I am looking to plant some creeping thyme on the edge of tall grass to keep it out of the garden, and am wondering if it will be able to push back against 4' tall grass bordering the garden.
My experience on sandy soil is that it will, in combination with mowing. The advantage of creeping thyme is that you can mow it hard to say two inches and it will bounce back and even flower abundantly if you and the bees are lucky, but the grasses give up after a while. As the thyme gets thicker you have to mow less, but the thyme will survive where the grass eventually gives up. Even couch/quack grass will stay away. I use it on a bank where mowing is possible but inconvenient; it works well.
In really droughty weather patches will die out, turn black and be somewhat unsightly but eventually fills in again where the grasses won't stand a chance.