The Henry Allan Gleason Nature Preserve is located in the extensive glacial sand deposits associated with the Illinois River in central Illinois. An extensive sand dune is present within the preserve on which undisturbed dry sand prairie, disturbed dry sand prairie, and blow-out communities are present. The undisturbed sand prairie is dominated by Schizachyrium scoparium (42% of the importance value [IV]), along with Tephrosia virginiana, Opuntia humifusa, and Ambrosia psilostachya. The disturbed sand prairie is dominated by Eragrostis trichodes (24% of the IV), followed by Heterotheca camporum, Ambrosia psilostachya, and Rhus aromatica. Common species in an active blow-out includes Aristida tuberculosa and Cyperus grayioides, while nearly stabilized blow-outs have a high diversity characterized by Bouteloua hirsuta, Ambrosia psilostachya, and Eragrostis trichodes. A total of 172 plant species were found: 4 fern and fern-allies, 3 gymnosperms, 39 monocots, 126 dicots. Thirty-one non-native species were found, comprising about 18% of the flora. The Floristic Quality Index for the nature preserve is 41.33 when the non-native species are included in the calculations.