Recently, the weed science program at the University of Illinois published recommendations for the management of Palmer amaranth in Illinois agronomic crops. One particular recommendation emphasizes the need for timely application of foliar-applied herbicides; in particular, foliar-applied herbicides must be applied before Palmer amaranth plants are taller than four inches. Most weed management practitioners are very familiar with the growth rates of waterhemp and many other weed species, but perhaps are less familiar with the growth rate of Palmer amaranth.
Cody Evans, a weed science graduate student at the University of Illinois, initiated work in the greenhouse to compare the growth rate of waterhemp and Palmer amaranth. This was not a replicated experiment, but simply a demonstration of the relative growth rate differences between these two species. Both species emerged on the same day, and the plants were photographed every other day for a month. As the photographs illustrate, Palmer amaranth plants reached a 4-inch height less than 10 days after emergence.
The growth rate comparison illustrated in the accompanying photographs was conducted under greenhouse conditions, but experience suggests a similar growth rate of Palmer amaranth should be expected under field conditions. Timely applications of foliar-applied herbicides will require vigilant scouting throughout a large portion of the growing season.