Waterhemp is quickly gaining ground in the Midwest, giving corn and soybean growers a new foe to battle.

"Waterhemp is now the dominant weed species in many fields," says George Kapusta, a weed specialist at Southern Illinois University.

Because it's a prolific seed maker, waterhemp may start out as a minor irritation one year and develop into a yield-threatening scourge the next. Just one plant can produce up to a million seeds, he says.

Waterhemp, a member of the pigweed family, was first discovered in the mid-1800s in parts of the Midwest. But it didn't pose a problem to corn and soybean growers until the early 1990s, when infestations began to build in Nebraska and Kansas.

Within a few years, waterhemp infestations started to appear in Missouri and Iowa. And by 1995, the weed had become a major problem for farmers in southern Illinois, too.

"Waterhemp infestations are moving east," says Tom Bauman, Purdue University extension weed specialist. "Here in Indiana, we are preparing our growers for potential problems by familiarizing them with the weed and encouraging them to scout for it."

Weed scientists recommend careful scouting for correct identification of waterhemp because it closely resembles other pigweed species. According to Kapusta, pigweed stems are covered with fine hairs and the color is much duller. Waterhemp has no hair and the stems and leaves are much glossier.

To tell waterhemp from Palmer amaranth, which also is hairless and shiny, check the leaves - waterhemp's are narrower.

Waterhemp populations have built readily over the past few years because of the weed's tolerance to ALS inhibitors, a class of herbicides that control most pigweed species but have little effect on waterhemp, says Kapusta.

If you find waterhemp, avoid ALS-inhibitor herbicides or back them up with a herbicide with a different mode of action, suggests Dan Childs, another Purdue extension weed specialist. Paying attention to chemical classes is especially important in soybeans, where ALS inhibitors have been widely used for years.

Broadstrike, Classic, Pinnacle and Pursuit are ALS inhibitors for soybeans. Accent, Basis, Beacon, Broadstrike, Exceed and Permit are ALS inhibitors for corn.

Waterhemp can germinate over a long period, so weed scientists recommend a combination of pre-emergence and postemergence controls.

Atrazine is highly effective on waterhemp in corn as long as rates aren't too low, says Kapusta.

"Use at least 1.5 quarts of atrazine per acre," he advises. "Atrazine, plus a grass herbicide, such as Surpass, Dual, Harness or Frontier, will give a good level of control."

For season-long control of waterhemp in soybeans, Kapusta recommends Authority or Canopy XL if soil pH does not exceed 6.8

Other options: "A grass herbicide, like Frontier, Lasso, Micro-Tech, Dual, Treflan or Prowl, will give 60-80% control and last a couple of months.

"A postemergence application of Blazer, Cobra, Reflex or Flexstar should provide good control, too" says Kapusta.

He points out that Roundup gives good waterhemp control for growers with Roundup Ready soybeans.