Here in Texas we have "oak suckers"... they run off the roots of the oak trees. With some oaks, like Red, there can be hundreds. It's a non-stop battle to cut them down in my gardens. If you use round-up or something like that, you're going to kill the tree. The only option, if you don't want to kill the tree, is to periodically cut them to ground level. I also have some cactus that send out runners and sprouts like that. All that said, and I am by no means any sort of expert here, but these root shoots are all looking for the same thing: sunlight. If they try to emerge and there is no light, they move on out until they come up and find light. They won't live without it. Assuming they already have moisture and some sort of medium to start their growth (like your basic 1st grade bean sprout experiment) the next most important thing is light. Yes, they will probably be running around under your liner, no matter what you put under it, but they aren't going to actually emerge until there is light. I live in the country in a heavily wooded area, so I don't have the option of clearing all the trees near the pool. I would think that would be the only way to prevent these suckers from coming up around the pool. I don't think I'd worry too much about them sprouting up through the pool underlayment and liner. A decent, standard installation should be good enough to prevent growth and sprouting. The plants are always going to take the path of least resistance.

On another note, my biggest concern where we are are stupid fire ants. They are all around the pool and I'm sure are tunneling mega-cities underneath it. I'm worried about parts of the ground collapsing underneath the pool. I did notice this year some new "trenches" under the liner when we walked in the pool that weren't there last year. The pool keeps that ground nice & moist underneath it... perfect environment for fire ant colonies. I haven't a clue what the long-term effects are going to be... just watching and learning for now.

Lori