I think its worth taking as much time to master background techniques as you would take for learning foreground techniques. The background should subtly enhance the foreground without overpowering it.
The photos below are crops, I`ve put some numbers on them to mark the places:
1. After you have finished painting the foreground leaves and flowers, quickly mark some interesting shapes and patterns that the leaves behind make with a pencil. You don`t have to draw all of them. Now take a very dark value of green and starting painting the area around these shapes (your mix should have enough water that it flows well from the brush) Once this dark value is established, its amazing how easy it becomes to put in the values for the middle layers. You can push the leaves backward or forward to any plane.
2. See how if you put in the background color and value on the top (or to the side) of the leaves, it gives the appearance of them emerging from the shadows. These leaves form the connecting layer between planes. This is important, not all leaves are facing directly towards the viewer.
3. You can also leave out small white shapes in the dark value. After the bg has completely dried out, you can brush in a very light yellow green or green to give the impression of small flickers of light that have managed to penetrate the shadows.
4. Another trick is to simply lift out color with a damp brush leaving behind soft-edged leave shapes.
Photo Below: Its important to put in shadow areas on veins and stems too. Compare leaf 1 to the same leaf in the first photos. Here I`ve put in a shadow color on the top and bottom part of the vein. The highlight is only in the middle area and that is where it is the brightest yellow-green. The same goes for leaf 2. The middle portion of the vein is almost white just as the rest of the leaf in that area.
For the stems and branches, don`t automatically reach out for the burnt sienna. You can get a wide range of green-browns, reddish browns and violet browns with the colors already used to make the branches look more interesting. Also you can take a little artistic licence and exaggerrate the colors whenever possible. In the reference photo, apart from the red of the roses, its largely a mass of uninterrupted green. I`ve used yellow brown and violets and reds for accents to make the area a whole lot more interesting.
Watercolor Journal 