My best guess would have to be a Bird of Paradise. Most likely a white Bird of Paradise. I had a white one before, and it resembled your plant.

Much of California is hotter than Hell (100s F.) in summer east of the outermost coastal fringe*, hence the chronic problem with dangerous fires. With a number of the native species - including various ceanothus - growing only close to the ocean in nature. Even a lot of the redwood area has a hot summer, with fog drip being a large part of what makes it possible for these giant trees to live there. *Death Valley is located in California

It is not knotweed......and it does look like a tree seedling (root shoot), what type I am not sure. But you will get faster and accurate ID's if you use the forum - Name That Plant - that is intended for plant identification. This is not the right place :-)

the very short varieties of landscape roses (like 12 inches or so) hardly have any "ellbow" or ability of concurrency against weeds, if you want resilient landscape roses, it is helpfull to pick a somewhat taller type