I've heard of people having success with two approaches:

1. Use ground layering: bend a branch over to the ground and cover the culm with dirt; you may get rooting from the nodes.

2. Use air layering: pack wet moss around a node and enclose it with clear plastic wrap secured with tape; roots may develop.

In both of these approaches, once roots form, you can separate and plant the new growth.

For more info on propagation, go to the BambooWeb link below and search on terms like "layering" or "propagation."

You may also find this writeup on culm propagation useful:

http://www.boonut.info/bamboohowto.html#propagate

Here is a link that might be useful: Growing Bamboo Forum

I've had good success propagating B. multiplex with your method, although I use perlite with a little peat moss as a medium, and I cover the pots with a plastic bag to keep the humidity at 100%. Definitely use rooting hormone. The leaves may all fall off after a week or two, but be patient. The cuttings may take a couple months or more to sprout new growth.

I do the trenching method with success. Have to wait until
January or February and cut off a 2-3 year old culm from
the mother plant. Cut this down to the lowest node that is missing branches. Now cut off all branches except one per
node (leave about 1 or two leaves per branch as without the root system to support it the leaves will dry out and die; you need to continue to have these leaves green and robust) and bury the whole culm/stem (do not cut into pieces
since the energy of the whole stem will insure survival
at each or most nodes). You will have to have your trench in 100% shade and keep it constanly moist but not standing in water. You may paint some rooting hormone (I like dip-n-gro) at the base of each node. I usually get roots at each node
by October/November and these little plantlets sending up
shoots from the gallon pots I put them in by the following
year. This is gonna take all your patience but you will
be handsomly rewarded. I use a combination of gravel/wood
chips and sunshine mix in the trench and you must not let
the trenches ever dry out completely or your work may all
be wasted. Don't know what impact freezing weather may have
on this process as I am doing this in a rainforest in a
subalpine climate in Hawaii. Max day temps all year are 75 F
and lowest night temps in February/March are 40 F.

My experience with the multiplex varieties is exactly the same as jjhawaii. Whole culm burial will work, but even this is a slow process with a marginal success rate. Since your mother plant is not very large maybe you can find someone that may have some donor culms that you can aquire.

Here is a link that might be useful: Bamboo propagation pics

Thanks to all of you. I will experiment with these methods to see what will work for me.. I am in the middle of Louisiana so we do not get extreme cold, but do get some frost in January - Febuary. I will probably have best success in spring.. BTW My bamboo is starting its 4th flush of canes this year! I didn't even fertilize except once with miracle grow, and they are planted in very high clay content soil..