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How to manage Palmer amaranth, waterhemp

Bill Johnson, Purdue University Extension weed specialist waterhemp
KNOW THE ENEMY: Here is a young tall waterhemp plant. This weed is showing up more frequently across Indiana than in the past.
Management starts with identifying the weed correctly.

By Allie Abney

“A weed scientist once told me, ‘Dead weeds cannot become resistant,’” says Sam Ebenkamp, owner of Ebenkamp Farms LLC in Ireland, Ind. To manage aggressive weeds, you must first be able to identify the weeds infiltrating your fields.

Two weeds that have developed resistance within Indiana and are highly destructive are Palmer amaranth and waterhemp. Bill Johnson, Purdue University Extension weed control specialist, says since the appearance of Palmer amaranth and waterhemp is similar, especially during the seedling stages, the weeds are easy to misidentify. It’s incredibly important to scout and identify the species that exist within each field.

Weed identification
Johnson describes some features of the weeds to help tell them apart.

Palmer amaranth:

• Leaves are wider compared to waterhemp and ovate to diamond-shaped.

• Petiole length is shorter than their long, lance-shaped leaves.

• An extended petiole exists on the first true leaves.

• Females have a long main terminal seed head that can reach up to 3 feet long.

• Females have stiff, sharp bracts that make seed heads prickly.

• Look for a white watermark shaped like a chevron or V on leaves.

• There will be a single leaf-tip hair.

Bill Johnson, Purdue University Extension weed specialist

MAKE THE ID: University weed specialists offer tips so you can differentiate Palmer amaranth (pictured) from tall waterhemp.


• Leaves are generally long, linear and lanceolate.

• Find multiple branched seed heads that are similar in length and lack the stiff, prickly bracts.

• Seedlings are hairless with leaves that look waxy or glossy.

If you’re unable to distinguish which weeds are taking over your field, consider tissue sampling. This can help you identify if your weeds are resistant to specific chemicals.

Once you know which chemicals your weeds are resistant to, you’ll be able to find a better mode of action to eliminate them.

“On our farm, we’re in 100% no-till and are using a cereal rye cover crop, which creates a mat that keeps weed pressure down,” says Ebenkamp. “Our key to preventing waterhemp is to burn down with residual before they come up, because even if they’re PPO-resistant, you can kill them when they’re really small.”

Weed management
After properly identifying weeds within your field, there are many ways to manage these two weeds. Here are a few ideas for winning the war on your weeds that Johnson recommends.

You could rotate crops, practice deep tillage, plant a cereal rye cover crop, hand-weed, monitor ditches and borders, and harvest heavily infested fields last.

Ebenkamp also notes that it’s incredibly important to keep weeds from coming up, spray weeds when they’re very small, use the appropriate rates, and avoid skimping to save money.

It’s incredibly important to properly identify weeds to be proactive rather than reactive. If you’re proactive with your weed management, you’ll be able to control the problem in each field, Johnson says. Remember, dead weeds cannot become resistant!

Abney is a senior in ag communication at Purdue University.

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