Waterhemp--What to Expect in 1999

April 9, 1999

Someone once said the only certainties in life are death and taxes. It seems reasonable to add "weeds" to this list of certainties. Although numerous weed species present management problems each year, waterhemp has emerged as one of the most troublesome weed species that Illinois corn and soybean producers face. Several theories have been proposed to explain why waterhemp has become one of the dominant weed species in Illinois agronomic production fields. Although these theories help understand the "why," producers are often struggling with the "how" when it comes to managing this weed. In this and the following issue of the Bulletin, we'll explore the biology of waterhemp and offer suggestions for management in both corn and soybean production systems.

Waterhemp Biology

Waterhemp belongs to the botanical Amaranth family, which also includes the other pigweed species found in Illinois. The Latin or scientific name of each pigweed contains the genus name Amaranthus; each respective species name differentiates among the genus members. Historically, plant taxonomists have recognized two distinct waterhemp species: tall (Amaranthus tuberculatus) and common (Amaranthus rudis). Even though the literature recognizes two waterhemp species, from a management standpoint there is little reason to be able to differentiate between these two species. We are not aware of any data that suggest that these two species respond any differently to herbicides. Additionally, the only way to accurately differentiate between tall and common waterhemp is to examine how the thin membrane surrounding the seed (utricle) fractures when abraded. Unfortunately, the time of the season when the plants are at a developmental stage to allow this identification procedure is also close to the time for harvesting operations.

Tall and common waterhemp (referred to collectively as waterhemp) are two of nine pigweed species that can be found in Illinois. Prior to the rapid expansion of waterhemp, smooth pigweed (Amaranthus hybridus) was probably the most prevalent pigweed across much of Illinois. During early vegetative stages, smooth pigweed is nearly impossible to distinguish from redroot pigweed (Amaranthus retroflexus), another commonly encountered pigweed species. Palmer amaranth (Amaranthus palmeri) can be found in the southern quarter of Illinois. Like waterhemp, Palmer amaranth is a dioecious (separate male and female plants) species and, from personal observations, appears to be moving northward in Illinois. Palmer amaranth may be the most aggressive pigweed species with respect to growth rate and competitive ability. Powell amaranth (Amaranthus powellii) is usually found in the northern portions of Illinois, but it can also be found in central regions of the state. Spiny amaranth (Amaranthus spinosus) is rapidly identifiable by grabbing the stem with bare hands. As the name accurately implies, sharp spines are present where leaves attach to the stem. Although not very common in agronomic cropping systems, spiny amaranth can commonly be found in pastures and around cattle feedlots. The two other pigweeds, tumble (Amaranthus albus) and prostrate (Amaranthus blitoides), are generally not considered to be as troublesome as other Amaranths.

Waterhemp plants are either male or female (dioecious). Thus male plants produce only pollen, whereas female plants produce only seed. This type of biology leads to cross-pollination, or the fertilization of female plants with pollen from one or more male plants. Cross-pollination can greatly increase the genetic diversity of a population, and with genetic diversity comes a wide range of morphological and biological characteristics. Seeds produced by female waterhemp plants are small and usually germinate from very shallow depths in the soil (0.5 inch or less). The number of seeds produced by female waterhemp plants can vary depending on numerous factors, but waterhemp is generally considered to be a prolific seed producer.

The germination and emergence patterns of waterhemp are characteristics that contribute significantly to management problems. Whereas the peak emergence of other, more familiar annual weed species generally occurs during the early portion of the growing season, waterhemp emergence can easily occur during the middle to late portions of the growing season. Data collected at Iowa State University in 1996 indicated that whereas velvetleaf emergence was nearly complete by early June, a significant number of waterhemp plants emerged well into July. Obviously, this type of emergence pattern presents management challenges for soil-applied-only or postemergence-only herbicide programs.


Accurate identification of the various Amaranths can be challenging, especially when the plants are in early vegetative stages. While each of the pigweeds previously described is recognized as a distinct species and has unique identification characteristics, hybridization among some of these species may produce offspring possessing characteristics of each parent, further complicating identification. The best time to accurately identify the various Amaranths is when the plants are at the reproductive stage with flowering structures present.

