The storage life of vegetable seeds varies depending on the type of vegetable. This page offers two downloadable vegetable seed storage life charts, one listing vegetable varieties in alphabetical order and the other by length of storage shelf life, as well as some general information about storing seeds.
As seeds age they eventually reach a point when they will not sprout when planted. The use by date listed on seed packets is not the point when none of the seeds will sprout but the point when the percentage of infertile seeds is likely to have risen high enough to make planting the seeds unviable. Seeds can be planted after the use by date has expired, but the further past that date the lower the percentage of fertile seeds.
Note that seed use by dates are only a rough guideline and are based on optimal storage conditions. Seeds exposed to damp or high temperatures will not last as long as the recommended use by dates.
Seeds can be used beyond their use by date. However, there are a couple of things that can be done when using old seeds to avoid unacceptably poor strike rates.
Plant seeds closer together than the recommended planting distances for the vegetable seed you are planting. Depending on the age of the seeds this could be as little as one-fourth of the recommended distance. If too many seeds sprout than thin out to the recommended distance.
To do this place a dozen or more seeds on wet paper towel and cover with cling wrap. After a few days check to see how many of the seeds have sprouted. If the strike rate is less than 50% then the seeds are considered unviable.
In place of cling wrap you can use another layer of wet paper towel, but the advantage of cling wrap is that the seeds can be more easily inspected.
TOP: Kale seeds sitting on top of wet paper towel and covered by cling wrap. BOTTOM: The same kale seeds six days later showing an almost 100% strike rate. This is an excellent way to test whether seeds that have passed their use by date are still viable.
To maximise the storage shelf life of your seeds it is important to keep them in a cool dry place away from direct sunlight. Seed storage life can also be enhanced by keeping seeds in airtight containers in the fridge.
It is also a good practice to do an inventory of your seed stock each year and throw out any seeds that are well past their use by date.