I think your photos are of the Spiny-leaved Sowthistle, Sonchus asper that bears yellow flowers and has spiny leaves. The Burnweed has white disc flowers that hardly open. I don't think it becomes really fluffy full of cottony seed.

I moved it to the correct entry since sadly, jmorth is no longer with us.

Calif_Sue said:I moved it to the correct entry since sadly, jmorth is no longer with us.

Sad news.

Good! Sad to hear of that person passing on