Hello Alan
Nice to have a 15 year old planting.
Yes, manure, fertilizer, milorganite and lots of water will help your bamboo size up.
Water seems especially important during shooting season. If you have not gotten over 1 inch of rain within the last 7 days, you should get out there and water. I seldom water, but this year we have had rain just about every day for 2 months. I have the tallest aureosulcata shoots I have ever had. I wish I had added fertilizer, but I didn't.
My planting is 30+ years old. It is a confined bed, and the rhizomes get very dense. My grove started performing poorly, especially in the oldest section, nothing real bad, but culms were smaller, and fewer new culms in spring. I had someone who wanted bamboo, so in early spring, I had him do the digging (saved my back). I marked out a section of the grove that included the oldest section. Told him he could take every single rhizome in there for his planting. The maniac did. He essentially took the top foot of soil too. So I filled in the bed with composted manure to replace the missing soil. By mid summer, rhizomes from the undisturbed portions had recolonized the dug up area, mostly with short, roughtly 3 to 8 foot culms. By the end of the second growing season, the dug up area was fully colonized, and culms that sprouted in the enriched soil had culms that were bigger than either side where the grove was undisturbed. By the 3rd year you could not detect where the soil had all the rhizomes removed, one end to the other the grove looked uniformly dense.
I was told about this in 1988, when I chatted with one of the workers in the bamboo garden at the Royal Botanic Garden at Kew, UK. At Kew they renew beds on a schedule. In the area where it is not the show garden, but rather the bamboo collection area, they roto-till 50% of a bed. Then 5 or so years later, they roto-till the other 50% of the bed. They chop up and pull out as much of the old rhizomes they can. Then the bed is left undisturbed for a decade or more, then 2 renewal roto-tillings. It was Kew gardens that first suggested this to me. Others have offered the same advice. It is a standard practice. You don't really need to do it for the first decade, but somewhere down the line it becomes a good maintenance practice.
Since that first experiment, I have had other similar sections of my grove removed, old rhizomes pulled out. (all to be replanted at new homes). The entire planting looks better for it. Well worth doing. I had a section removed about every 4 years, so in 12 years I have renewed 90% of the planting. There are sections at the ends, because of the fence the bamboo is holding up on one side, and obstacles on the other, so the ends stayed undisturbed. But 90% of the grove have been removed, fresh manure and soil dumped in and the bamboo from the other sections completely recolonized it. As long as you renew in sections, the undisturbed portions will quickly re-colonize the renewed areas.
I think this should be done in my zone 5b climate, about every 10 to 20 years. I think a 15 year cycle for renewing a bed is near ideal. Funny you should post photos of your 15 year old planting.
And definitely manure and even chemical fertilzers really help, especially if you get enough rain, or water often too.
It is a luxury, having an old, well established grove, where you can causally talk about chopping out 30% or 50% of it out, and can relax, because you know in 3 years, the renewed portion of the bed will be just as dense as before.
