The safest way to get rid of the grass is just to use a shovel and scrape off the top layer of grass. If you have some thin spots, you can just pry it up and use it like sod.

You can use roundup around trees if you want. Pick a day with very little wind and hold the sprayer low to the ground. If you don't spray the leaves of the tree (or get any wind drift to the leaves) you are very unlikely to see any damage.

Really, I would recommend you digging it out though. That will give you an opportunity to create a nice edge to the soil that will hold the mulch in place and allow you an edge to string trim.

Thanks for the info, I think I will stick to digging,its more work but I feel safer doing that. I am dangerous when it comes to using weed killer.

I wouldn't dig anything up as it will harm tree roots. Scalp them with the lawnmower at the lowest setting, use garden rake to rake them out and mulch 3 inches deep around the tree as long as nothing touches the trunk/root flares.

maybe think about whether to put down a weed blanket and put the pine straw on top? The grass shouldn't be able to grow through the cloth and the pine straw on top should block out any sunlight.

Once you do that just cut an edge around the tree.

I think you're making it too hard. Just scalp it and cover it with 2 inches of sand and the St Aug will die. I can show you a house in my neighborhood where they applied too much compost in March and the St Aug is gone. It really doesn't take anything to smother out St Aug.

Another way to kill St Aug is to not water it for three weeks.

i put down a couple of layers of newspaper and then held it down with a nice layer of mulch, every once and a while i pull some grass that grows in but it works well. the newspaper eventually breaks down.