With William Ashbrook Kellerman he edited the exsiccata Kansas fungi (1889), a specimen series which is widely distributed in major herbaria.

[1][3] In 1927, botanist Elmer Drew Merrill published Swinglea, which is a genus of flowering plants from the Philippines, belonging to the family Rutaceae and named in Walter Tennyson Swingle's honor.

[5] He made several visits to the Mediterranean countries of Europe, to North Africa, and to Asia Minor, from where he introduced the date palm, pistachio nut and other useful plants, as well as the fig wasp, to make possible the cultivation of Smyrna figs in California.

Swingle also traveled to Asia, bringing back 100,000 Chinese volumes on botany to the Library of Congress.

[6] Much of his research is published in the five-volume book, The Citrus Industry, of which he wrote a significant portion.