The varied ecosystems and biological communities of San Juan Island National Historical Park support a diverse variety of plant life. Prairie, fir-hemlock-cedar forests, Garry oak woodlands, thickets, intertidal, lagoon and wetlands are plant communities encountered in the park.

The final retreat of the glaciers of the last ice age beginning about 13,000 years ago created a landscape ready for plant pioneers. Prehistoric peoples probably helped maintain the prairies with fire to enhance the growth of camas, an important part of their diet.

Euro-American settlement, which began in the 1850s, brought accelerated change to the Island’s plant life. Old growth forests were logged to feed the lime kilns. over 100 species of non-native plants were introduced by accident or for utilitarian or aesthetic purposes. Today both native and non-native plants vie for the necessities of survival: space, nutrients and moisture.