Because of the varied ecosystems and biological communities of San Juan Island NHP, you'll find a diverse variety of plant life, including prairie, fir-hemlock-cedar forests, Garry oakwoodlands, thickets, intertidal areas, lagoons, and wetlands.
Prairie spans nearly half the acreage at American Camp, from the bluffs along South Beach to the south-facing slopes of Mount Finlayson. Non-native species have infested the prairie, but patches of native grasses and wildflowers still exist.
On the northern slopes of Mount Finlayson are Douglas fir, western hemlock, western red cedar, grand fir, and lodge pole pine. The understory includes evergreen salal and western sword fern.
South-facing slopes are drier, and though Douglas firs still dominate, the understory is much thinner. Other trees found here are the big leaf maple, Pacific madrone, and Pacific yew.
English Camp is dominated by mature Douglas firs and grand firs, big leaf maple, red alder, Pacific madrone, and a few western red cedars and Pacific yews. A remnant stand of open Garry woodlands remains on the south slope of Young Hill.
Because non-native plants such as thistle, tansy ragwort and blackberry are crowding out native grasses and wildflowers, efforts are underway to restore the prairie.