Butternut and all other winter squash varieties, such as acorn, pumpkin and spaghetti, can be frozen for up one year at 0 F before quality starts to diminish. Butternut squash can be cubed or mashed before freezing to accommodate different recipes.
Prepare butternut squash by peeling it and removing seeds and pulp. Cut cubed squash into 1- to 2-inch pieces, and lay the pieces in a single layer on a baking sheet. Freeze the cubes for several hours, and transfer to freezer-safe, airtight bags or containers for final storage. When freezing mashed butternut squash, allow it to cool completely before transferring it to the freezer bags or containers, and allow at least 1/2 inch of room at the top of the container for expansion. Label containers with the freeze date using freezer labels and a permanent marker.