Illustrating foliage and flowers can be quite challenging and daunting. So I have developed a technique where you can draw them using pattern brush in Adobe Illustrator.
Pattern Brush Options appears on the screen. Select Hue Shift on Colorization. Leave the rest of the settings at default.
Select the brush on the Brushes Pallet. Using pencil tool or brush tool, Do short and quick strokes.
Using brush (B) or pencil tool (N), repeatedly do quick and short brush strokes with different shades on top of the dark green shrub. Do the dark ones first and draw the lighter colored leaves afterwards.