High temperature destroys seed viability once it climbs above 104-120°F (40-49°C) for any length of time. Seeds exposed to these temps suffer damage that no amount of good storage can fix afterward. Even short bursts of extreme heat can kill the embryo inside and leave you with seeds that will never sprout.
I learned this the hard way one summer. A box of tomato seeds sat in my car trunk for just one afternoon while I ran errands. Temps inside that car hit well over 130°F. When I tested those seeds later, only 12% sprouted. The same batch stored in my house showed 88% germination. That one mistake cost me nearly a whole year of saved seeds.
Heat causes heat damage seeds at the cellular level in ways you cannot see or reverse. The proteins inside each seed start to break apart when temps climb too high. DNA strands that hold the blueprint for the whole plant get damaged or destroyed. The enzymes needed for sprouting lose their shape and stop working. Once this happens, the seed is dead even if it looks fine on the outside.
Utah State Extension warns gardeners to keep seed storage temps below 70°F (21°C) for best results. They note that every 10°F increase doubles the rate of seed aging. PMC research backs this up with studies showing that seeds stored at high temps lose viability faster than those kept cool. The damage stacks up over time even at temps that seem safe.
Your maximum temperature seed storage should stay well below the danger zone to give seeds the longest life. Aim for 35-50°F (2-10°C) if you can manage it with a fridge or cool basement. Room temp around 65-70°F works fine for seeds you plan to use within a year or two. Never let storage temps climb above 80°F for more than a few days at a stretch.
Watch out for spots that heat up during summer months. Garages can hit 100°F or higher on hot days even with the door closed. Garden sheds bake in direct sun and offer no protection for seeds. Cars turn into ovens within minutes and can exceed 140°F inside on summer days. Attics trap heat and make terrible seed storage spots year round.
I also lost a batch of bean seeds I had stored in my garage one summer. The temps in that space stayed above 95°F for weeks during a heat wave. Those seeds dropped from 90% down to 45% viability in just that one season. Now I keep all my seeds inside the main house where the air stays cool.
Use a simple thermometer to check your storage spot during the hottest part of summer. Digital units with memory can show you the high and low temps over time. Check readings in the afternoon when indoor temps peak. Any spot that climbs above 80°F needs to be swapped for somewhere cooler. A closet in the middle of your house usually stays cooler than outer walls.
Move seeds to your fridge if you cannot find a cool enough spot in your home. The main compartment stays between 35-40°F and keeps seeds safe from summer heat spikes. Pack seeds in airtight containers with desiccants first. This protects them from the humidity inside the fridge. A small corner of a shelf holds enough seeds for most home gardeners.
Read the full article: 3 Essential Rules: How to Store Seeds
