It might seem impossible for grass to grow in a swimming pool, but it happens. Vinyl-liner above-ground swimming pools, unfortunately, sometimes fall victim to grass pushing up through the liner. There are a couple of different types of grass hardy enough to grow in a swimming pool once they've gotten into it. Most solutions to this problem take some effort but can be successfully completed.
Deal with grass that's growing into a pool's vinyl liner by eliminating the grass under the pool. To get at grass growing through an above-ground pool's liner, you'll have to drain the pool and move it. Once you've done this, dig up all the grass, so that it can't grow back. Once you've eliminated the grass, apply a layer of sand on which the pool can sit.
To prevent grass from growing back into a swimming pool, you'll also need to establish a grass-free zone around it. Treat the ground surrounding the pool with weed and grass killer, then lay down plastic sheeting and cover the now-grass-free area with decorative rocks. Creating a grass-free zone around a pool helps to prevent splashed-out water from supporting any possible new grass growth.
Handle a pool liner that had grass growing through it carefully after pool drainage. Slowly lift the liner up to prevent the grass that's grown through it from tearing it. Remove any grass blades and inspect the liner carefully for holes and patch any. Unfortunately, if too much grass has grown through your pool's liner, you may have to replace it.
Two types of grass, Bermuda and nutgrass, are hardy enough to push through an above-ground pool's vinyl liner over time. If you don't want to create a grass-free sand floor beneath your pool, there are commercial pool-floor underlayment pads available. At time of publication, an 18-foot round underlayment pad cost about $85. Underlayment pads are made of rubber, an effective barrier against grass that might try to grow into a pool through its floor.
One method of removing grass growing in an in-ground swimming pool is to remove it blade by blade. Once you've removed a blade of grass growing through your pool's liner, use a syringe to inject grass killer into the hole. After, you'll need to patch the hole. Never just add grass killer to your swimming pool in order to kill grass growing in it, though, as doing so can be hazardous to swimmers.