Another thing you might want to try is stepping into each color channel, say the green one(might be red or blue, just depends on color information) and setting that as a alpha matte for your text layer. Or try a chromakey effect, sample the grass color and use that as the alpha matte inverted.
It’s possible that neither of these work, it really depends on the color information of your footage, but I’ve used these techniques occasionally and have gotten lucky. Otherwise it’s rotoscoping, get some good music and some coffee.
Johnny Cuevas, Editor
http://www.thinkck.com