The Blackland Prairie of east-central Texas, the southernmost tip of the once vast North American Tallgrass Prairie, now covers only 1% of its original 6.8 million hectares. Thus, there is considerable interest in restoration and re-establishment of this endangered ecosystem. In a randomized complete block field experiment conducted near San Marcos, Texas, we tested the effectiveness of the post-emergent herbicide imazapic on weed control, establishment, and growth of four C4 warm-season grasses native to the Blackland Prairie (Andropogon gerardii, Sorghastrum nutans, Schizachyrium scoparium, and Bouteloua curtipendula). Treatment with three different levels of imazapic (0.07, 0.092, and 0.138 kg ai/ha) all significantly increased seedling density of native grasses, and low-to-medium levels of imazapic increased seedling shoot growth and flowering relative to water controls. End-of-season aboveground biomass of native grasses also increased with imazapic treatment while that of broadleaf species decreased. However, shoot biomass of exotic C4 grasses (principally Bothriochloa ischaemum and Dichanthium sericeum), the dominant species of this site prior to study, was not significantly affected by imazapic treatment. Despite vigorous site preparation and pretreatment with a broad-spectrum herbicide (Roundup®), these exotic C4 bluestem grasses continued to dominate all treatment plots. These findings indicate that while imazapic is beneficial for native warm season grass establishment, it is inadequate to effectively control these exotic bluestems, which are abundant on many sites once occupied by Blackland Prairie. Successful restoration of native grasses in the Blackland Prairie will, therefore, require more effective means of controlling or eliminating these aggressive exotic species.
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Vol. 31 • No. 3