BIOLOGICAL WEED CONTROL VIA NUTRIENT COMPETITION: POTASSIUM LIMITATION OF DANDELIONS

Abstract

Weedy plants are often controlled by the application of herbicides. Here we explore an alternative method of control. We suggest that the abundance of an undesired plant species (here dandelions: Taraxacum officinale) may be controlled by modifying interspecific competition via changes in resource supply rates. This hypothesis is supported by several lines of evidence. First, analyses of effects of different patterns of fertilization on plant-species abundances in the 140-yr-old Park Grass Experiment at Rothamsted, England, show that Taraxacum abundances were highly dependent on potassium fertilization and on liming, but not on addition of other nutrients. Potassium fertilization led to a 17- to 20-fold increase in Taraxacum abundances in the classical Park Grass data, and to a 4- to 7-fold increase in the modern data. Liming led to a 2- to 3-fold increase for classical data and to a 3- to 4-fold increase for modern data. Second, in a greenhouse study in Minnesota, Taraxacum had a higher requirement for potassium and had its biomass more limited by potassium than any of five common grass species of Park Grass. This suggests that Taraxacum may be a poorer competitor for potassium than these grasses, but this mechanism has not yet been tested. Third, in a series of Minnesota lawns that had not received fertilizer or herbicides, both Taraxacum density and abundance were significantly positively correlated with its tissue potassium levels. This demonstration that desired and weedy plant species can differ in their resource requirements suggests that adjustments in resource supply rates may determine the outcome of interspecific competition, allowing desired species to competitively control weedy species. In particular, for soils with low potassium levels, the use of potassium-free lawn fertilizer is predicted to decrease Taraxacum because of competition from grasses like Festuca rubra.