A method of improving the growth of plants belonging to the family Cannabaceae sensu stricto by providing a plant nutrient solution about the plant roots and a gas mixture circulating about the plant shoot； by selecting the gas mixture and plant nutrient solution temperature independently of the other； and providing a plant nutrient solution to gas mixture temperature differential of approximately 0° F. or of at least approximately 15° F. during different phases of plant development, in order to change the plant development to improve a desired plant organ for industrial, scientific, and medical purposes. Infection of plants by Pythium, Fusarium, Verticillium, and Rizoctonia are treated or prevented by selecting a gas mixture temperature that is approximately 65 to 85° F. and a plant nutrient solution temperature that is approximately 35 to 65° F.
