Polystichum andersonii is an allotetraploid (D. H. Wagner 1979); its diploid parents are P . munitum and P . kwakiutlii . The triploid cross, P . munitum × andersonii , has been analyzed cytologically (W. H. Wagner Jr. 1973). It is the only sterile hybrid in the genus that develops large colonies through vegetative propagation by its bulblets. Hybrids look very much like some of the more deeply incised forms of Polystichum munitum except that they have abundant filiform scales, abortive sori, and nearly triangular lowermost pinnae with ± equally incised acroscopic and basiscopic auricles.