Throughout the poem, the poet uses vivid and evocative language to describe sights, sounds, smells and temperatures. For example, he describes the "hazel shells" as looking "plump," the sound of the gnats as "a wailful choir," and the summer air as "clammy." Words like "plump" and "clammy" are much more vivid and evocative than, for example, synonyms for those same words, like "big" and "humid." Indeed the words "plump" and "clammy" are slightly onomatopoeic, and thus the senses being described are richer and more immersive.
The images described in the poem are described in the present continuous tense, and this makes those images more appealing to the senses because they seem more immediate and more dynamic. For example, the wind is described as "winnowing," the gnats are described as "sinking," and the flowers are described as "budding." The "ing" suffix in these examples indicates a continuous tense and gives the impression to the reader that the images being described are very much alive, immediate, and dynamic.
The images in the poem also appeal more fully to the senses because they draw upon different senses at the same time. It is much more appealing to imagine a sight accompanied by a sound or a smell, for example, than it is to imagine just a sight on its own. Thus in the poem, the poet describes "moss'd cottage trees" and "barred clouds," and also "the fume of poppies," the "clammy" air, and the "whistles" of the birds. The images of the trees and the clouds, in this instance, are much more appealing because we can also smell the poppies, feel the humid air, and hear the birds. The poet appeals to four different senses at the same time and thus allows the readers to immerse themselves more fully into the scene.