University of Illinois

No. 4/April 18, 1997

Waterhemp--Biology of a Troublesome Species

Few weed species have garnered so much attention over the past few years as waterhemp has. Waterhemp has gone from obscurity just a few years ago to widespread distinction at the present time. How has this problem developed? What factors have contributed to waterhemp's rise to prevalence? In this issue of the Bulletin, we'll cover waterhemp identification, biology, and some of the factors responsible for the increased severity of waterhemp infestations across much of Illinois. A second article related to waterhemp will appear in the next issue of this Bulletin.

Identification. There are actually two species of waterhemp in Illinois: common (Amaranthus rudis) and tall (A. tuberculatus). These are distinctly separate species, yet it is not possible to differentiate between them while they are in the vegetative stage. Only when the plants have produced seeds can tall and common waterhemp be differentiated. Is there a need to be able to identify tall from common? To our knowledge, no data exists to suggest that these two similar, yet distinct, species respond any differently to soil-applied or postemergence herbicides.

Similar to the other pigweed species in Illinois (there are nine pigweed species commonly encountered in Illinois), waterhemps have notched leaf tips and reddish colored roots. When pigweeds are small, it can be very difficult to identify accurately which species you actually have. Waterhemps generally have no hairs on the stem or leaf surface and so feel fairly smooth to the touch (Fig. 3). Redroot and smooth pigweeds, the more familiar pigweed species, have small, fine hairs that cover the stem and leaf surfaces (Fig. 4). These species feel rough when you run your finger along the stem or leaf surface.

Figure 3

Figure 4

The leaves of waterhemp are typically glossy and often more elongated or lanceolate (Fig. 5) compared to redroot or smooth pigweed. Stem color of more mature waterhemp plants varies greatly; most are a shade of green, but others can be some shade of red. Plants with green stems and red petioles or red stems with green petioles can be found (Fig. 6).

Figure 5

Figure 6

For further information on pigweed identification, consult an excellent guide developed jointly 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 guide contains color photographs of the various pigweed species, as well as brief descriptions of vegetative and reproductive characteristics.

Biology. Both common and tall waterhemp are species native to Illinois. In years past, these species have been prevalent primarily south of Interstate 70, but during 1996 waterhemp could be identified growing about as far north in Illinois as Interstate 80.

Waterhemp plants are either male or female (dioecious). Male plants produce only pollen while female plants produce only seed. In contrast, redroot and smooth pigweeds have both male and female reproductive structures on the same plant (monoecious). What is the significance of this? Dioecious species are cross-pollinated, meaning that pollen from more than one source can fertilize a female plant. Cross-pollination can greatly increase the genetic diversity of a population, and with genetic diversity comes a great range in physical appearance and other biological characteristics.

Like other pigweed species, waterhemps are annuals that produce an abundance of very small seeds. These small seeds are ideally suited for shallow germination (likely less than 1/4 inch). High seed production and small seeds make waterhemp well-adapted to reduced tillage systems, in which the soil is not disturbed and the seeds remain near the soil surface.

Seed germination and plant emergence of waterhemp often extends later into the growing season than for most other annual weed species encountered in agricultural systems. The extended germination and emergence present the potential for several flushes of waterhemp during the course of a growing season. Soil-applied herbicides can provide effective control of earlier season flushes but may not persist long enough to control later-emerging waterhemp. Conversely, postemergence herbicides are most effective when waterhemp are small, but few provide sufficient residual activity to prevent future emergence.

Why the increase in waterhemp? Many producers who have been confronted with waterhemp have asked why this weed has increased so rapidly and extensively. From our perspective, there does not appear to be one single reason for this spread. Rather, several factors appear to be responsible and many of these likely work in concert.

1. Increased adoption of no-till production systems. Why would no till systems tend to favor species such as waterhemp? Three features of no-till systems likely contribute:

a. Lack of tillage allows the small seeds of waterhemp to remain near the soil surface, a zone ideal for their germination. Placement of waterhemp seeds in the soil deeper than about 1/4 inch, would likely decrease emergence.

b. Soil-applied herbicides applied several weeks prior to planting often do not provide adequate residual activity to control later-emerging flushes of waterhemp. Early preplant applications are often made to increase the likelihood of receiving sufficient precipitation to incorporate the herbicide, as well as to extend applicator workload. Generally, however, the earlier a herbicide is applied, the earlier within the growing season weed control begins to decrease.

c. Strict adherence to no-till eliminates the option for mechanical cultivation to control waterhemp. Without the option to cultivate, you may find that waterhemp escapes remain uncontrolled or additional postemergence herbicides are necessary.

2. Reduced use of effective, soil-applied herbicides; more postemergence applications. Many older, soil-applied herbicides possess substantial activity on small-seeded broadleaf species such as lambsquarters and pigweeds. When broadcast applications of soil-applied herbicides were used in conjunction with one or more cultivations, pigweeds of any species only occasionally escaped control and developed into substantial problems. However, with the reduced use of soil-applied and incorporated herbicides that control grasses and small-seeded broadleaves, pigweeds have escaped control more frequently in recent years. Many of today's postemergence herbicides control a wide spectrum of weeds, but few have adequate residual activity to control weeds, such as waterhemp, that may emerge several days or weeks following application.

3. Variable response to herbicides. Biotypes of waterhemp have been identified in Illinois that are resistant to herbicides with particular modes of actions. In particular, biotypes of waterhemp that are resistant to ALS-inhibiting herbicides, in addition to biotypes resistant to triazine herbicides, have been identified. Which type of resistant biotype is more prevalent? It appears that ALS-resistant biotypes are more widespread than biotypes resistant to triazine herbicides. Is every waterhemp plant encountered resistant to some herbicide? Although there are confirmed instances of waterhemp biotypes that are resistant to ALS or triazine herbicides, by no stretch of the imagination is every waterhemp plant resistant to some herbicide. Waterhemps vary widely in appearance and also apparently vary quite a bit in their response to herbicides. This variation in response can range from highly susceptible to very resistant. The instance of resistant waterhemp does, however, appear to be widespread enough to warrant good herbicide management practices that may slow the further development of resistant weeds, such as rotating herbicide modes of action and tank-mixing herbicides with different modes of action.

In the next issue of this Bulletin, we'll present some principles of waterhemp control, as well as some specific herbicide recommendations for corn and soybeans.

For those interested in obtaining further information on waterhemp, a color brochure has been developed through a collaborative effort between the University of Illinois and the USDA/ARS. Waterhemp Management in Agronomic Crops (publication number X855) is available for purchase ($2 per copy) from Vocational Agricultural Service, Information Services, College of Agricultural, Consumer and Environmental Sciences, University of Illinois, 1401 S. Maryland Dr., Urbana, IL 61801; fax, (217)333-0005; phone (217)333-3871.

Aaron Hager, Loyd Wax, and Marshal McGlamery, Department of Crop Sciences, (217)333-4424