Common Ragweed (Ambrosia artemisiifolia) Growth as Affected by Plant Density and Clipping

During the past century, common ragweed has spread from its native eastern North America to Europe, where it has become an increasing problem from both an agricultural and a human health perspective. Two field experiments were performed over a 2-yr period in a naturally infested fallow field in northern Italy to evaluate the effects of common ragweed plant density on its growth dynamics and to study its response to clipping. In the first experiment, three plant densities were tested (4, 12.5, and 25 plants m(-2)) and plant height, aboveground biomass, and leaf area were assessed. Intraspecific competition had a substantial negative effect on leaf area and aboveground biomass on a per plant basis in both years, but did not affect plant height. However, the high-density (25 plants m(-2)) treatment resulted in the highest total aboveground biomass (1,428 and 4,377 g m(-2)) and leaf area index (5.6 and 12.6 m(2) m(-2)) in 2006 and 2007, respectively. In the second experiment, common ragweed plants were clipped at reaching 20 cm (four clippings during the season), 50 cm (three clippings), or 80 cm (two clippings) plant height. Number of surviving plants, flowering plants, and aboveground biomass were assessed before each clipping. Clipping resulted in a partial reduction in the surviving plants and did not prevent flowering. Under the most stressing condition (clipping at 20 cm height), more than 67% of plants survived to the last clipping and, among these, more than 97% flowered, whereas before the last clipping at reaching 80 cm height from 50 to 100% of plants survived and 100% of them flowered. These findings in northern Italy confirm that common ragweed is a fast-growing annual species, capable of producing considerable aboveground biomass at various pure stand densities and that plants can still flower from plants clipped at various frequencies.