For distant puffball trees, I use the time-honored technique of making balls out of polyfil fiber, and then add ground foam (different sizes and colors) with matte medium to add a little personality. However, if you've bought green polyfil fiber prepackaged in small bags, you quickly find the whole process gets quite expensive if you have a large area to cover. Here's my tip:
Buy a large bag of polyfil fiber at your local department or craft store - usually this stuff is used to fill pillows, comforters, etc. You can usually get a big bag for less than $10. However, it's one color - white. Unless you're modeling snow or want to make clouds from the stuff, there's very little on a layout that can be modeled with this color. Solution - paint it black first. Pull out a couple handfuls at a time, and spread it out into a loose bat that covers an area of about 1 square foot. Get some cheap spray paint (99 cents a can) and paint your bat. Allow that to dry and spray with your desired color (I use greens to model summer). You can then use these bats directly on your layout, drape them over tree armatures, or pull them apart and make your puffballs. You'll find that some of the white polyfil will show through once you've make puffballs, but once you add the ground foam the white will disappear.