This protocol describes a quantitative analysis of the morphology of small plants from the moss Physcomitrella patens. The protocol can be used for the analysis of growth phenotypes produced by transient RNA interference or for the analysis of stable mutant plants. Information is presented to guide the investigator in the choice of vectors and basic conditions to perform transient RNA interference in moss. Detailed directions and examples for fluorescence image acquisition of small regenerating moss plants are provided. Instructions for the use of an ImageJ-based macro for quantitative morphological analysis of these plants are also provided.