24" deep should be enough, that's 2x75 ft = 150 of trench. ugh

B. oldhamii is a tropical clumper, it doesn't run, doesn't need to contained.

Will-
A couple of random observations...

1. If you're only going to use B. oldhamii, as unautre points out, it's a clumper and needs no barrier. However, if you want a 75' long hedge fairly fast, you're going to have to plant lots of it.

2. It's not always a good idea to plant a large bamboo hedge using only one species. If it flowers, there goes your whole hedge. Threrefore, I suggest you use several species: other clumpers if you don't want to worry about containment, and runners if you want the hedge to fill in faster.

3. If you plant runners, one compromise with the barrier would be to only do it on the side that separates you from your neighbor, and do rhizome pruning on your side.

4. If you do any kind of concrete barrier, use reinforcing wire because runners can get through cracks if they're big enough. Also, if you're going to the trouble of putting in a barrier, I'd go for 30" to be on the safe side.

"If it flowers, there goes your whole hedge" ???

I'm new to bamboo... what do you mean by the above statement?

And, if I mix another with oldhamii... what would you suggest? (I want straight growing, very tall, very thick, very fast growing.)

thanks - Will

Bamboo is "monocarpic". Flowers once and dies.

But bamboo only flowers once in decades, so it lasts longer than many trees.

I've got a Bambusa tuldoides that I really like more each day.

http://www.bambooheadquarters.com/photoweb/batu.htm

http://www.bamboo.ws/bambusa%20tuldoides.htm

I bought it new last July. It's now putting up 10 new culms, some about 1" in diameter, which will probably reach 20+ ft high in several weeks, which is pretty fast. Last year is put up new culms from July to Nov.

It's not perfectly erect, but the lean is not too bad. You could always prune the worst leaners.

your problem is populating 150 linear feet with a clumper. That will take a lot of separate plant divisions, eg, one plant every 6 ft, at most. about 15 - 20 plants per 75 ft.

Not all bamboos die with flowering. Underground barriers may have to be pretty serious, in Seattle there is a freeway sound barrier wall with Pseudosasa japonica on both sides.

IJMO, but you don't need container to keep a clumper in order. In fact, there has been some reports of larger clumper (and Oldhammi is a relatively large clumper) busting through fully grouted (i.e. filled with concrete) 8 inch cinder block containers. It takes a lot of force for a root ball to do that...apparently, Oldhamii can do it.

Will-
Bamboo flower unpredictably. Some might start flowering tomorrow, and some might not do it for 150 years. The point is that it generally happens globally for a particular species over a 1-3 year period. No one can predict the cycle or spacing between flowering for any bamboo, so it could never happen to your bamboo in your lifetime, or....
RonB is right in saying that all bamboo don't die when they flower, but some do. All that can be said for sure is: some just partially die, some just look really ratty for several years, some die and regenerate after a few years, and some don't seem to be very affected. Since we don't know what would happen with B. oldhamii, I was just warning you to be conservative and plan for the possibility. If you do use more than one species, and you use runners, it's not a good idea to mix them as they can compete with one another. Rather, plant each in a discete section. With clumpers it's not that much of an issue.

Here's something you may want to read about bamboo flowering:

ABS - When bamboo flowers

Thanks everybody for the kind assistance.

Unautre, you mentioned Tuldoides at...

http://www.bambooheadquarters.com/photoweb/batu.htm
http://www.bamboo.ws/bambusa%20tuldoides.htm

I like the picture in the first link as it is nice and thick... good privacy... but in the 2nd link it is only a few culms ?? Which is normal? I'm looking for a very quick privacy hedge, very tall.

To others: I was considering a containment wall along our 75 foot back property line to keep the oldhamii from getting two wide. The ones I've seen get 6-10 foot wide at the base. I was hoping to put in a double contaiment wall to limit it to 75 foot long and 3 or 4 feet deep.

I'm posting another idea seperately.

thanks again for all the help.

Will

B. tuldoides is a pretty tight clumper. Any manture clump that is "open" has probably been culm-pruned for openness.

You can also prune the lower branches, say below 7 ft, to keep them out of your face when the clump is near a walkway.

I don't have any exerience with barriers but just thought i would mention, A barrier is similar to a big pot. eventually bamboo in a pot becomes root bound and will suffocate if it is not taken out and divided. i could be wrong but i do think the rhizome pruning is maintenance but a barrier is not the "set it and forget it" scenario either. down the road there will be some kind of maintenance with a barrier. I would barrier your property line only....especially with using a clumper variety. Just some thoughts

Kurt