Individuals of the cactus Cylindropuntia fulgida, are adapted to long periods of environmental stress with ephemeral periods of resource abundance (e.g., seasonal rains and subsequent nutrient mineralization) which requires this slow-growing cactus to act quickly to acquire nutrients. A short-term phenotypic response to varying levels of a fertilizer pulse was investigated in this species. Phenotypic plasticity could be a mechanism whereby this plant can modify its phenotype to allow for high resource intake during ephemeral nutrient pulses. The phenotype of a clone grown across fertilizer treatments was described for several morphological growth characteristics: root biomass, shoot length growth, weight increase, and number of new stem sprouts. Root concentrations of carbon (C) and nitrogen (N) were used to measure plasticity in nitrogen uptake capacity and allocation. Significant differences between clones were found in all response variables except root biomass, when each response variable was considered individually, but fertilizer treatment had an effect only on root nitrogen concentration and C:N ratio. However, when covariation among response variables was accounted for using principal components analysis, fertilizer treatment was shown to significantly impact root nitrogen allocation, above-ground growth and root biomass. In no case was a significant genotype-by-environment interaction found, indicating that there was not a diversity of responses among clones. These results demonstrate that this slow-growing species is able to respond to brief changes in the environment, which fact has important implications for the ecology and evolution of this species in a highly variable environment.