Waterhemp is one of the most common weeds across the corn belt and temperate climates of North America. Classified as a summer annual weed, it germinates throughout the season. It begins in the spring, flowers mid-summer, can cross pollinate easily with relatives, then sets its many seeds in late summer and lies in waiting for soil temperatures to begin warming in the spring to sprout again. Tied often to row crops and small grains, and hence most commonly associated with corn belt crop systems, waterhemp can thrive from Texas to Maine. Over the years, farmers have relied on several cultural and chemical options, which are best implemented within a good rotation management strategy.
A member of the pigweed (Amaranth) family, there are many variations that result from its aggressive and prolific ability to cross breed with same-species varieties of the plant. Some examples include Palmer amaranth, redroot pigweed, and smooth pigweed. Distinguishing between the many resulting types of Amaranth is especially challenging--particularly at the seedling stage, when weed identification is crucial in an integrated weed management program. Waterhemp can reduce yields in soybeans by as much as 44%.
Pigweed not only is an aggressive grower, but boasts tremendous seed viability and long-lived persistence as a seed population in the soil, germinating over several years. This characteristic causes great frustration because in a given acre-slice, you may have literally hundreds of potential genetic parentage foundations germinating. Modern day resistance management of pigweed must address this issue through a well-managed integration of cultural, chemical, monitoring, and control regimes.
Researchers continue to develop compounds and genetic material in search of how farmers can prevent this weed from robbing yields and profits.
Here are some key differences between and among the various brothers, sisters, and cousins of waterhemp:
- The cotyledons of waterhemp are more “egg-shaped” than the longer, narrower, and more linear cotyledons of other pigweeds.
- The first true leaves of the waterhemp plant tend to be more lance-shaped and longer than other pigweeds.
- Waterhemp seedlings are more glossy and are hairless, whereas other pigweeds are hairy.
As with other summer annuals, it becomes easier to identify waterhemp as it matures. Ranging in ultimate height from a few inches to as tall as 10-12 feet in fallow fields, it’s most typical in row crops to see it reach a height between 3-6 feet.
Along with the characteristic waxy, elongated and narrow leaves as a seedling carrying through to full maturity; the stems and leaves tend to be various shades of green, with some random members of a population exhibiting a distinctly red stem or leaf color.
Prevention and Control of Waterhemp
The evolution of reduced tillage systems, herbicide-resistant crops, and continuous annual crop rotations have compounded the challenge to controlling waterhemp and other related pigweeds. It is one of the most prolific cross breeders and germinators, and can travel across field boundaries easily. This makes resistance management even more challenging when relying only on herbicide-based control programs.
However, research, genetic science, variety development, and field equipment technology are up to the task. Controlling pigweeds in grass crops like corn and small grains is far simpler, as they are monocots. In soybeans and other broadleaf dicots, the challenge becomes more complex. Development of dicamba and phenoxy herbicide resistant crops has helped. However, this weed is especially adept at finding ways to survive.
Hence, a cultural perspective is also important for managing waterhemp. Rotation to perennial crops; fallow and disk programs; cover crops; row spacing; and tighter rotations of annual agronomic crops can all come together along with a similar rotation of chemistry in your herbicide choice, timing, and method of application. Rotating tillage choice is also beneficial to reducing the accumulation of Amaranth populations in the acre-slice. Finally, and perhaps most important; nutrient management of the crop can keep weed populations below actionable thresholds as much as any other activity.