I feel like the most sensible answer to how plant souls work when plants can have cuttings is to say plants are single souled and the entire species is controlled by one soul. This makes sense with Hindu stories like the curse on the Ketaki flower somehow applying to all of them. Is there any scripture that contradicts this idea?
In case you don't see the conundrum, how does the cutting survive when the origin plant dies? The new soul also can't rely on a small vessel otherwise the soul wouldn't be able to exert enough control to control the body, especially for organisms without brains, like plants.