Non-native plant species often colonize retired agricultural lands, creating monocultures with low species diversity that provide poor wildlife habitat. We assessed whether sowing a mix of 29 locally adapted native species reduced invasion of non-native plant species compared to allowing vegetation to colonize naturally following tillage. There was a sampling date × treatment interaction for canopy cover of perennial exotic plant species. Plots that were not sown to natives had two to six times greater canopy cover of exotic species than did plots with both preparation (woody vegetation removed, plowed, and disked) and control (no preparation or sowing) plots. Canopy cover of exotic plants was similar in prepared-only and control treatments from October 2008 to June 2010, ranging from 8 to 40%. Percent absolute canopy cover of native vegetation was 10–20 times greater on prepared and planted plots than on prepared-only plots during March 2009 to June 2010. Sowing a mix of locally adapted native species may inhibit encroachment by non-native species for up to two years after sowing on retired agricultural land in the Lower Rio Grande Valley of Texas.