Watercolour has been the traditional medium for botanical art since the Middle Ages, and for good reason. They have a beautiful translucency which enables the subtleties of a plant's colour and form to be realistically rendered, and for the detail to be accurately depicted.
It's a tricky medium to use, however, and much practice (ongoing!) and patience is required - challenging in the heat and humidity of Brisbane's summer when dried blobs of paint on the palette soften without needing the addition of water.
I use professional grade watercolours from the Daniel Smith and the Windsor & Newton ranges, and whatever good-quality brush I've been able to pick up on sale. I use Arches Hot Pressed 300 gms Watercolour paper.