Our stand of black bamboo that has flowered is dying back. It was already escaping its barrier, but now I am concerned about the flowers producing seed that take hold everywhere. When we cut it down, should we be careful about not letting seeds scatter? I’ve heard that when one bamboo flowers, it coincides with other bamboos flowering. Is there going to be a massive die-off of bamboo?
You can certainly lay out a tarp for your cut bamboo, its flowers, and any potential seeds. If you are curious about seed viability, you can put some in containers and wait to see if they germinate. Some bamboo species have larger seeds that are easier to see, while others are small and easily obscured by decaying flower parts. Seeds collected before they are mature are unlikely to germinate. Based on all of these details, I don’t think your Phyllostachys nigra will be sowing itself all over the garden or the compost pile.
According to The Gardener’s Guide to Growing Temperate Bamboos (Michael Bell, Timber Press, 2000), flowering may be partial or complete. With partial flowering (which sometimes precedes full flowering), some culms will keep on going and not die. “When a bamboo flowers completely, most of the leaves are replaced by flowers, transpiration is largely interrupted, and this triggers natural responses that hasten the aging of the culm,” eventually resulting in death.
Depending on the species of bamboo, flowering is a fairly infrequent occurrence, and there are multiple theories about what prompts it. It can happen once every 30-60 years up to intervals of over 120 years. (There are just a few unusual species that flower yearly.) Bell says anecdotal accounts suggest that bamboos rarely if ever set seeds but, in his experience, “it is very rare that bamboos flowering in earnest do not set some seed during one of the years of their flowering cycle.” Clumping (sympodial) bamboo species will flower in winter and produce seed in spring, while running (monopodial) species like your black bamboo will flower in summer and produce seed in fall.
You mention the phenomenon of many bamboos flowering in unison. This is sometimes called mass synchronous flowering, or gregarious flowering, and can occur across the globe. According to Bamboo by Robert Austin and Koichiro Ueda (Weatherhill, 1970), “practically every bamboo of the same species, young or old and however widely separated they may be […] will flower in or about the same year.” Flowering in bamboo is complex and incompletely understood. A more recent scientific article, “The Bamboo Flowering Cycle Sheds Light on Flowering Diversity” by Xiao Zheng et al., classifies flowering into four categories: sporadic, massive synchronized, combined massive synchronized and sporadic, and partial flowering. Depending on the species of bamboo, regeneration can occur through sexual reproduction (seeds) or asexual reproduction (rhizomes forming small, weak shoots at first, as “the proportion of flowering bamboo generally first rises and then falls, while the proportion of non-flowering bamboo falls and then rises.” If your black bamboo dies, it is still possible you may observe some regeneration that follows this pattern.