Waterhemp is a weed indigenous to the midwestern United States and is problematic in agronomic crop production. This weed is well suited to inhabit minimally tilled environments and is increasing in prevalence across many agricultural production areas and systems. A common garden experiment was established in Indiana in 2014 and 2015 with waterhemp populations from Indiana, Illinois, Missouri, Iowa, and Nebraska to compare the growth and development of waterhemp from these regions. Three establishment dates (May, June, and July) were used each year to simulate discontinuous germination. Mean biomass accumulations from the May (1,120 g plant⁻¹) and June (1,069 g plant⁻¹) establishment dates were higher than from the July (266 g plant⁻¹) establishment date. There were no differences in biomass accumulations between the five populations in the May and June establishments, but biomass accumulations ranged from 195 to 338 g plant⁻¹ in the July establishment. Mean seed yields were higher from the May (926,629 seeds plant⁻¹) and June (828,905 seeds plant⁻¹) establishment dates compared with the July (276,258 seeds plant⁻¹) establishment. In the May and June establishments, seed yields ranged from 469,939 seeds plant⁻¹ to 1,285,556 seeds plant⁻¹. The Illinois population flowered the latest of all the populations yet also grew the tallest. The July establishment flowered the most rapidly after establishment, accumulated less biomass, and also had the largest seeds. This study demonstrated differences among waterhemp populations when grown in a common environment and the effect of establishment timing on waterhemp growth and development.

Nomenclature: Amaranthus tuberculatus (Moq.) Sauer (syn. rudis) AMATA