Moxie Mulch is the prize for winning the Bounce game with Katherine de Claw in The Garden Gazebo. It is used to grow plants, so you can have Potted Plants.
It is also needed by the Garden Sprites, when they help you turn the crystal critter statue into a kitty cat.
You can have a maximum of 10 Moxie Mulches in your backpack.
NOTE: Moxie Mulch used to be a prize for playing the Word Race game against the Leaf Sprite in Wildwood Glen, but that is changed.