Charles Manoras wrote:
If only because the rhizomes sometimes emerge
from the ground, in which case I add a layer of
good top soil to bury them again.

When rhizomes poke out and then go back under, it doesn't seem to have
any adverse effect on the plant. I am not sure you are actually solving
a problem by adding top soil, but I am sure the plant likes it anyway.

FYI, if you are going to add material on top, composted steer manuer is
probably better for it's higher nutrient value (which will leech down
into the root system over time)

I also mulch the ground around and between the
culms a good deal.

Bamboos love mulch.

Would it be effective to also add some fertilizer
in order to stimulate future and further growth
(spread, height and diameter)?

If so which one?

I have been told that a 3-1-2 ratio is best... It doesn't matter
whether it is a 3-1-2 fertilizer and you put a lot on, or a 24-8-16 and
you put just a little.

If you are going to fertilize only once or twice a year, get a slow
release fertilizer. If you don't mind the extra work, a weak water
soluable fertilizer like Miracle Grow, applied every two weeks or so is
great.

Also, those who really know what they are doing change the fertilizer
with the season. In the spring, bamboo is shooting and leafing out. So,
a high nitrogen fertilizer like a 28-3-3 grass fertilizer is a good
choice. However, in the fall there is a lot of root and rhizome growth.
So a "root-blast" type fertilizer (I forget the numbers) is better.

And, note that the three numbers in fertilizer are not the only things
a plant needs. They ARE the most important, which is why they get their
own numbers. But some fertilizers like Miracle grow (and many others)
have a lot of the other minerals that plants need.

BTW why do the rhizomes not always stay
underground?

This is pure speculation, but I suspect it is part of their attempt to
stay near the surface. They grow very quickly and if pointing up
because they think they are too deep, they might very well shoot out a
few inches before being able to turn around.

Cheers!