The physical properties of aquatic sediments may affect both the ability of aquatic plants to establish and their probability of being dislodged subsequently by hydraulic forces. We investigated the influence of sediment cohesion on the survival and root establishment of newly germinated seedlings of the rare submerged aquatic plant Najas marina. Germinated seeds were placed on three submerged substrates of different cohesive strength. Survival was high (55–65%) in all treatments for 4 weeks and there was no effect of sediment type on seedling survival. Root establishment (penetration of the sediment) was greatest on the sediment of lowest cohesive strength and declined substantially with increasing cohesive strength. Sediment type did not alter the allocation of resources to roots and shoots (root mass ratio). Although seedlings of N. marina may survive without being anchored to the substrate in experimental conditions, they would be unlikely to survive in the wild as free-floating plants. Soft, weak sediments generally predispose macrophytes to uprooting and loss but, in contrast, seedlings of N. marina are unable to become established on firm sediments. Thus, the cohesive strength of sediments could be important in determining its colonisation and distribution, both within and between lakes.