A Change In Plan
They would grow on the monster's back, exchange energy with it in exchange for giving it what the monster most needed, a water canal!
The monster used water to survive, but weirdly enough it didn't have the ability to absorb water itself. So, it had to use the help of this herb, in exchange for giving part of its power on a regular basis.
It was a mutual benefit relationship between these two. And William was happy for his luck to find such an adult monster here thanks to the herbs grown on its back.
"These herbs are all mature, at least a thousand years old or more…" he examined the bushes' leaves before storing them inside one of his rings.
He knew keeping these inside his ring was a big and fatal mistake. But he had no other choice but to do so for now.
Rings couldn't help to store any living herb or plants for long durations before these would decay. But William knew keeping these inside for a week or so wasn't going to be a problem for him.