Horticultural therapy is a complementary therapy by trained professionals utilizing plant and horticultural activities as media to pursue the positive change of clients, which resulting from stimuli of plant․nature, activities, and interaction with other people. Outdoor horticultural activities provide clients with the opportunities of the contact to nature. such as fresh air and sunlight as well as plants. But the importance of plant․nature to client's health has not been fully acknowledged compared to that of therapist's role and activities in horticultural therapy in Korea. In horticultural therapy it is possible for client to participate on the various levels from passive viewing to active gardening according to the client's ability. Therefore it is necessary for horticultural therapists to consider the therapeutic factors of plant․nature to maximize the therapeutic effect of horticultural therapy. The purpose of the research is to clarify the therapeutic factors in the contact of client with plant․nature and to emphasize those importance in horticultural therapy. From the review of the experimental researches five therapeutic factors were revealed: Natural light, plant․natural view, natural sound, aroma(volatile oils of plants), and plant itself. This paper raises the importance of outdoor activities in horticultural therapy and the necessity for severely handicapped clients to the contact with plant․nature, though passively.