I've been reading Hardy Bamboos, Taming The Dragon, by Paul Whittaker. He writes that "seed may form 4 or 5 months after the hanging anthers are seen". Then cautions that you have to be vigilant to get to the seeds before the birds do. Apparantly cutting down a culm with seeds and placing it indoors with a sheet under it (to catch the seeds when they fall, can work). He also writes that it is sometimes necessary to put the seeds in the fridge, as a cold snap is sometimes needed to break open the husks. I'm just reading this book now, so I don't know if there is any further info.