When you see a plant boll hung under the eaves, it is a sign of sake. It is usually brown but sometimes green. This ball, made from cedar leaves, is called sugi-dama (杉玉), literally means a cedar ball, and its color represents the matureness of sake.
Originally, sake was stocked in a barrel made from cedar tree and sake makers made a ball with cedar leaves. When the sake is freshly stored in the barrel in winter, the ball is still green. And as sake gets mature, the cedar ball turns to brown. However, now days less pubs hang a cedar ball to show the matureness of sake. It is used just as an ornament to add an authentic atmosphere.
So, when you come to Japan in winter and find a fresh green cedar ball under eaves, you are very lucky! And perhaps, that pub is authentic and would serve good sake, as green cedar ball suggests that pub keeps tradition to replace a cedar ball every winter.