The old forest that Otis had created was abandoned, left to rot and die. Creating a forest was the way he was used to fighting. However, Otis came to the realization that sticking to old ways wouldn't always work.
He needed to change.
And that started with using the environment to his advantage instead of letting it hamper him. Otis had grown a mushroom in the toxic wasteland before, but didn't see a way to attack with them. That was why he didn't use them earlier.
However, his old strategy clearly wasn't going to work.
And so, he adapted to the situation.
Otis didn't have to create anything new for these mushrooms. The mycelium, or roots of the mushroom, did not go incredibly deep. However, they spread through nearly every inch of the toxic wasteland.
