^{(Photograph = Barrel cactus fruit and seeds,  Desertoirisebotanicals.com)}

Introduction

Some seeds, like avocado seeds, don’t last too long. They can dry out and die in a few months or a year. Other seeds retain viability for years; they are orthodox.

The Details

Orthodox seeds are seeds that can survive a long time when dry and cool. In contrast, if you dry an avocado seed it will die. Orthodox seeds are often small, sometimes tiny and sometimes large (like corn). But, all orthodox seeds can survive for a long time. Cactus seeds are orthodox and so survive years if treated properly.

One rule of thumb for estimating the longevity of orthodox seeds takes moisture and temperature into consideration. For each 1% you can lower the amount of water in an orthodox seed, you can double its lifespan. Thus, a seed with 14% water that is dried to 6% water will live longer, by a factor of seven doublings. That is about a 125-fold increase in lifetime. If an orthodox seed will naturally survive for 1 year, then it will survive for 125 years if you dry it to 6% moisture content.

The second part of the rule states that orthodox seeds will double their lifespan for each 10F that you lower their temperature. If seeds start out at 100F and you drop the temperature to 90F, it will live twice as long. If you cool the seeds to just above freezing, their lifetimes will be increased by a factor of six doublings (about 60 years if their original lifetime was 1 year).

The really interesting thing is that the results seem multiplicative. If you performed the two operations  above  (lowering moisture and reducing temperature) you could increase the life of orthodox seeds by 125 × 60. That is 7,500! Theoretically, you can increase the longevity of orthodox seeds by thousands of years.

You can keep dropping the temperature and seeds may be viable for tens of thousands of years. But, no one has done the experiment, so we don’t really know. Cactus seeds in the desert are subjected to winter rains and summer heat, and they probably only live a few years in desert soils.

Keep those cactus seeds cool and dry and they will last a long time.

Additional Reading: Temperature and Seed Storage Longevity

Additional Reading: Longevity of Cryogenically Stored Seeds