Get your side edges matched up first so the image can tile horizontally. You will get the best results there if the image is around 700px wide, as the seams will be largely hidden behind the main content.

Make sure the image is tall enough to fill an average monitor's content window (say 700 or 800px), and then only large monitors will have to deal with the grass blending anyway. You will not have any vertical tiling.

In Photoshop (adjust instructions as required for any other program), enlarge the canvas say 100px on the right side, then copy a 100px strip from the left edge and paste it onto the right edge so it meets the original right edge of the image. Work the joint (rubberstamp can be helpful) until it is well concealed. Be sure not to change the right side of the copied strip, only the left. When it looks good, cut the 100px strip from the right and paste it back where it came from on the left, then shrink the canvas back to where it was.