It's not random but he has no real control over them beyond hiding them, the plant part of him operates on instinct based on his own feelings, they manifest plants or flowers as raw emotion or subconscious needs.(such as shade from sunlight, water, as well as an assortment of nutrients)
The flower on his hair is probably the closest thing to its 'true form', but really it is so mutually weaved into his biology that it both acts with a mind of its own but also as an extension of the very same body and mental chemistry.
