Some years, no matter how diligently you pull, your backyard garden is always covered with weeds. Other years, with the minimum of effort, your garden remains weed-free. What is the cause of these oscillations? A group of weed scientists based at the Spanish National Research Council (CSIC) spent fifteen years studying flixweed a member of the mustard family commonly found in areas where the ground has been cultivated or disturbed in an attempt to identify the processes underlying these fluctuations.

"The failure to recognize the intrinsic nature of many weed population changes may result in over-application of control inputs, with subsequent negative economic and environmental effects," says Jose Gonzalez-Andujar, who co-authored the study, forthcoming in The American Naturalist, with Cesar Fernandez-Quintanilla and Luis Navarrete.

Many populations exhibit cyclic oscillations. Everybody can recall a summer where mosquitoes hindered attempts at al fresco dining. These cycles can be produced by climatic conditions or by internal feedback mechanisms. However, in contrast with studies of insect and animal populations, little attention has been directed at the study of cycles in plants. What happens with your garden weeds?

The researchers demonstrate that there are some intrinsic mechanisms that explain observed plant oscillations more specifically, evidence of cycles produced by delayed density dependence in a plant population growing under field conditions. This study can have a capital importance in crop protection.

"Traditionally, the major objective in weed management has centered on how weeds can be controlled. The emphasis on control, however, has obscured the overriding question of why weeds are so abundant at certain times and places," write the authors. "This is an ecological question that may lead to a better understanding of the agroecosystems and to the development of more sustainable agricultural systems."