The Western Tiger Swallowtail is often found near riparian corridors as that is where all their larval plants grow. These plants include Cottonwood, Populus spp,Pacific Blackberry, Alder, Alnus spp., Willows, Salix spp., Birch, Betula spp, and California Sycamore, Platanus racemosa. These are all deciduous riparian trees.
The Western Tiger Swallowtail gets nectar from a wide variety of plants. They like Lobelia dunnii serrata a sky blue Lobelia that grows in most areas in part sun.
They also like sages like Salvia Pozo Blue. This is a sage with light green foliage and and purplish blue flowers. It is very showy and very drought tolerant.
Another flower that is very popular with butterflies including the western Tiger Swallowtail, is Woolly Blue Curls, Trichostema lanatum. This is an open shrub found in the chaparral. It has fuzzy purple flowers and is also popular with Hummingbirds and the California Dog-face butterfly.