Warm, most and fertile is what plants in this genus like during the growing season. Some kinds may shoot from spring to fall where happy.

tjchermack,
At this time of year all Phyllostachys send out rhizomes underground to expand their growing area. Some of those rhizomes are pointed upwards and after surfacing they bend further towards vertical and transform into shoots. They are called whipshoots because they are usually thinner than regular spring shoots. This behavior is normal especially with younger plants. The problem is that these new shoots will not survive the winter. They will not have enough time to harden sufficiently. If they do not detract from the overall look of the bamboo, leave them and maybe they will survive.
Eric