Azolla filiculoides Lam. causes serious weed problems in Britain, but its long-term survival might be limited by winter death. The aim of this study was to establish the low temperature responses and limitations of A. filiculoides sporophytes.

In the laboratory, normal vegetative growth was shown to continue at 5°C. Reddening of plants was a response to low temperature and high light conditions which could be prevented by shading. Adult plants died after short (18 h) exposure to −4°C but survived sub-zero temperatures >−4°C. Evidence was found of seasonal changes in chill tolerance, but not in freeze tolerance.

In outdoor culture, plants survived encasement in ice and air temperatures to −5°C. Additional evidence suggested that natural populations can readily survive air temperatures much lower than this. Microclimatic effects are likely to be responsible for this discrepancy between laboratory and outdoor culture results.

Three phenotyes were identified; survival, colonizing and mat forms.