The rapeflower plant or nanohana in Japanese is a plant in the mustard family. It is tough and spreads easily.
Linda Inoki’s article has more (see below)
By LINDA INOKI
By Matsuo Basho (1644-1694),
The cheerful yellow flowers of rape plants are a sure sign that spring is here. Nanohana is the general name for several plants in this group, first introduced from Europe. Originally these were grown for the edible oil extracted from their seeds, as they still are in the West. But Japanese people appreciated their pretty flowers, so nowadays nanohana grown here are mostly intended for display as cut flowers or for consumption as spring greens.
The Japan Times: March 7, 2002