The soil additive Ascophyllumnodosum (Norwegian kelp) can increase drought tolerance of plants, and promote seed germination, yet there has been no published research testing this amendment on green roofs. Liquid A. nodosum extract in two volumes and a slow release fertilizer were tested in a pot culture experiment for annuals, perennial cultivars, and three native species. Kelp extract did not improve the germination percentage of any of these species. In drought conditions both on a green roof and in the greenhouse, kelp treatments increased plant biomass relative to controls for some species only; plant health and longevity were generally improved when kelp was applied as an amendment. Soil water content was higher in kelp treated pots compared to fertilizer treated pots, and higher than controls in kelp treated pots for some species. Application of this local renewable resource to green roof plants could help to improve plant and substrate health while improving drought tolerance of plants on green roofs.