Short speculation here based off most recent manga chapter. My speculation is that Green Bull does not actually have a plant based devil fruit but instead is a master of a power shown earlier in the series.
Kumadori (funny pink haired guy from CP9 that Chopper fought) had an ability we have not seen since called “Life Return” he said this power, not any devil fruit, allowed him to control his hair at will and he could through force of will alone cause his hair to move. He did this through control of his cells and manipulating them, but I think this could be used other ways.
My speculation is that Green Bull is a master of Life Return to the point of controlling how his cells consume energy that he has stored. We see him lounging in a chair which shows he is probably someone who is laid back or taking it slow. So I think he essentially functions like a sloth where he conserves energy for long period (shown by him not eating for 3 years) and can make use of the stored energy whenever he needs to. He thinks eating is a “pain in the ass” because he is able to balance the flow of energy to his cells easier when he has less energy (assumption).
Basically I am getting at the fact that Oda has introduced this power long ago and done nothing with it. I think it would be cool to see someone who has mastered their own body to the point of being able to control the flow of energy to the cells and thereby is able to refrain from eating.
I am also leaning towards a photosynthetic plant based devil fruit since that would make perfect sense, but since it would make use of a power we have already seen I could see Oda doing something along those lines since Oda tends to expand on previously utilized powers, such as Shanks using Haki in chapter 1, but not explaining it until around Amazon Lily/Timeskip.
*Theory by Shalebale