The work of Julian Onderdonk is divided into several periods. His style changed somewhat in his later years, but regardless of the period in which they were done, all his pictures had "Roots deep in the soil of his native State". The divine spark urged him to express his emotional reactions to nature and fired him with an undying enthusiasm.

          Onderdonk married at an early age. Living in New York and sometimes working at his art day and night to support his family.

          While living in New York, he traveled to Dallas, Texas to help organize The State Fair of Texas Annual Exhibitions which he continued doing so for several years. He returned to San Antonio in 1909 and painted only what he loved and wanted to paint. The bigness of Texas with it's characteristic subjects; Hillsides covered with bluebonnets & dusty roads,  which captured the atmospheric conditions of misty mornings in the bluebonnets with the full character of the twisting native oak trees & headwaters of different streams, where he found the colors wonderful in varying lights. These are the subjects Julian Onderdonk loved and longed to put on canvas. Julian  refused to follow his fathers vocation as a teacher, but always gave his time and influence to art interests and activities at the San Antonio Art League.