The aluminium sulphate (Al2(SO4)3) effect in Rosa vase fluid consists of three factors: (1) at pH 5 or more a weak biostatic activity, and at pH 4 or less a weak biocidal activity towards Bacillus subtilis cells added to the vase fluid; (2) obstruction of B. subtilis cells to infiltrate (and plug) into open xylem vessels influenced by co-precipitation of Bacillus cells with Al(OH)3 and formation of a layer of precipitated Al(OH)3 on the cut surface, acting as a bacterial filtre; (3) a slightly decreased daily water uptake and Rosa bud development, slightly extending the vase life of the roses. Cryo-SEM investigations showed in addition that the Al2(SO4)3 and B. subtilis cells added to the vase fluid did not affect the xylem ultra-structural morphology of the xylem vessels of the roses within 2 days of vase life. X-ray microanalyses of the cut surface and of the xylem vessels in 20 cm height of the stems showed that Al migrates to at least 10 cm in the stem of the roses only at pH 4 or less of the vase fluid.