"MICHELLE H." wrote:

So does grass seed really go bad after its expiration date?

There are several schools of thought about that.

There are those that believe that commercial grass seed is treated with
pre-emergence inhibitors the concentration of which might depend on when
(during the year) the actual seeds were harvested. There are two
factors that are looked at when it comes to grass seeds and
germination: The actual rate of germination, and the percentage or
total yield. Some put more importance on the germination rate, and they
observe that older seeds tend to germinate faster - presumably because
the pre-emergent treatment has worn off or degraded.

The storage conditions of the seed will play a huge factor in shelf life
and viability. Seeds that are exposed to a few hours of high heat
(sitting in the sun in the back of a truck) such that the seed
temperature reaches 130 -140 F will be enough to cause significant
reduction in yield regardless the age of the seeds.

It's universally quoted that there is a germination reduction of such
and such percent with every year on the shelf, and such stats are
generally correct because of the uncertainty of the storage conditions
of any given sample.

Keeping seeds dry and refrigerated will at least double the shelf life,
and keeping them frozen will extend the life by a factor or 4 or 5. If
the seeds have been stored at ambient temperature (ie no special
cooling) the entire time they've been in storage, then putting them
through a freeze cycle for a few days prior to planting will increase
germination rate and possibly yield.