Autumn is the season of seeds as well as "mellow fruitfulness" (see last entry). Keats was picturing vines and apple trees. In the botanical sense the term 'fruit' refers to the organ of a flowering plant that contains seeds be it edible or not.
 The photograph above shows the fluffy seed heads of a once prickly thistle. There is an old saying that "One year's seeds is seven year's weeds". True enough I suppose but here is another saying which I have just made up: "One year's seeds is seven year's wildflowers".