The answer is: the places where harvests are stored.
Garners means barns, storehouses or granaries, which are places used to store threshed grain when it is separated from the plant.
In the excerpt from "When I have fears that I may cease to be," the author John Keats reflects on what would happen if he died today. In that respect, he mentions he still has a lot of books to write. Thus, he compares the works in his mind to wheat grains, as if they were the natural outcome of his brain.