One of the most widely established invasive alien plant species in Sweden is the Japanese knotweed. A range of control measures exist, and there is great need of methods to evaluate their effectiveness. No single method currently guarantees eradication of the Japanese knotweed, hence there is considerable interest to develop more efficient techniques. One is heat treatment using steam or hot water. The effects of heat on above-ground biomass are easy to evaluate but it has proven to be more problematic to evaluate vitality of below-ground rhizomes, which can stay dormant under unfavorable conditions. We have therefore evaluated rhizome vitality using combined greenhouse trials and fluorescence microscopy observations with the vital stains fluorescence diacetate (FDA) and propidium iodide (PI). Fluorescence diacetate was effective in separating live rhizomes from rhizomes that were autoclaved at 121°C. However, while propidium iodide is routinely used to identify dead cells by staining their nuclei, unexpectedly few nuclei were stained in the heat-treated rhizomes. When temperature was reduced to 80°C for 1 hour, FDA indicated some live meristems. However, at 80°C for longer periods, no fluorescence was visible which indicated dead rhizomes. This shows the importance of treatment longevity in order to terminate persistent rhizomes of Japanese knotweed. Type of presentation: Flash talk.