Minecraft: Can't place down Mushrooms?

I'm playing minecraft and I wanted to grow some giant red mushrooms. I tried placing them on dirt in places the had plenty of room but it won't work (tried a couple of different places). I can't place the mushrooms down anywhere. The only place I've noticed I can place them are right next to swamp trees.
Did they change the way mushrooms work?

It needs to be completely dark for you to put down any mushrooms. You can build during the night. Or you can make a huge square in the sky to cover up the area where you want to work at. Watch YouTube tutorials they'll help!