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The Dark World of Waterhemp

Larry Steckel Larry Steckel
Graduate Research Assistant
Department of Crop Sciences
(217) 333-4424
steckel@illinois.edu
  Aaron G. Hager Aaron G. Hager
Weed Science Extension Specialist
Department of Crop Sciences
(217) 333-4424
hager@illinois.edu

Common waterhemp is a species native to Illinois. Examination of herbarium specimens from the Illinois Natural History Survey indicates that waterhemp collections were made in Illinois as early as 1948. Historically, common waterhemp has been more prevalent in the western and southern parts of the state. However, over the last decade, waterhemp has become one of the most troublesome weeds across most of Illinois. Several factors have contributed to its predominance.

Waterhemp is a prolific seed producer, so in only one year a few newly introduced plants have the ability to create a huge soil seed bank that will last for years. Research at the University of Illinois found that in the absence of competition, a single female waterhemp can produce over 1 million seeds.

No-till production systems have aided the proliferation of waterhemp. A six-year study at the University of Illinois examining the influence of tillage on waterhemp emergence found a 34 percent increase in yearly waterhemp emergence in no-till compared with conventional tillage systems.

Waterhemp has the ability to germinate and emerge later into the growing season than is common for most other summer annual weed species. University of Illinois research has shown that waterhemp emergence begins in late April and continues well into July. Prolonged emergence challenges soil-applied herbicides to provide late-season control and makes it difficult to time postemergence applications for optimum control. Research examining application timing of certain soil-applied corn herbicides has shown that more consistent late-season waterhemp control was obtained when two-thirds of the final rate was applied at planting, with the remaining one-third applied at early postemergence, compared with applying the full rate at planting (Figure 1).

Waterhemp control with preemergence vs. split applications

Figure 1. Waterhemp control with preemergence vs. split applications
of various soil-applied corn herbicides.

The development of herbicide-resistant biotypes has also contributed to the spread of waterhemp. Populations of waterhemp resistant to acetolactate synthase-inhibiting (ALS) and triazine herbicides have been documented. Recent reports from Kansas indicate that a waterhemp population resistant to several diphenyl ether herbicides has been discovered. This past year, we monitored several waterhemp populations from Illinois that matured after being sprayed with a 3x rate of glyphosate.

Shade can be an effective tool for managing waterhemp. In field research during 2000, common waterhemp that emerged in late May with no competition for light produced just over 1 million seeds per plant, while waterhemp subjected to 98 percent shade produced only two seeds per plant. Shade at the soil surface of 95 percent or more can be achieved sooner with soybeans planted in narrow rows. Cultivation is a good management option for soybeans or corn planted in 30-inch rows. Crops with thin stands should be monitored for late flushes of waterhemp, even into July. In the shade experiment, waterhemp that emerged in late June and received only 33 percent of full sunlight still produced over 92,000 seeds per plant.

Waterhemp management has proven to be challenging. Understanding waterhemp biology can help to develop an effective management plan that includes optimum crop row spacing, tillage, and herbicides. The most consistent, long-term approach to managing waterhemp consists of an integrated approach combining cultural, mechanical, and chemical programs. The biology of waterhemp makes consistent control with only one herbicide application, particularly when crops are unable to shade the soil effectively, unlikely to succeed.

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