Seeds begin to die at temperatures above 108°F and are completely sterilized at 140°F (which usually happens in hot compost piles). However, it only takes consistent high temperatures over 90°F to affect the embryo inside a seed and lessen the chances of germination. Avoid storing your seeds in an attic or uninsulated garage, or inside a hot car on a sunny day. If ideal storage conditions below 40°F with less than 10 percent humidity aren't possible, keep your seeds in the coolest (and driest) part of the house, like a closet in a north-facing room or a dehumidified basement.