In Keats' poem "When I have fears that I may cease to be," he makes references to nature, and some vague, literal references that can be specifically associated with autumn.
The word "glean" in line two, is often used with regard to a harvest, though in this case he refers trying to harvest the "teeming" ideas in his brain:
Before my pen has glean'd my teeming brain...
Likewise, in line four, Keats refers to "garners" which is another name for "granaries or storehouses for grain." In this case, he has used it in a simile to refer to completing his work to his potential as one stores grain in the granary until it is full, another task of autumn.
...Hold like rich garners the full ripen'd grain...
Overall, I believe that Keats' references to autumn in this poem are figurative, as he looks toward the end of his life, feeling he is in the "autumn" of his years—a sick man who will not have the time to live as he would wish (because of an illness that took the lives of several of his family members—tuberculosis).
The next piece is an ode, which is a kind of poem. Keats' ode "To Autumn," was the result of a Sunday evening walk, where he was deeply impressed by the warmth of the season's appearance, so much more so than the "chilly green of the spring." In the first stanza of the poem, Keats pays particular attention to the ripened fruit in trees and on the vine.
Season of mists and mellow fruitfulness!
Close bosom-friend of the maturing sun;
Conspiring with him how to load and bless
With fruit the vines that round the thatch-eaves run;
To bend with apples the mossed cottage-trees,
And fill all fruit with ripeness to the core...
In the second stanza, Keats uses several examples of personification that likens autumn to a "harvester." However...
Autumn is not depicted as actually harvesting but as seated, resting or watching.
For example, autumn as the harvester (without showing the actual action of harvesting) is seen in the following examples:
…whoever seeks abroad may find
Thee sitting careless on a granary floor
Or on a half-reaped furrow sound asleep,
Drowsed with the fume of poppies...
And sometimes like a gleaner thou dost keep
Steady thy laden head across a brook;
Or by a cider-press, with patient look,
Thou watchest the last oozings, hours by hours.
The last stanza speaks to the glory that autumn has, a music all its own—far different from the "songs of Spring." This stanza notes the passage of autumn as it nears its end and awaits the coming of the winter.
While barred clouds bloom the soft-dying day
And touch the stubble-plains with rosy hue;
Then in a wailful choir the small gnats mourn
Among the river sallows, borne aloft
Or sinking as the light wind lives or dies...
The first couple of lines provide beautiful imagery of the end of a day, and the sunset that casts the "stubble-plains" (harvested fields) with a rosy color; then the gnats "mourn" (for the ending autumn, or even their own lives…) as they rise or fall on the wind.
Keats' poem "When I have fears that I may cease to be" speaks more figuratively of autumn in terms of the place where Keats is in his life, as he looks at his imminent death. (He will die at the age of twenty-five.) The ode, "To Autumn," is different in that Keats clearly describes the beauty of autumn, making it more literal, though it too recognizes the passage of time.
In and of itself, an ode is a poem of praise and celebration. In the very title of the latter poem, Keats informs the reader that he will be extolling the virtues of the season. As is typical of the form, the ode is full of imagery and is written to affect the senses. The language that he uses to promote this imagery is extremely positive: "bosom-friend," "load and bless," "swell the gourd," "sweet kernel," "thou hast thy music," and "rosy hue" are just a few examples. Keats likens the season to a woman in the second stanza ("hair soft-lifted by the winnowing wind"), which lends the implication of life at its ripest--just as the gourds of the season are swelling, so too might a woman in the last stages of pregnancy, when life is about to burst forth, or even in the "autumn" of her life, when she is experienced and confident. The same imagery is used in "When I have fears that I may cease to be," comparing again a stage in later life to this fruitful and "full-ripen'd" season.