Growing conditions influence growth and development of weed plants. Each of the past several growing seasons has seen unique challenges and triumphs. In 2012, favorable soil conditions allowed for early planting, soon followed by drought conditions. In 2013, record-setting spring rains delayed planting. While in 2014, mild temperatures and timely rain events led to record-setting corn and soybean yields. What has the corn-belt experienced in 2015?: record-setting precipitation in June and July in many areas that led to flooded or ponded water in areas of many fields.

Every weather condition that our crops have experienced, have also been experienced by the weeds that compete with our crop for limited water, light, and nutrient resources. This concept was evident at one tour stop at the recent University of Illinois Department of Crop Science's Agronomy Day program in Urbana.

Weed scientist Dr. Aaron Hager and a graduate student working with Dr. Hager showcased many living examples of the various pigweed species that one might find in a typical Illinois corn or soybean field. To prepare for this demonstration, seeds had been sown into soil in various sized pots and maintained in the Crop Sciences greenhouse. An interesting observation was that the weeds appeared to grow according to the resources that were available to them – those that were planted into pots that held more soil and could support a larger root system tended to grow larger than those that were grown in smaller pots.

Later conversation revealed that numerous weed science professionals have made similar, important observations across the corn belt this year regarding both waterhemp and Palmer amaranth plants. Although it can be found growing under many different conditions in fields from Texas to Maine, waterhemp, as its common name implies, thrives in wet areas of fields. The wet conditions throughout June and July this year have helped waterhemp plants to thrive, allowing them to grow larger than they have in less wet years. Conversely, the center of origin of Palmer amaranth is the desert in the southwestern U.S. and northern Mexico. While able to survive and thrive under water-limited conditions, the mid-2015 growing season has been a little bit wetter than Palmer amaranth plants would prefer.

Although now that the reproductive structures of both Palmer amaranth and waterhemp plants can aid in identification efforts, one should not be confident in using plant height to positively identify the various pigweed species. While pictures from past, drier growing years have shown Palmer amaranth plants towering over tasseled corn plants, Palmer plants may be a lot shorter statured in 2015.

Importance of weed ID. Knowledge of with which weed species one is dealing can affect both harvest operations and future weed management decisions. It is therefore important before weeds mature and harvest operations begin to take to the field an identification guide with good pictures and descriptions and positively identify weeds. See the many resources below that can help towards that effort.

Resources

Hager, A.G. 2014. Management of Palmer amaranth in Illinois. University of Illinois Department of Crop Sciences Weed Science publication.

Hager, A.G. 2014. Remain vigilant for Palmer amaranth. Bulletin article. – has nice pictures that help distinguish Palmer from waterhemp.

Palmer amaranth ID form – if there are still healthy plants that are suspected of being Palmer amaranth you may still have time to submit a sample for ID.

Hager, A.G. 2015. A quick pigweed identification exercise. Bulletin article. – contains pictures of leaves and flower parts that can help in identifying different pigweed species.

Nordby, D., Hartzler, B., and Bradley, K. 2013. Biology and Management of Waterhemp.