Weed Densities in Perennial Flower Mixtures Cropped for Greater Arable Biodiversity

The cropping of perennial wildflower mixtures to produce biomass for use in biogas plants is one option for breaking maize’s dominance as a bioenergy feedstock. The aim of this study was to investigate the development of weeds in commercially produced perennial wildflower mixtures. Weed control during the establishment of perennial wildflower mixtures is very difficult to manage and raises the issue of the development of new weed problems when cropping perennial wildflower mixtures. In a three-year field experiment with staggered starts in four sites in northeast Germany, a perennial crop mixture was sown each year. The weeds and sown crops were surveyed annually in June. Plant densities and species numbers were counted and diversity indices calculated. Data were analysed using linear mixed models. Across all sites, weed density decreased significantly over the years of use (=cropping seasons), while weed species richness in the perennial mixture remained unchanged. The sown crop species richness significantly decreased, while the sown crop densities increased with cropping seasons. Weed density did not increase at any of the experimental sites. Although weed densities were high and crop establishment was poor in the first growing seasons, the perennial mix was able to suppress weeds in the following growing season. It was concluded that the cropping of perennial flower mixtures could contribute to biodiversity without causing new weed problems.