Sulfur cinquefoil, a perennial invasive weed of many different habitats in the United States, reproduces and spreads predominately through seed production, making seed bank survival and successful germination essential steps in the invasive process. To evaluate its potential to invade water-stressed environments, field and growth chamber studies were conducted in two areas of sulfur cinquefoil seed ecology: (1) monitoring the seed bank population and (2) determining the effect of salt and water stresses on germination of two sulfur cinquefoil seed populations differing in site characteristics of precipitation, temperature, and soil conditions. Field sampling demonstrated that a 2-yr average seed rain of 73,700 seeds m−2 resulted in an average 26,600 seeds m−2 annual increase in the seed bank. Regression analysis showed seed rain accounted for only 7 and < 1% of the variation in seedling density in 2003 and 2004, respectively. Seeds from the two populations showed a difference in the level of decrease in germination in response to increasing water and salt stresses. Managers should be aware that some populations of sulfur cinquefoil may be able to survive under drier or saltier conditions than previously thought. It appears that several years of control of mature sulfur cinquefoil would dramatically reduce the sulfur cinquefoil seed bank.