All plants are harmless and they won't be doing any sin during their plant birth.
Then what determines which birth they are going to take up in their next janma?
Probably the next birth will be as another plant or as an insect or as an animal.
The Vishnu Smriti(Chapter XLIV) says:
- Now after having suffered the torments inflicted in the hells, the evil-doers pass into animal bodies.
- Criminals in the highest degree enter the bodies of all plants successively.
- Mortal sinners enter the bodies of worms or insects.
- Minor offenders enter the bodies of birds.
- Criminals in the fourth degree enter the bodies of aquatic animals
Now,since the gravest sinners are getting born as plants,it is highly unlikely that they will be humans in the next birth.
Because even those with relatively less sins are getting animal births,it is natural to expect,that they(gravest sinners) will have to endure possibly a lot of more suffering before being born again as a human being .
I think,they have to pass through all the intermediate species of life, like other plants,insects and animals ,of various kinds, before attaining human birth.