It’s always nice to trip over something on the Internet and find a hunk of gold. This is the case with this awesome Groot cosplay by Calen Hoffman.
The main body is pipe insulation foam, hot glued (low temp) to a suit of under armor(on my duct tap mannequin). The bark plates and mask are carved EVA foam sheets. The bark texture is kleenex soaked in Tacky glue and water. I recommend that you use something a little thicker than kleenex though, maybe tissue paper. Then painted with rattle cans and acrylics
Calen has done a fantastic job! You can see more of his work at Propcustomz.