Imagery is the use of sensory details to create an image in the reader’s mind. Sensory details are descriptions that involve the five senses of sound, smell, touch, taste and vision. This poem includes several sensory details. Many of them are related to heat, which is touch. For example, line 2-4:
When all the birds are faint with the hot sun
And hide in cooling trees, a voice will run
From hedge to hedge about the new-mown mead;
Hot sun and cooling trees are contrasting images. New-mown mead is another. Note that there will be many sensory details in a poem describing nature. Picture it in your head, and explore it through your senses.