Try sketching the desired plants first as a plan. This will give you a good idea of what the outcome will be. A little planning is always helpful. And using some photographs as reference points would help, too.

In fact, if you're a good drawer, you could draw the plants, scan them in and color them.

If you are new to painting in PS, don't expect good results first time around. It takes practice - a lot of it.

There is also a variety of leaf brushes out there. However, I find the ready-made brush shapes give poor results for realistic painting. After all, every leaf on a bush is not identical.

Good luck.