Iowa State University

Attack of the Killer Weeds,
or The Effects of Rising CO2 on Weeds 
by Bob Hartzler

July 31, 2007 - Just when I was thinking I had the upper hand in the perpetual battle against undesirable plants, I read that changes in the atmosphere may provide two of the more dreaded weed species an upper hand. It has long been known that an increase in atmospheric CO2 may benefit certain plants since current atmospheric CO2 concentrations are below optimum levels for plant growth. Two papers in the recent issue of Weed Science discuss possible effects of increasing CO2 on poison ivy and dandelion:

• Ziska, L.H., R.C. Sicher, K. George and J.E. Mohan. 2007. Rising atmospheric CO2 and potential impacts on the growth of poison ivy. Weed Sci. 55:288-292.
• McPeek, R.M. and X. Wang. 2007. Reproduction of dandelion in a higher CO2 environment. Weed Sci. 55:334-340.

Poison ivy is a common weed of woodland edges best known for the toxic oil (urushiol) that causes severe rashes in persons that come in contact with the plant. Ziska et al. (2007) reported that recent changes in atmospheric CO2 concentrations enhance the growth of poison ivy, and that further increases in CO2 will continue to promote growth. An earlier study at Duke reported similar increases in poison ivy vigor due to changes in CO2, and the Duke study found that the changes in CO2 also increased the toxicity of the poison ivy. Although the Ziska study did not find a change in relative toxicity of the poison ivy, they reported that more urushiol would be produced due to an increase in poison ivy biomass. Poison ivy is an understory vine typically found on the edges of wooded areas. Woody vines such as poison ivy are expected to benefit more from increases in CO2 than the plants it competes with since it puts a higher percentage of its resources into leaf production rather than into a self-supporting stem.

Dandelion is the bane of many homeowners striving for the perfect lawn, and an increasing problem in agricultural fields. Earlier studies have shown that dandelion responds favorably to increases in atmospheric CO2. McPeek and Wang (2007) report that elevated concentrations of CO2 resulted in a 32% increase dandelion seed production. Not only were more seed produced, but the pappi (the feathery attachment on the seed that facilitates wind dispersal) were altered in a way that the authors speculate would allow the seed to be carried longer distances. Other research has shown that dandelion is better adapted to take advantage of increases in CO2 than other plants, thus changes in the atmosphere would provide dandelion a competitive advantage.

It is difficult to predict how changes in the atmospheric CO2 concentration will influence plant communities since so many other factors (temperature, moisture, growth of competing plants) will be impacted. However, one trait of weeds is the ability to adapt to change (this is how they survive our efforts to kill them), thus it should be expected that many of our weed species will acclimate to the changes in climate better than other plants in the landscape.

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