I've limited the list to the most common plants, but there are others that will produce what you want.

The following combinations will produce a Nox flower:
Daisy and Bluestar
Mystic and Painted
Spotted and Rosaceae

The following combinations will produce an Orchid:
Grass and Astera
Scandens and Gladiatus
Reptans and Fern

So, for example, you could cross a Spotted Grass with a Rosaceae Astera and get a Nox Orchid.