These bright yellow flowers are called na no hana (rape blossoms/field mustard) and they’re more than just eye candy—they’re food. The stems are blanched and then drained and seasoned.
This photo is of a huge field of na no hana growing on a hill by a zoo we visited the other day, but they can be found pretty much everywhere in spring. They often grow beside train tracks along with daikon no hana, a purple flower that looks a lot like phlox to me. They’ve been brightening my spring commute to work for nine years now.