|Zinnia 'Sombrero' from seed purchased in 1996!|
I acquire a lot of seeds, and most of them end up in the refrigerator. The seeds of many plants seem to retain their viability indefinitely if they are stored cold. I used to think "cold" meant frozen - some seeds stored in the freezer do remain viable for decades - and that's from my own experience. But seeds stored in the refrigerator at temperatures above freezing also last and last.
In the image above you see germinating seeds of Zinnia 'Sombrero' : those seeds were purchased in March 1996. That makes them over twenty years old (and they were probably a year old when packaged). As you can see in the image, they are giving good germination. And they germinated quickly: it took only four days for them to show signs of life.
Some of you might be horrified by the potting medium. I use unsterilized soil mixed from whatever is available. The pots stand outside in the fresh air and sunshine, exposed to whatever weather occurs. Fresh air and sunshine keep the bad guys in check.
Update, September 4, 2016: when these plants matured and bloomed they proved to be misnamed. They are zinnias, but they do not have the 'Sombrero' color pattern.