I received a phone call yesterday from a homeowner bemoaning, “there’s corn coming up in my lawn!” I asked if the lawn was newly seeded and the caller said it was not; the lawn had been established several years ago. It’s a useful question because wheat is notorious for springing up from errant seed in straw used to cover turf seed.
However, the homeowner e-mailed some very good images today that revealed the “corn” was Johnsongrass (Sorghum halepense, family Poaceae). This coarse-bladed warm-season grass strongly resembles young corn plants and may present a weed identification challenge in turf.
Johnsongrass is a perennial grassy weed native to the Mediterranean region. It was exported worldwide primarily for erosion control and can now be found growing on every continent except Antarctica. Its common name references Alabama plantation owner William Johnson who sowed seeds on his river-bottom land sometime in the 1840s to control water erosion.
This non-native grassy weed may reach a mature height of 6 – 8'. It's a prolific seed producer with seed viability lasting more than 20 years. Plants will also spread by underground stems called rhizomes making it stubborn recurring weed. Dense colonies will outcompete preferred plants including landscape annuals and perennials as well as field crops.
Johnsongrass was long considered a problem confined to southeastern states. However, it has been gradually creeping north to present a serious weed management challenge in Ohio's agricultural fields, nurseries, and landscapes.
Seed may occasionally drift into turfgrass from surrounding infestations. The newly established Johnsongrass plants become very apparent in the spring to early summer as they grow much faster than the turfgrass plants. Thankfully, even though this weed is a heavyweight in other locations, it's a wimp in turfgrass. Johnsongrass will not survive continues mowing at turfgrass cutting heights.
Control in landscapes and nurseries is more difficult owing to long-term seed viability, underground rhizomes, and the development of herbicide-resistant biotypes. Johnsongrass biotypes have been identified that are resistant to glyphosate (e.g. Roundup), fluazifop-p-butyl (e.g. Fusilade, Ornamec, Grass B Gon), sethoxydim (e.g. Poast, Vantage), and quizalofop (e.g. Fusilade II).
There are no effective pre-emergent herbicide options for nurseries or landscapes. Post-emergent herbicides including glyphosate are effective on young plants as long as you're not dealing with a resistant biotype. Otherwise, cultivation and physical removal are the best options. Unfortunately, rouging and destroying plants may still leave behind rhizomes allowing plants to spring forth another day.