We do not recommend canning summer squash or zucchini. We recommend freezing summer squash or zucchini.
USDA has withdrawn recommendations for canning summer squashes, including zucchini, that appeared in former editions of So Easy to Preserve or USDA bulletins. The reason for withdrawal is uncertainty about the determination of adequate processing times. Squashes are low-acid vegetables; they require pressure canning for a known period of time to destroy the bacteria that cause botulism. Documentation for the previous processing times cannot be found, and available reports do not support the old process. Attempts to reproduce the old process did not result in adequate heating to ensure safety. Slices or cubes of cooked summer squash will get quite soft and pack tightly into the jars. The amount of squash filled into a jar will affect the heating pattern in that jar.
It is best to freeze summer squashes or pickle them for canning, but they may also be dried.