Imagery is description using the five senses of sight, sound, touch, taste, and smell.
Young uses visual imagery when he likens "Time" to an "aged gardener." An aged gardener is a person we can visualize, perhaps seeing in our imagination a man bent over, with white hair.
Another visual image is "the scentless flowers of the frost." The patterns of frost on the ground can look like flowers, and "scentless" is an image that reminds us of smell or a lack thereof.
Young describes the bare tree limbs spread out against the night sky, another picture we can see, with the stars in the background. He also uses the visual image of the stars and the moon hovering over the sea.
Young relies primarily on visual imagery to paint a lovely picture of winter. Other images could make winter seem gray, deathly, and dismal but Young does not use those. His positive images support his point that although winter is simpler and starker than summer, it is still beautiful in its own way.