Aims Higher plants are an understudied component of the global silicon cycle; they absorb silicic acid (dSi) which is stored as biogenic silica (bSiO2). Si is believed to alleviate physical, chemical, and biological stresses such as storms, high salinity, heavy metal toxicity, grazing, and disease. We investigated a Si-accumulating invasive species growing in the tidal marshes of the Bay of Brest (France), viz., Spartina alterniflora. Our objectives were to determine (1) where and when bSiO2 accumulates in the plant during its life cycle, (2) whether this accumulation varies with abiotic factors: wave action, estuarine salinity, and duration of immersion, and (3) if the accumulation was limited by dSi availability in marsh porewater. Methods A 2 years field survey permitted to sample plants which were analyzed for there bSiO2 concentrations. Sediment cores were sampled seasonally and the dSi concentrations in the porewater were measured from 0 to 10 cm. Results bSiO2 accumulated more in mature leaves than in other organs. There was a strong linear relationship between bSiO2 concentration and plant length. bSiO2 concentrations did not increase, but rather decreased as a function of exposure to the three abiotic factors tested. dSi availability was not significantly different for each of the tested sites and dSi profiles did not exhibit huge losses in the root zone. Conclusions Our evidence suggests that dSi availability did not seem to be a limiting factor. bSiO2 did not increase with increasing abiotic stresses but was strongly correlated with growth. Hence, S. alterniflora is likely to have other adaptive strategies for dealing with environmental stressors but it did not exclude the possible role of Si in alleviating these stresses. If this is the case, there remain intriguing questions about Si uptake, its availability, and its role in silicification and growth.