It was reported that fuel usage during the flight appeared to be excessive and the burner flames were more yellow coloured than normal. The pilot suspected a problem with the fuel gas and decided to land before his supply ran out. During the approach, the pilot had difficulty in controlling the descent accurately because of an unusually slow response to control burns. Following three landing attempts, the balloon was landed in pine trees. Eleven envelope panels were torn. Investigation with the gas supplier revealed that the fuel mixture was not the 97% Propane requested but a mixture of 35% propane and 65% propylene. It was not the first flight using this fuel mixture but it was the first time that the outside air temperature was as low as +5 degrees Celcius. Subsequent tests by the operator showed that the burners were inefficient with this gas at the lower temperatures.