Fungal Invaders!

I looked out at the garden yesterday and wondered why there were large clumps of dirt on top of my flower garden bed. Today I went out to water my Larkspur seeds and discovered a nasty infestation of mushrooms growing. This is one of the piles dug up:

Where did they come from and how do I prevent them from coming back??

 Is it the old tree stumps?

Somebody help!

In other garden news, I trimmed up my rose bushes and they are started to bud and turn a pretty shade of green:

You like my one instance of pretty yellow flowers that previous owners planted?