Think of your feed as you might a loaf of bread in your cupboard. Bread doesn't take very long to begin to mold. The air this time of year is extremely humid and moist. The air is also very warm as summer winds down. Hot and moist is a recipe for mold grow. I suppose putting a bag of scratch in the freezer might work if you can scoop out just want you need in portions.
If feed grows molds, it is potentially a very bad and very dangerous thing to feed. If your flock is too small to finish a bag of feed within a month, consider buying smaller bags.