The non-certificated pilot/owner reported that he was self-taught in his unregistered powered parachute, as he believed it was an "unregulated machine" and had performed 10 flights prior to the accident flight. The pilot stated that he was flying in the evening, it was getting dark, and he feared he would not return to his home airstrip before complete darkness so he decided to perform a precautionary landing in a field. Due to the decreased visibility, and the field conditions, he felt his approach angle was too steep, and initiated a go-around; however, the powered parachute was unable to out-climb the trees at the departure end of the field. The subsequent collision with trees and terrain resulted in substantial damage to the airframe, and serious injury to the pilot/owner, who reported there were no mechanical deficiencies with the powered parachute that would have precluded normal operation.