Effectiveness of summer mowing for weed control in plantings of native C4 prairie grasses

Land managers often face seemingly conflicting needs to use land for species conservation or to produce economically valuable goods. Diverse prairie plantings harvested for biomass offer an opportunity to achieve both goals. Persistence of these disturbance-adapted prairie communities requires management strategies that promote desirable native plant diversity and reduce abundance of undesirable weedy species. This is sometimes achieved through prescribed burning or mowing, though mowing effects have not been well studied. To address this knowledge gap, I compared two summer mowing treatments to a no-mowing control in experimental prairies on both loamy and sandy soils for two years. The communities examined contained mature (4- and 5-yr-old) prairie grasses (Andropogon gerardii Vitman, big bluestem, and Schizachyrium scoparium Michx., little bluestem), as well as young colonizers of both desirable and weedy species. Overall, I found that June mowing reduced weed biomass and seed production on loamy soil. Biomass of native grass colonizers was greatest in mowed blocks, suggesting that mowing stimulated native colonization. However, these benefits of mowing came at the cost of some reduction in the size and reproductive output of mature prairie grasses. June and July mowing substantially reduced size and flowering of native grasses without additional weed control benefits. I therefore suggest that June mowing could be a useful component of adaptive management in tallgrass prairie in the Great Lakes region, with careful consideration of effects on summer-nesting bird species, but I advise against mowing in both June and July.