives it away. This upright summer annual is native to North America and is one of the most common weeds in Pennsylvania gardens and farm fields.

Seeds sprout once the soil warms in late spring and keep coming through midsummer. Plants grow fast, flower from July on, and each one can produce a huge amount of tiny black seed before frost kills it. Those seeds can last for years in the soil.

It’s also important because it belongs to the same group as Palmer amaranth and waterhemp, two far more serious weeds. Knowing pigweed well helps you spot those if they show up.

Redroot pigweed identification: key features

• Root: a shallow taproot that’s pink to red near the soil line.
• Stem: stout, upright and branching, covered in short, rough hairs, especially on new growth near the top.
• Leaves: dull green, oval to diamond-shaped, with wavy edges and long stalks. Undersides often show reddish veins.
• Seedlings: narrow seed leaves, often reddish underneath, and a first true leaf with a small notch at the tip.
• Flowers: tiny and green, crowded into stiff, bristly spikes at the tip of the stem and in the upper leaf joints. The spikes feel prickly.
• Seed: very small, round, shiny and black.

Where redroot pigweed grows in Pennsylvania

Redroot pigweed loves warm, sunny, fertile soil that’s been disturbed. In central PA it’s a staple in vegetable gardens, new beds, fields, and around barns and compost piles. Rich soil with plenty of nitrogen makes it grow even faster.

Construction sites and freshly graded yards often see a big flush of pigweed in their first summer, since old seed gets brought to the surface. It rarely survives in a thick, mowed lawn.

Pigweed seed is also a common hitchhiker in bulk topsoil, unfinished compost and manure. If a brand-new bed fills with pigweed in its first summer, the soil you brought in is a likely source.

Redroot pigweed look-alikes

The pigweed family has a few members you should know:

• Palmer amaranth has smooth stems, leaf stalks often longer than the leaf blade, and very long seed spikes. It’s a Class A noxious weed in Pennsylvania. If you suspect it, contact your county Penn State Extension office or the PA Dept. of Agriculture.
• Waterhemp also has smooth, hairless stems and long, narrow, often shiny leaves. It’s also a Class A noxious weed in PA.
• Common lambsquarters grows in the same spots, but new leaves have a white, mealy coating and the root isn’t red.

The quick check is the stem. Redroot pigweed’s is rough and hairy; Palmer amaranth and waterhemp are smooth.

How to get rid of redroot pigweed

Hand and mechanical. This is a weed where pulling it yourself is the right answer. Seedlings are easy to hoe or pull, especially after rain. Larger plants pull out fine too, since the taproot is shallow. The key is timing: get them before those bristly spikes start making seed in July. If a plant has already gone to seed, bag it and put it in the trash instead of composting.

Cultural.

• Mulch vegetable and flower beds 2 to 3 inches deep once the soil warms. Pigweed seed needs light to sprout.
• Plant crops closely so they shade the soil.
• Keep bare soil to a minimum. Seed or cover graded ground promptly.
• Make sure compost piles get hot enough to kill seed, and be cautious with manure.

Chemical. Most home gardens don’t need herbicides for pigweed. In ornamental beds, a preemergence herbicide labeled for your plants can stop seedlings if applied before the soil warms in spring. In lawns, it’s rarely a problem, and common broadleaf products containing 2,4-D or dicamba control young plants. In vegetable gardens, stick with hoeing and mulch unless a product is specifically labeled for the crops you’re growing. Read and follow the label (the label is the law), and confirm it’s labeled for your site. In Pennsylvania, applying herbicide for hire requires a PA Dept. of Agriculture pesticide applicator certification, so larger jobs should go to a licensed company.

Keeping redroot pigweed from coming back

The rule with pigweed is simple: never let it go to seed. A single season of letting plants seed can fill the soil for years. Each summer you keep it from seeding, the seed bank shrinks.

Keep up with weekly hoeing in June and July, when new flushes keep appearing. Mulch and dense planting do the rest. After a few clean seasons, you’ll notice far fewer seedlings.

For new beds, starting with clean soil and a good mulch layer makes a big difference. If you’d like help, bed installation and mulching sets the garden up to stay cleaner.

Questions we get about redroot pigweed

Similar weeds

Palmer AmaranthAmaranthus palmeriWaterhempAmaranthus tuberculatusCommon LambsquartersChenopodium album