To Find a Pigweed

Identify Palmer amaranth early to stop an infestation.

By the time a Palmer amaranth seedling has eight to 10 leaves, it is already showing key differences between other look-alike weed species, like waterhemp and redroot pigweed, Image by Aaron Hager

We’ve all seen them--the startling pictures of giant Palmer amaranth plants towering over soybean fields, with their long, spiky seed heads jutting toward the sky.

But, the prime time for spotting Palmer amaranth plants is when they are still small enough to kill. The problem is Palmer amaranth seedlings can look an awful lot like other plant species, particularly waterhemp and other types of pigweed.

North Dakota State University Extension weed scientist Joe Ikley has some tips on how to distinguish this weed--which may be new to many in the Midwest--early in the season.

IT’S ALL IN THE HAIR

So, you’ve found a suspicious seedling. The first thing to do is see how hairy it is, Ikley says. By the time they are about 2 inches tall, certain pigweed species such as Powell amaranth and redroot, or smooth, pigweed will sport fine, tiny hairs on their stems, known officially as pubescence. The stems and leaves of Palmer amaranth and waterhemp, however, are smooth and hair-free.

These hairs can be hard to see, especially if your eyes aren’t the sharpest, Ikley admits. “One trick is to hold the plant up to the light and--especially if the plant is wet--the hairs will stand out better,” he says. If you have a smartphone, try taking a focused photo with it and zoom in on the stem to spot them, he adds.

Finding smooth, hairless stems and leaves effectively rules out other pigweed species, so it’s down to Palmer amaranth and waterhemp.

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PETIOLE LENGTH

Now, it’s time to check the length of the petiole--the narrow, branchlike structure that connects a leaf to the stem.

Look for a seedling with eight to 10 leaves, and pluck one of the oldest, mature leaves near the bottom of the plant, Ikley says.

Fold the petiole over the length of the leaf blade, and see how long it is. A waterhemp seedling will have short petioles that will not be longer than the length of the leaf. A Palmer amaranth seedling will have long petioles that will be as long as the leaf, if not longer.

Because of the length of these petioles, as well as their alternating pattern on the stem, most of the leaves of the Palmer seedling are visible when you peer down at them from above, giving the plant a rosette appearance, much like a Poinsettia.

THE LOOK OF THE LEAF

In general, waterhemp and Palmer amaranth seedlings also have different leaf shapes. Waterhemp leaves tend to be longer and narrower, like little lances. Palmer leaves will be fatter, particularly in the middle, giving them a more oval or diamond shape.

Some Palmer plants also have a white chevron or V-shaped pattern in the middle of the leaf but not all. So, while it can rule out a different pigweed species if present, it is not a definitive identification tool.

There is a lot of genetic diversity among both Palmer and waterhemp plants, and even some hybridization between the two, Ikley notes. So, treat leaf shape and appearance as a useful but not final indicator of plant species.

CHECK THAT NOTCH

The first two or three true leaves on a Palmer amaranth plant often sport a single, stubby hair emerging from the notch at the leaf tip.

Most common waterhemp plants don’t have this, which makes it a generally useful sign of Palmer. But, some waterhemp plants have been observed with leaf tip hairs in the western Corn Belt, and some Palmer plants have been observed without it, Ikley cautions.

Like leaf shape, use the notch hair to help confirm a Palmer amaranth seedling, but don’t rely on it exclusively. The safest and surest signs of a Palmer amaranth seedling remain smooth, hairless stems and leaves, and long petiole length on the first true leaves, Ikley says.

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