The one thing that you should never do is pick sakura petals or branches. There are several reasons for this.
Common SensePopular hanami spots are filled with thousands of people night after night. If everyone picked the flowers they'd be gone quickly. Everyone hopes that hanami will last as lost as possible.
Cultural and Religious Significance of SakuraSakura are deeply engrained in Japanese culture.
According to Japanese Buddhist traditions, falling sakura petals represent the impermanence of life. Sakura petals only live a week. They bloom brilliantly and fall with the wind. The beauty of falling sakura has been the topic of countless Japanese poems and songs.
For hundreds of years, sakura petals have also symbolized warriors. Samurai ideals stated that a warrior should live passionately and die young. This was symbolized by sakura petals.
According to Japanese cultural traditions, falling sakura petals represent the reincarnated souls of warriors who fell in battle.
Likely ReactionIf you pick a sakura petal in Japan people will likely feel uncomfortable but probably won't say anything. If you pick a branch (even a small one) people will likely speak up.
HistoryIn the Edo-era people did pick Sakura as part of hanami celebrations. This died out some time in the Meiji-era (1868 ~ 1912).