The color is obviously the strongest point of contrast here. The vibrant, red hue of the flower against the lighter, tanner background brings our eye right to the flower.
This may not immediately seem like a balanced image, but the vastness and depth of the negative space here makes it it's own thing. It balances out the image because the flower brings intensity, and the negative space brings depth.
The harmony in this image comes from the tone and saturation of color. The colors in this image are all warm, and very saturated (even the colors in the background), and that causes this image to have harmonious elements. The texture of both the flower and the background are very similar. Maybe its the soft light that makes the background feel as velvety as the flower petals.
On a completely different note: I think I'm going to try to keep my blog posts within a certain color scheme... you know, for fun. Wish me luck!