Iowa State University

Weeds in the Concrete Jungle
by Bob Hartzler

March 10, 2008 - Seeds of many weeds are equipped with mechanisms that enable long-distance dispersal, such as the pappus (feathery appendage) on dandelion and horseweed seed. These mechanisms are not as important for agricultural weeds as they once were due to the efficiency of farm equipment (especially combines) at dispersing weed seed over relatively long distances. However, the rapid spread of glyphosate resistant horseweed (marestail) across the mid-Atlantic region and Cornbelt illustrates the benefit of seed dispersal mechanisms in agroecosystems.

Weeds also are found in urban settings, a habitat that differs considerably from the Iowa countryside. Hawksbeard (Crepis sancta) is a winter annual in the sunflower family that produces two types of seed: big, heavy ones that drop straight to the ground and light, wind-blown seeds. Researchers in France reported that hawksbeard introduced into an urban environment rapidly evolved to produce a higher percentage of heavy seeds than plants grown in rural areas (Cheptou et al. 2008). The advantage for the large seed is that in a city, wind-blown seed are likely to fall on uninhabitable areas (e.g. concrete streets or parking lots). While a shift to non-windblown seed provides a short-term benefit for the plant in that the large seeds are likely to fall onto the patch of soil that the mother plant grew in, it leads to genetic isolation (inbreeding) of the plants. This isolation may make the population vulnerable to future changes in the environment due to the lack of diversity within the population.

In order to survive, weeds continually adapt to the environment they encounter. This is why it is critical to use integrated weed management systems in order to reduce the likelihood that weeds will develop mechanisms that allow them to survive the 'hot' weed management tool of the time.

Cheptou, P,O., O. Carrue, S. Rouifed, and A. Cantarel. 2008. Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta. PNAS http://www.pnas.org/cgi/content/abstract/0708446105v1

Prepared by Bob Hartzler , extension weed management specialist, Department of Agronomy, Iowa State University