I played with it for a few minutes, and came up with this:

I could have worked harder on blending the plants and blurring them slightly to fit with the surroundings, but I'm thinking the matte line is key. Once I adjusted it to more realistically fit the contour of the cliff, the image started looking better. The default weed clump does leave a lot to be desired, but with only slightly better tools than mine (this was done in Photoshop Elements, so I couldn't load a selection from an alpha channel), more could be done to fit it in.