How Do I Make A Custom Plant Generate In A Custom Biome?

Did you finish you biome?

Well if you did you need to put restrict to biomes, and  the first biomes are custom, so click your biome.

And if you did already that, you need to go to generation and set the dimension of spawning to the dimension/s where your biome is generating, if you have restricted blocks make sure you grass block replacement is good. If that does not work or you already did it, you need to look at the top, and search for add tag, and add forge:dirt, and add your grass block to it. If that does not work, try checking spawning per chunk, and add to it more, if that does not work, send pictures of everything about your plant (properties, generation, and the tag etc.)

might be a bit late, but try your cave plant with "generate at any height" enabled