Harrison's practice of Zen Buddhism was important to his poetry, in part because it kept his "head from flying off". He became aware of Zen inspired poetry "by way of poets like Clayton Eshleman and Cid Corman, and most powerfully of all through Gary Snyder""." He wrote that his long poem "The Theory and Practice of Rivers (1986)" was "basically Zennist". His sequence of 57 poems "After Ikkyū" (1996) is entitled after the Zen monk Ikkyū and was occasioned by his study of the Zen sages Tung-shan and Yunmen.