Climbing plants utilize external structures as support for upright growth. Because climbers lack the capacity to hold themselves upright their performance in natural vegetation will be restricted by the availability of suitable supporting structures and by their efficiency in ascending these structures. In a greenhouse experiment we studied the effect of external support on height growth and biomass partitioning in three herbaceous tendril climbers and three herbaceous twiners from the temperate zone. To simulate the success and failure of encountering support one third of all plants was supplied with support directly at the start of the experiment one third remained unsupported during the whole experimental period (3 months) and the remaining third was offered support after they had attained their maximum self-supporting height (approximately half way through the experimental period). Supported plants were longer than unsupported individuals of the same species. Total plant dry weight was not affected by support availability. However unsupported plants allocated more plant mass to shoots relative to roots and invested more above-ground mass in branches relative to the main shoot than did supported plants. Furthermore unsupported plants produced a larger number of branches that presumably were foraging for external support. Overall responses were consistent in all climbers. Plasticity in allocation patterns and morphology may be interpreted as adaptations allowing herbaceous climbers to forage for external support.