Longevity of effectiveness of stem-injected phosphite against Phytophthora cinnamomi was determined in two ways. Injected and not injected Banksia grandis and Eucalyptus marginata were challenge inoculated with P. cinnamomi at different times over an 8-year period. The mortality of injected and not injected B. attenuata and B. grandis was monitored over a 16-year period on active disease fronts. Duration of phosphite effectiveness was determined from the time after stem injection to 50% inhibition of either lesion length or girdling following challenge inoculation with P. cinnamomi and time to 50% mortality following injection of trees in disease fronts. For both B. grandis and E. marginata, time to 50% inhibition of lesion length increased from 4–5 years for 50 g phosphite/L to at least 10 years for 200 g phosphite/L. Time to 50% inhibition of girdling was significantly less than that for lesion length. In B. grandis, time to 50% inhibition of girdling increased with phosphite concentration from 2.9 years for 50 g phosphite/L to 10 years for 200 g phosphite/L. Time to 50% inhibition of girdling in E. marginata was 4 years. Predicted response surfaces of the effect of phosphite concentration and time on inhibition of lesion length and girdling showed that there was a much more gradual decline in predicted inhibition in E. marginata than the more susceptible B. grandis. An injection of 100 g phosphite/L of Banksia trees on disease fronts delayed time to 50% mortality by 3 to 4.8 years (mean 4.1 ± 0.6 years). At least 4 years of effectiveness of phosphite against P. cinnamomi in native communities is much greater than the ≤2 years found in agriculture. The effects of phosphate competition and adaptation to infertile environments on longevity of effectiveness of phosphite are discussed. Demonstration that the effectiveness of an injection of phosphite against P. cinnamomi will last at least 4 years in native flora confirms the practical utility of using phosphite injection for the control of P. cinnamomi in threatened communities. A greater understanding than available at present of the differences in phosphite uptake, within-plant distribution, loss and effectiveness between targeted plant species can optimise current phosphite prescriptions.