i want to plant bamboo and create a huge forest of them behind my house.


Where do you live? The main problem is that if you plant a forest of bamboo
behind your house, it will also spread to your neighbors' houses and clear into
the next county.

i have some from a thai store that i have in water they have roots.

That is not bamboo. It is misnamed. It is actually a Dracaena.

can i plant them outside?

The Dracaena, only if you are in Hawaii, southern Florida, or southern
California. Maybe Corpus Christi. There are bamboos that are hardy all the way
to Zone 5, but they are still vicious.

will 2 bamboo stalks create more and more?

Like the brooms in "The Sorcerer's Apprentice."

how do they spread?

Most bamboos, called running bamboos, spread by underground runners. If you are
in a warm climate, there are some bamboos, called clumping bamboos, which stay
put. They are the only ones you can use in landscaping without putting in deep
barriers to keep the running bamboos from taking over the world.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)