The Wood Card is not as dangerous as the other cards. It is a peaceful, calm spirit who is simply looking for friends. In Episode 4, it filled Sakura's house entirly with branches, causing her to get stuck in some places.
After capturing Rain, Wood revealed itself in it's true form, and allowed Sakura to seal it. When used as a card, Wood will appear in it's true form, then transform into a barrage of branches and trees, or it will do that automatically after being summoned.
