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How to Manage Palmer Amaranth

“We don’t need any more weed problems; we have enough of our own,” says Jeff Gunsolus, Extension weed scientist at the University of Minnesota. Palmer amaranth, the native desert Southwest weed, is why he's concerned. 

The drought-tolerant weed shares characteristics with waterhemp, so knowing what to look for can help in identification. Palmer amaranth grows faster – at 2 inches per day – and is more competitive, warns Gunsolus. It’s been perceived as a Southern weed, but it’s on the move. There have been reports of Palmer amaranth from Indiana to Wisconsin and even southern Iowa. He believes it is spreading through livestock manure, CRP seed, and infested field equipment.

“We are concerned about Palmer amaranth because we know once it gets established, it has a bigger effect on yield,” says Bob Hartzler, Iowa State Extension weed specialist.

The yield ding comes because it’s more competitive, has more biomass, and uses more resources than waterhemp. When scouting, trying to differentiate Palmer amaranth from waterhemp can be a challenge. Following are a few tips.

Check the Details
Palmer amaranth has a long petiole, and the leaf is more ovate or round, says Gunsolus. Waterhemp has a short petiole and a longer, narrow leaf. At the end of a Palmer amaranth leaf is a tiny notch with a little spike. Palmer amaranth also looks waxier than a waterhemp plant, and it can have a little thumbprint where the leaves unfurl, says Gunsolus.
Another key to look for is the spiky seed head. One plant can produce 1 million seeds. Palmer amaranth has long bracts (¼ inch), which are twice the length of the flower, says Hartzler. The bracts stick out from the other parts of the flower and from the seed capsule.

“Palmer amaranth has long, prickly structures that become sharp,” says Hartzler. “When you grab those female plants, you know it’s not a waterhemp because the waterhemp is very soft.”

If Spotted
Keep an eye out for the pesky weed. Hartzler recommends looking for it in field entrances. As equipment moves from field to field, there’s a greater likelihood of it being deposited in the entrance first.

“Don’t leave it in the field. Bag it so if there is any seed, it doesn’t spread. Pull it out of the field and burn it,” says Gunsolus.

“The cheapest way to manage a weed is to prevent it from getting established in a field,” says Hartzler. “At this time, 99.9% of fields don’t have it. If you can identify it, get it out of the field. In most fields, you can stop it from getting established.”

Since it’s not a widespread problem yet, Hartzler expects infestations to be small. He encourages you to hand weed at that point, especially with the female plants.

Palmer amaranth calls for an aggressive management plan with soil preemergence herbicides and effective postemergence herbicides, says Gunsolus.

“What we are concerned about is that a lot of Palmer amaranth is already becoming resistant to a couple of classes of chemistry – often to Roundup, the site of action 9, and often to the site of action 2, like the chemistry in Pursuit. So you’ll start to lose tools right from the get-go. That makes it even more challenging.”

The first step when you choose a weed-control program is to use multiple effective herbicide groups, says Hartzler. Be sure to consider the species, weed density, and known resistances. You’ll get into trouble when you let weed densities build up, he says.

“Small changes can make significant differences in selection pressure and can prolong the herbicide’s effectiveness,” says Hartzler.

He also recommends altering your herbicide program each year and adding mechanical tactics to your weed-control program. If you struggle to get weeds under control, incorporating new weed-control tactics is not as simple as altering herbicide programs. When faced with herbicide-resistant weeds, however, you’ll want to use every tool available.

Mechanical Tactics
If herbicide programs are no longer sufficient for weed control, Iowa State Extension weed specialist Bob Hartzler recommends that you incorporate different forms of mechanical options. Here are four tips he recommends considering.

1. Consider preplant tillage.
“There’s no doubt that tillage provides benefits in managing weeds,” says Hartzler. “Any seedbed tillage you do is going to bury a significant amount of seeds at a depth where they can’t germinate.”

It has its drawbacks, though. You’re putting the seeds into deep storage, and the next round of tillage will bring them back to the surface. 

“In most situations, I wouldn’t recommend preplant tillage just for the sake of managing weeds, unless it’s a total control failure,” says Hartzler.

2. Incorporate cultivation.
Cultivation will allow you to target problem areas in specific fields. “What we like to remind you is that you don’t have to do all of your acres,” says Hartzler. “You don’t even need to do all of your field. Resistance problems are patchy in nature.”

3. Consider the planting date.
If you have a problem field with a significant weed infestation, hold off on planting that field, says Hartzler. Waiting will reduce the time that the herbicide needs to work, he explains.

4. Narrow the row spacing.
“As long as we are at 30-inch rows, waterhemp is going to be a big problem,” says Hartzler.

Narrow-row soybean spacing will reduce the time until canopy closure. It should be a consideration if you have the ability to make it work, he says.

Other tips from Hartzler include stopping weeds from entering your field. Do a better job controlling the weeds in the ditches and waterways, he says. If you have resistant weeds in certain fields, take the time to clean your equipment.

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