Growing an Oasis based on Falloff Object

I’m looking for some help/advice.

I want to try and get the effect of growing an oasis on a landscape, similar to the effect seen in Scorpion King

It doesn’t need to run in real-time, But I need to be able to control where the growth starts and its speed.

Initially, I’m thinking the trees could be done using speed-tree but would prefer to not have to hand place. It’s just the Grass and small foliage that I’m not sure how to do it and get its growing on the landscape.

Cheers

Hello, has this problem been solved? I have a similar idea and hope it can be realized