Yellow Violet Viola glabela
The yellow violet shown above is probably a stream violet (Viola glabella). It is abundant in the Native Plant Garden and other moist places in the park, where it spreads to form extensive patches. Several other yellow-flowered violet species are native to the Willamette Valley, but most have leaves that are divided into segments or are otherwise distinctive. The leaves of V. glabella are entire and heart shaped, with pointed tips. Thanks to Rachel Foster for image.
Another native species coming into bloom now in the Native Plant Garden is Oregon Grape (Berberis aquifolium), which we featured last year and is pictured below.