PPDL Picture of the Week

March 5, 2018

Waterhemp

Joe Ikley, Weed Science Program Specialist, Department of Botany and Plant Pathology, Purdue University

Even though the calendar still says it is winter, our spring growing season is just around the corner. One of our top weeds we battle in corn and soybean across Indiana is waterhemp (Amaranthus tuberculatus (= A. rudis)). This plant is one of our most problematic weeds in the pigweed (Amaranthus) family. Other weedy species in this family that we find in Indiana are Palmer amaranth, redroot pigweed, and spiny amaranth.

Waterhemp can begin germinating in early to mid-April across most of Indiana, and will also germinate well into September. During the growing season, waterhemp can also grow as fast as 1 inch per day. Waterhemp is a dioecious summer annual weed that is also a prolific seed producer. In direct competition with soybean, an individual female plant can produce upwards of 200,000 seed per plant. Larger plants without competition, such as those that can thrive in drowned-out areas of a field, can produce 1,000,000 seed per plant. Due to high seed production, a few escaped plants per acre can turn into a major weed problem in the future.

           Waterhemp is known to be resistant to multiple herbicide modes of action. In Indiana, we have known populations with resistance to glyphosate (group 9), ALS-inhibiting (group 2) herbicides, and PPO-inhibiting (group 14) herbicides. Several populations have resistance to all three modes of action within individual plants. Populations with this stacked resistance are difficult to control in soybean, and will require a trait other than Roundup Ready or non-GMO soybean for postemergence control. This summer Purdue Weed Science will once again be offering herbicide-resistance screening for waterhemp plants across Indiana.

More information on biology and management of waterhemp can be found on the Take Action waterhemp factsheet: http://iwilltakeaction.com/uploads/files/54403-01-ta-factsheet-waterhemp-update-lr_1.pdf​   

​Click image to enlarge

Figure 1. Waterhemp and spring-emerging horseweed in a no-till field in late April.

Figure 2.          A susceptible vs. resistant waterhemp plant after application of glyphosate.

Figure 3. Waterhemp can grow over 10 feet tall and produce 1 million seed with no direct competition.