Mushroom Biome's Main Block (the weird grass bock type thing) Cant have Saplings planted on it (RESOLVED)

Hello!

I was Playing the Latest Version of Minecraft and i wanted to have a Tree Farm, So i tried to Plant a Sapling and it did Nothing, My Sapling was still in my Inventory. I had to place down Dirt and make a Cobblestone Square around the Dirt to make it so I didn't have to move Biomes, I'f you could fix this  that would be great