Whether you save your own or buy them, as a gardener, you’re going to have to deal with older-than-ideal seeds. Some seeds are notoriously hard to germinate anyway, but put a year or two on them and notorious can seem impossible. I can’t claim 100% success with the methods below (any gardener who makes a 100% claim about anything in the gardening realm is a mendacious, untruthful, dishonest, duplicitous fibber!) but they often do work.
Create the ideal environment. With fresh seed, you can get away with less than ideal conditions; toss some seed at the ground, sprinkle on a little water and stand back. With older seed, perfection matters. Look up the ideal germination conditions for whatever it is you’re trying to grow and figure out how to create those conditions. When the directions say to scarify or cold-stratify or soak overnight, don’t skip that step; you can’t get away with it when you’re dealing with older seed.
Tinker with the temperature. If it’s 100 degrees in the shade and you’re trying to get lettuce going, it’s a losing cause to direct seed. Odds are the seed won’t sprout until fall, when you’ve already said the heck with it and planted something else in that space. Although lettuce seeds supposedly germinate best at about 75 degrees, it’s a very narrow window. Once the temps climb a mere five degrees to 80, the seeds sulk and refuse to germinate. Place them about two inches apart on a moist paper towel, roll up the towel and put it in a plastic bag, then germinate in the refrigerator for about four to six days. Rather than disrupt the tiny root hairs, I then cut the paper towel in small squares and plant seed plus towel.
Cover them. If you just HAVE to plant in place for some reason, water seed in well and cover the soil with a board, burlap or anything else that will help keep the soil cool. I’m not particularly fond of that method because you have to watch it like a hawk and take the board off the minute the seeds germinate. And in my garden, the pill bugs and sow bugs congregate under the board and find the seedlings before I do.
Treat with bleach. You can also try soaking the seeds in 10% bleach (1 part bleach to 10 parts water). Soak the seeds for two hours at 40-60 degrees, then drain and rinse four times with plain water. This speeds up the germination process and increases germination rates.
Add water. Older seeds are often drier than fresh seed. Try soaking them overnight (but no longer, or they may rot instead of sprouting). You could also make sure the soil is thoroughly soaked and then keep it wet for the first 24 hours. Don’t keep watering heavily every day, as you’ll encourage rot and damping off rather than germination, but do water if the soil seems dry. The idea behind these techniques is to make sure the seed is fully hydrated, which promotes germination.
If all else fails and it’s the last available seed of something you really want to grow, direct seed in an out-of-the-way corner or pot and just leave them. Remember, lots of plants manage to get themselves growing without human intervention. It might be next spring, but you could very well find yourself with some seedlings.