True pampas grass, Cortaderia selloana, gets to be a big plant in a pretty short time in climates where it is hardy. I wouldn't plant them any closer than 3' and 4' apart would be better to develop and appreciate their arching form. They have extremely sharp edges as well, so make sure they are far enough away from walkways or any type of pedestrian activity.

I live in Las Vegas Nevada...recently moved into new home....am landscaping my backyard which is small 16 by 25 FT. My landscaper gave me two choices for the tree I want in a corner of the yard....the bottle tree or a flowering plum....I need a tree whose roots want spread out but that will grow straight up....are there any other options...I didn't much like either one of those choices.???

marilla - what does your question have to do with pampas grass? You should be posting this as a new thread.

And have you conveyed your concerns to your landscaper - he/she will be more familiar with your requirements and what might fit best and work well in your climate. That's what you are paying them for!