'Phyla canescens' (Kunth) Greene (Verbenaceae) is an introduced invasive herb of floodplain pastures and wetlands in the Murray-Darling Basin. Its germination response to 81 constant and alternating temperatures was studied using a two-way thermogradient plate. The experiment was repeated for seed aged four months and 16 months, as well as seed collected from a population in another catchment. Germination prediction surfaces were generated using Geographical Information Systems-based techniques. All seed batches exhibited a 'homothermophobic' response, in which virtually no seed germinated at constant temperatures. The ecological significance of the homothermophobic germination response is discussed.