If you haven’t seen waterhemp on your farm yet, Bill Johnson says you likely will. And while waterhemp is in the pigweed family, it can prove tougher to control than redroot pigweed. That’s the type most people picture when they think of pigweed.
Take a look at the picture (above) to see if you recognize it. If you’re not used to looking at weeds that small, Johnson, Purdue University Extension weed control specialist, suggests you start learning to recognize tough weeds in that size range. That’s when they’re easier to control.
Here are eight facts about waterhemp that illustrate why you may want to get a jump on it now. You can find more information about this weed in the 2017 Weed Control Guide for Ohio, Indiana and Illinois. Mark Loux of Ohio State University and Aaron Hager of the University of Illinois contributed to the guide, in addition to Johnson.
Fact 1: Competitive through sheer numbers. Seasonlong competition from waterhemp at 20 plants per square foot can reduce soybean yields up to 44%. That’s a lot of waterhemp plants!
Fact 2: Just keeps coming! Weed scientists say they have data that shows even if waterhemp plants don’t emerge from the soil until soybeans are at the V5 stage, they can still reduce yields up to 10%.
Fact 3: Fast grower. Waterhemp gets its competitive edge because it grows faster than most crops or weeds, Johnson says. It can grow from 1 to 1.25 inches per day during the growing season. The taller the plant, the more sunlight it can capture.
Fact 4: Late-emerger. A residual herbicide can’t control a plant that germinates after the herbicide loses its weed-killing activity. A higher percentage of waterhemp plants emerge later in the season compared to most other annual weeds. That allows it to escape some residual herbicides and also flourish after postemergence applications of herbicides without residual activity.
Fact 5: Prolific producer. Waterhemp plants typically produce 1.5 times as many seeds as most other pigweed species. They generally produce 250,000 seeds per plant, but can produce up to a million seeds if there is little or no competition. That’s similar to the seed production capacity of Palmer amaranth.
Fact 6: Creates genetic diversity. Also like Palmer amaranth, waterhemp comes in male and female plants. That gives it an edge in creating genetic diversity compared to many other plants. More genetic diversity means it’s easier for waterhemp to evolve and spread plants with resistance to various chemicals.
Fact 7: Remarkable ability to adapt. Waterhemp has evolved resistance to six chemical classes. Group 5: triazines; Group 2: ALS inhibitors (Pursuit, Classic); Group 14: PPO inhibitors (Ultra Blazer, Cobra, Flexstar); Group 9: glyphosate; Group 27: HPPD inhibitors (Callisto, Laudis, Impact); and Group 4: 2,4-D.
Fact 8: Multiple resistances in one plant. Many waterhemp populations in the Midwest now exhibit multiple herbicide resistances. Resistance to groups 2 and 9 in the same population is fairly common. In fact, resistance to as many as five chemical groups now occurs within some populations, Johnson concludes.