I think it's a draw order problem, and not a transparency issue. Try changing the backdrop colour to something more distinct. If the border colour around the leaves is the same colour as the backdrop, it's a draw order issue.
If that's the case, DBP is detecting the origin of the plant objects is further away from the camera than the origin of the white grid object the plants are on. It is therefore drawing it first. This means the alpha channel around the leaves is blending with the backdrop colour. The grid is then rendered next, and since it is behind the leaves, it isn't drawn at all where the blended edge pixels are, and you therefore get your outline.
What you need to do is get DBP to draw the grid first and the plants next. Since we don't have a lot of control over draw order in DBP (I never got the Z Bias command to work), you can use a trick I have used before. Offset the base limb of the ground object so it is really high (e.g. OFFSET LIMB groundObject, 0, 0, 100, 0). Then position the ground object at the correct height again (now at 0,-100,0). By doing this, DBP will detect the ground object's origin is at -100 on the y, assume it is further away and draw it first. The leaves will then be drawn afterwards and will blend with the ground image.
Obviously, this only works in the camera is above the ground, but it should work for most circumstances.
