What is waterhemp?

Waterhemp is a tall pigweed native to the Midwest that has become a major problem in corn and soybean fields across the country. Good waterhemp identification matters here because Pennsylvania lists it as a Class A noxious weed, with the goal of keeping it from getting a foothold.

Isolated populations have turned up in Pennsylvania and neighboring states over the past several years. Plants can produce hundreds of thousands of seeds each, and many populations resist glyphosate and other common herbicides. Male and female flowers grow on separate plants, and pollen can pass herbicide resistance along to the next generation.

For homeowners, the concern is a plant slipping into a vegetable garden, a new meadow planting or the edge of a field near the house.

Waterhemp identification: key features

Young pigweeds are hard to tell apart, so look at plants that are at least several inches tall and check more than one.

• Hairless: the stems and leaves are smooth. Redroot and smooth pigweed have fuzzy stems.
• Narrow leaves: leaves are longer and narrower than other pigweeds and often have a glossy, waxy shine.
• Short leaf stalks: on mature leaves, the stalk is shorter than the leaf blade. That’s a key difference from Palmer amaranth.
• Stem color: ranges from light green to dark red, sometimes with several shades on one plant.
• Seed spikes: slender and often branched at the top of the plant. Female spikes feel soft rather than prickly.

If you think you’ve found it, take photos and contact your county Penn State Extension office to confirm the ID.

Where it grows in Pennsylvania

Most finds are in crop fields, especially soybeans and corn, along with field edges and other disturbed soil. Seed travels in contaminated crop seed, feed, hay, bedding and manure, and combines carry it from one field to the next.

Waterhemp seedlings emerge over a long window, from late spring well into summer. That’s part of why it slips past a single herbicide pass or one round of weeding. Gardens and plantings near infested farm ground in our valleys are the most likely places a homeowner would run into it.

It has also been known to hitch a ride in wildflower and pollinator seed mixes that weren’t properly cleaned, so a brand-new meadow planting deserves a careful look its first summer. Tall, smooth, narrow-leaved pigweeds that you didn’t plant are worth checking.

Waterhemp look-alikes

Palmer amaranth is its closest relative and also a Class A noxious weed. Palmer has wider, diamond-shaped leaves on long stalks, and female plants have sharp, prickly bracts. Waterhemp’s leaves are narrower and its leaf stalks are shorter.

Redroot pigweed is the pigweed most gardeners know. It has hairy stems, broader leaves and short, dense, bristly seed spikes. Lambsquarters can look similar from a distance, but its leaves have a whitish, mealy coating on new growth.

How to get rid of waterhemp

In a home garden or yard, pulling by hand is the most reliable control. The whole game is making sure no plant drops seed.

• Pull small plants: seedlings and young plants come out easily when the soil is moist. Use a digging fork on bigger plants to get the taproot.
• Bag flowering plants: if a plant already has seed spikes, bag it on the spot and put it in the trash. Don’t compost it or leave it lying on the ground.
• Scout all summer: because waterhemp keeps emerging into summer, check the area every week or two until fall.

Cultural practices make a real difference. Mulch garden beds to block new seedlings, keep lawns thick so there’s little bare soil, and avoid leaving freshly worked ground uncovered. Clean tillers, mowers and boots before moving from an infested area to a clean one.

On farms, waterhemp control depends on layered herbicide programs with residual products and several modes of action, applied before plants reach about 3 to 4 inches. Many populations resist glyphosate and ALS-inhibitor herbicides, so relying on one product usually fails. Any herbicide must be labeled for the crop or site, and you must read and follow the label, which is the law. Applying herbicides for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, so larger infestations are a job for a licensed applicator working with Penn State Extension.

Keeping it from coming back

Waterhemp seed doesn’t last forever in the soil, and most of it is gone within about four years. That means steady work pays off: if you keep every plant from setting seed for a few seasons, the seed bank drops sharply.

Gloves make the job easier on your hands, and pulling after a soaking rain gets more of the root. Buy tested seed for meadow and pollinator plantings, be careful about hay and manure brought in from outside, and keep watching areas where you’ve found it before. Report suspected plants to your county Penn State Extension office.

Questions we get about waterhemp

Similar weeds

Palmer AmaranthAmaranthus palmeriRedroot PigweedAmaranthus retroflexusCommon LambsquartersChenopodium album