Collingsworth, P.D., R.A. Oster, C.W. Hickey, R.C. Heidinger and C.C. Kohler. 2009. Factors affecting water willow establishment in a large reservoir. Lake Reserv. Manage. 25:200–207.We evaluated the transplanting potential of an emergent macrophyte, water willow (Justicia americana), into Rend Lake, a large, relatively unvegetated reservoir in southern Illinois. We transplanted water willow over two years into sites varying in physical conditions and using four propagule types. Transplanting in 2001 was performed to compare the durability of root crowns, plugs, and stem fragments, as well as to measure the effect of open water versus backwater sites on water willow survival. Water willow was transplanted in 2002 to explore how different planting protocols, such as planting in different months, planting different propagule types, and shoreline slope affect water willow survival. We found that water willow transplanted early in the summer had better post-winter colony survival and vigor (stem density within colonies) than transplants conducted later in the summer (Kruskal-Wallis test; P = 0.017). Rooted propagules had significantly greater first-year colony survival and ultimately exhibited greater vigor in newly established colonies. Lastly, we found that shoreline slope had a significant influence on the vigor of colonies following winter dormancy but was not significantly related to first-year colony vigor. In the summer following their initial transplanting, established water willow colonies planted on shorelines with steeper slopes (>5°) produced more plants than those planted on shallower shorelines (Kruskal-Wallis test; P = 0.003). Water willow seems to have high potential as a transplant species for reservoir management and riparian zone enhancement.