colorThresholder seems to do pretty well using the LAB color space. I generated the attached mask function using colorThresholder. Then use regionprops to get a bounding box around the white pixels. If you can ensure there is some white around all leaves first it gets a lot easier. You may want to use labeling functions like bwconncomp to find small outlying regions of white pixels to discard. Then if you can get one bounding box that includes all the white area including leaves, you can copy that portion of the image into a smaller image or just set all the pixels outside the bounding box to black.
See imfill and other "Morphological Operations" in order to try to do things like replace small holes in a mask.
A = imread('leafImage.jpeg');
maskA = createMaskWhiteBackground(A);
s = regionprops(maskA,'BoundingBox');
>> bb = s.BoundingBox
bb =
1.0e+03 *
0.6225 6.5175 0.0020 0.0050
