However, you can use the comparative "lusher" if you want to compare two things in terms of lushness, but it is not as common as using "more lush."
Here are some examples:
* Superlative: "The rainforest is the lushest place on Earth."
* Comparative: "This garden is lusher than the one next door."
* More Common Comparative: "This garden is more lush than the one next door."