In zone 9a, you are right on the margin of a suitable climate for many subtropical clumping bamboos. This means that just a few degrees one way or the other can mean the difference between little damage and topkill.
In the Houston area, the early to mid 2000's saw a string of mild winters. People decided that those winters would last forever, so they planted many marginal plants. Because clumping bamboos mature so quickly, there were many marginal bamboos that had achieved impressive stature. Nurseries sold things like
Bambusa oldhamii as "bullet-proof" plants.
In 2009, there was a severe advective freeze that dropped the area to around 20 degrees F. The daytime temperatures also stayed pretty cold for several days. This event caused massive damage to many clumping bamboos. The plants that performed best (as far as I recall) were
Bambusa multiplex,
B. textilis,
B. chungii,
B. ventricosa, and
B. emeiensis. These plants mostly experienced minor leaf loss, or maybe a little dieback. Plants that I saw perform poorly included
B. lako,
B. oldhammii,
B. odashimae,
B. pervariabilis,
B. eutuldoides, and
B. tuldoides. These varieties experienced severe to total topkill. I believe all of them returned from underground buds, except for the many that were removed by people who did not want to occasionally have to deal with the enormous task of cleaning up freeze damaged bamboos.
Since the 2009/2010 winter, there have been several more winters with somewhat milder but still damaging freezes that have continued to injure the more sensitive plants listed above.
A very knowledgeable member of this forum (Boonut) posted some pictures of his
B. lako after the winter of 2009. He lives far south of here, in an area with much milder winters. The temperatures he experienced should be quite common in zone 9a. See his series of photographs here:
http://www.bambooweb.info/ShowPictures. ... ton=Search
I have not grown any
Dendrocalamus, so my knowledge of them is quite limited, but the ones that I have seen are definitely not as hardy as the hardier
Bambusa species. I do not know of any
Dendrocalamus in the area that have not topkilled several times recently.
So....what is my recommendation? It depends on your goals and limitations. I think a large bamboo that looks good after 3 out of 5 winters is worth having. There are some plants that I enjoy enough to buy again if they freeze kill, because I am willing to gamble on a few mild years, and a backup specimen can always be kept in a pot. However, I am a collector, and I like to grow everything. If someone really just wants a beautiful, low maintenance landscape plant, I would never recommend a marginally cold hardy clumping bamboo. For the average urban person, clearing a mature, topkilled clumping bamboo plant is a pretty large cleanup job. Only a serious collector would knowingly risk having to take on this task, when hardier, less uncertain landscape choices exist.
As for microclimate and being near the river.....be careful. Many Houston residents have gotten used to the heat island, which gives them enormous protection during the normal radiational freezes. Over the past decade, the occasional severe advective freeze has caused much grief for gardeners who previously felt a false sense of security. On those cold, windy nights, temperatures across broad areas reach similar minimums, and there is nowhere for a huge, subtropical bamboo to hide.
As for maturity making the plants more cold hardy...here are my thoughts. During radiational freezes, the top of a dense, mature plant provides a thermal blanket for the lower foliage and branches. This will work for approximately one severe freeze event. Once the dead leaves from above fall off, the lower part of the plant is more exposed. Therefore, each successive freeze event during a winter will cause damage closer to the base of the plant. With subtropical bamboos, I have not found that a leaf or branch on an older plant is noticeably hardier than the equivalent on a younger plant. It is true that actively growing shoots and branches that have not yet hardened are vulnerable to freeze damage.
Maybe someone with more knowledge will chime in here, but I know of no "bullet-proof" black clumping bamboo for zone 9a. I would love to have one myself, so I will be happy to see evidence that one exists.