They couldn't absorb back the plant energy that they had already stored in the plant, and the energy could only be used for the plant to grow or do something depending on how they use it in the future.
And they couldn't even use it to consciously plant because the conscious plants know how to resist and forcibly use the energies they stored in the plant.
So, other ability users in his past life had thought that only plant users could probably do it because the plant is a living thing that they were controlling to grow using their power; that's why the energy they stored had some use for the plant.
However, other people who had different superpowers used something different.
For example, fire users use fire they just conjure, and for metal users, they use metal around themselves.
