Very large cardboard box
Camouflage duct tape ($5)
Hot glue gun and glue
Brown paper (can use postal paper $4/roll, or a free brown paper bag)
Foam piece 6"x3"x2" (optional for padding for your forehead)
4"x2" Velcro ($3)
Bark (thinner is better)
Fake ($2) or real leaves and/or moss (free)
Step 1: Cardboard Cutout
With your large cardboard box laid out flat, outline the shape of Groot's head, but leave an extra length of cardboard to the left and right of his face that you will fold around behind the face to make a cylindrical mask shape. Then cut it out. (See the next step for a better picture of the back of the mask.) You could probably use posterboard instead if needed. I added some padding to the nose area, since Groot's nose sticks out some. Then I covered it with postal paper to make a nice even blank slate.
Step 2: Form Back of Mask
Cut the cardboard on the back side of your cylindrical shape to form a band, long enough to overlap two pieces. Cover the back of the exposed cardboard with camouflage duct tape.
Step 3: Optional Foam Padding
I added a piece of foam duct-taped to the inside of the forehead area. This helped with comfort, but also helped the mask stay a few inches off my face too.
Step 4: Add Velcro
Place your Velcro on the band of the back of your mask. This makes it adjustable for multiple users.
Step 5: Glue on Bark!
This is the tedious step. Glue thin bark pieces all over the front of the mask using a hot glue gun. Do this with your mask already formed into the cylindrical shape, not lying flat. Thinner bark is best to keep the weight of the mask as light as possible. It will still be a little heavy.
Step 6: Add Foliage
Add a few pieces of leaves (I used fake ones) or moss (I will add this right before wearing it so it doesn't dry out and turn brown). And done!