How do you choose what Y layer your plants will spawn in?

I started making a mod a long time ago when I first discovered MCreator. It was a mod that added new ores and other cool things to the underground. (This was before the Caves and Cliffs update was announced, so don't judge me) Recently, I decided to continue working on it. One of the things I recently added to it was some new plants. I created the plants and set them to "cave" type, and then I created another Generation Test world. Unfortunately, there were plants all over the surface. Can someone please tell me how to adjust the Y level for where they spawn or tell me another way to make them appear underground? And if there isn't a way to do so, then MCreator Staff, please add that to the next update.