Technical Abstract: Diffuse knapweed is a deep tap rooted species that was introduced to North America about 1900 and by the mid 1990s occupied 1.2 million hectares in the western United States. Diffuse knapweed invasion has reduced biodiversity, forage for livestock and wildlife, and deteriorated watersheds characteristics. This exotic species is highly invasive in rangeland plant communities, from heavily grazed areas to even those in high ecological condition. Diffuse knapweed is thought to be native to southeastern Europe, with the natural range extending to north central Ukraine. It is difficult to determine the native range because the species now occurs as an invasive weed in much of Central Europe. The objective of this study was to further define the environmental requirements for safe sites for germination of achenes (seeds) of diffuse knapweed. Germination temperature profiles were developed for diffuse knapweed seeds collected from sites in the Great Basin and Colorado. Each profile consisted of seeds germinated at 55 constant or alternating temperatures from 0 through 40 C. Some germination occurred from 71 to 96% of the temperature regimes depending on the accession being tested. Maximum observed germination ranged from 85 to 98%. Optimum germination occurred at a wide range of temperatures from cold periods of 0 through 20 C alternating with warm periods of 10 through 35 C. The temperature regimes that most frequently supported optimum germination were 5/25 C (5 C for 16 hours and 25 C for 8 hours in each 24 hour period) and 10/25 C. Germination of diffuse knapweed seeds was generally higher at alternating than constant temperatures. The only constant temperatures that supported optimum germination were 15 C (33%) and 20 C (17%). What is the ecological significance of limited germination at very low, high, or widely fluctuating temperature regimes? It has long been recognized that three basic germination strategies exist in seeds of invasive weeds: 1) simultaneous, 2) continuous, and 3) a combination of both simultaneous and continuous germination. Simultaneous germination is the first species to germinate and preempt environmental potential in the seedbed which has an advantage in subsequent competitive relations or potentially in interference for actual safe sites for germination. The risk with this strategy is the entire seedling population can be lost to drought, weed control, or predation without a seed bank to give the species a second chance. Continuous germination avoids the pitfalls of a portion of post emergence disasters at the ecological deficit, of by definitions, not completely winning the race for preemption of environmental potential in seedbeds. One method of accomplishing the combination approach is to produce seeds with the inherent potential for simultaneous germination if they find sufficient safe sites in seedbeds. Those that fail to find safe sites acquire a dormancy that breaks down in response to time and environmental stimuli producing a de facto state of continuous germination. This is the germination system found in the exotic, invasive cheatgrass (Bromus tectorum). A second way to accomplish the combination of simultaneous and continuous germination is for the plant to produce seeds with phenotypically variable germination requirements. This system is well represented by the native perennial Indian ricegrass (Achnatherum hymenoides. Continuous seedling emergence may allow diffuse knapweed to occupy all available safe sites for germination. Perhaps, diffuse knapweed accomplishes continuous germination by producing seeds with a wide variation in amplitude of temperature tolerance for germination. Give them the right temperature and they virtually all germinate, but a few can germinate at such extremes as 0/2, 35/40 or even 0/40 C.