Waterhemp plants typically have no hairs (pubescence) on their stem and leaf surfaces. In contrast, smooth and redroot pigweed have small, fine hairs on both stem and leaf surfaces that make the plant feel rough to the touch. The leaves of waterhemp plants are often glossy and more elongated (lanceolate) compared to redroot or smooth pigweed. Stem color of waterhemp can vary from light green to dark red, with multiple shades sometimes on the same plant. There does not appear to be a strong correlation between stem color and sex of the plant. Female plants may be completely red, completely green, or some combination of red and green. Male plants may exhibit a similar color pattern.

Table 4 contains information for identification of the various Amaranth species. Other identification sources are available, and many contain color pictures that some individuals may find helpful. One of the best pigweed identification references is a guide that was jointly developed by Kansas State University, USDA/ARS, and the University of Illinois. Pigweed Identification: A Pictorial Guide to the Common Pigweeds of the Great Plains is available from the Kansas State University Cooperative Extension Service Distribution Center, 16 Umberger Hall, Kansas State University, Manhattan, Kansas 66506-3406. This reference guide contains color photographs of the various pigweed species described in this article as well as brief descriptions of both vegetative and reproductive characteristics.

Management Considerations

What is the best way to manage waterhemp in corn or soybean production systems? Although there may not be any one "best" way, some methods may be much more consistent than others. Whereas waterhemp may, in some instances, be adequately controlled by a single soil-applied or postemergence herbicide, this is generally not considered the most consistent method to manage this weed. The most consistent waterhemp management programs in either corn or soybean production systems consist of a sequential management approach. By sequential we are referring to utilization of multiple control options, including tillage, cultivation, soil-applied herbicides, and postemergence herbicides. Although a single postemergence herbicide application may sometimes provide acceptable waterhemp control, this is the exception rather than the rule. Waterhemp may well be the "poster weed" for an integrated weed management program.

Considerations with Soil-Applied

Herbicide Programs

Numerous soil-applied herbicides possess good activity on waterhemp and other small-seeded species. Time of application can have a significant impact on the success of soil-applied herbicides for waterhemp control. A common practice in no-till systems is to apply a herbicide several weeks prior to planting in order to receive sufficient precipitation to incorporate the herbicide. Keep in mind, however, that the earlier a herbicide is applied, the earlier within the growing season the level of weed control begins to decline. Waterhemp can emerge much later in the growing season than is common for other summer annual species. If the herbicide was applied several weeks prior to planting, it may not have sufficient residual activity remaining to control a late-emerging species such as waterhemp.

What can be done to extend the length of control afforded by soil-applied herbicides? Here are three options:

  • If allowed by label, increase the rate when applications are to be made several weeks prior to planting.

  • Apply the herbicide in a split application (generally two-thirds early with the remaining one-third at planting).

  • Apply the herbicide closer to planting time.

In our research, we have had better and more consistent results with soil-applied herbicides that were applied within 1 to 2 weeks of planting or at planting compared to the same herbicides applied several weeks (up to 5 weeks) prior to planting. It is not reasonable to assume that all soil-applied herbicides can be applied immediately before planting due to time and equipment restraints, but fields with a significant waterhemp problem would be excellent candidates for soil-applied herbicide applications immediately before planting.

Considerations with Postemergence Herbicides

Similar to soil-applied programs, several postemergence herbicides are very effective on waterhemp. The factors governing the effectiveness of postemergence herbicides are critically important when dealing with waterhemp. Herbicide rate, application timing, and spray additive all influence how well postemergence herbicides perform against waterhemp.

Producers often like to wait as long as possible to apply postemergence herbicides, especially those that lack any significant soil residual activity, in order to have as many weeds emerged as possible. Because waterhemp can germinate and emerge for an extended period of time, a wide range of plant sizes typically exists by the time postemergence herbicides are applied, which can present problems with spray interception by smaller plants under the protective canopy of larger plants. Adjustments in spray volume and pressure can help to overcome some of the problem with coverage. Spray volumes of 20 gallons per acre with application pressures of 40 to 60 pounds per square inch generally provide a uniform coverage of the target vegetation.

The next issue of the Bulletin will contain more information about waterhemp management in corn and soybean production systems.--Aaron Hager, Loyd Wax, Marshal McGlamery

Author: Aaron Hager Marshall McGlamery Loyd Wax