Sugarcane is a vegetatively propagated crop and hence the production of seed and its fate in the environment has not been studied. The recent development of genetically modified sugarcane, with the aim of commercial production, requires a research effort to understand sugarcane reproductive biology. This study contributes to this understanding by defining the abiotic limits for sugarcane seed germination. Using seed from multiple genetic crosses, germination was measured under different light regimes (light and dark), temperatures (from 18 °C to 42 °C) and water potentials (from 0 MPa to −1 MPa); cardinal temperatures and base water potential of germination were estimated based on the rates of germination. We found that sugarcane seed could germinate over a broad range of temperatures (from 11 °C to 42 °C) with optima ranging from 27 °C to 36 °C depending on source of seed. Water potentials below −0.5 MPa halved the proportion of seed that germinated. By comparing these limits to the environmental conditions in areas where sugarcane grows and has the potential to produce seed, water, but not temperature, will be the main limiting factor for germination. This new information can be taken into account when evaluating any risk of weediness during the assessment of GM sugarcane.