i started to work up some of my virginia seeds last night and the humidity is running in the 90s.
added some rice that was dried out in the microwave. That lowered the humidity sum.
So my question is how dry is dry enough for storage?
The drier the seed is in storage, the longer it remains viable. Pods of seed just hanging in a barn may last 3 or 4 years. Very dry seed can last well beyond 10 years.
I really don't know any moisture content or relative humidity numbers to go along with this.
My suggestion is to wait until the humidity is quite low, prior to collecting the seed from very dry pods. Then store it in a completely air-tight container. Cooling a closed container will increase the relative humidity of its contents.
For a typical home grower, a tablespoon of seed will be sufficient for a decade or more of planting, so a much smaller container can be used. I use tiny Ziplock bags, 1" x 3" (available in the arts and crafts section of Walmart or specialty hobby stores), for each variety that is refreshed, and place them, labeled with variety and year, into a freezer Ziplock, which then goes into a recreational dry-proof box into which I also place a packet of desiccant. These boxes are stored in a small refrigerator that I seldom open.
When the time comes to use the seed for another season, the chilled container should be allowed to come to room temperature before it is opened, otherwise the cold of the container will draw moisture out of the air, and condense it as soon as you open it.
It's a good idea to refresh the seed for each of your varieties at least every 10 years, more often if the seed or its storage conditions were not ideal. (At ARS-GRIN, where they hold well over 2000 varieties of Nicotiana tabacum, they refresh 10% of their varieties each year.)
In studies of seed storage in India, where refrigeration is sometimes not available to rural tobacco farmers, they found that seed stored as dry as possible would last over 10 years, regardless of refrigeration. We do know, though, that dry seed stored in a laboratory deep freeze (way colder than your home freezer) is likely to still be viable for 40 years or more.
,,,"I really don't know any moisture content or relative humidity numbers to go along with this. "...
In a perfect world, you want a 7% moisture content. But under 10% is generally good enough. The relative humidity of the air around the seed doesn't necessarily indicate the moisture content of the seed.
I leave the seed heads on our plants until well into October. I want them fullymatured and as dry as possibly before picking. I never pick earlier unless the pods are starting to split and dump seed. After picking, the seed heads go into 18 gallon totes or paper bags and come indoors to dry more. It can take up to a month for them to become fully dry. I often have fans running on them to help with drying. The pods should then feel like bone dry, out of case tobacco does. Crunchy dry when handled. Then they go off to be threshed, screened and blown. The seed gets bagged into 1 gal zip locks and goes back to our drying room where it sits with the bag openfor another 3-4 weeks before being sealed and going off to the storage room.
The key things in storing any type of seed is to keep it in a dry, dark, and cool spot. We store all our seed in sealed plastic bags inside heavy cardboard file boxes. The boxes help keep the seed from being exposed to light, and are stacked in rows and alphabetized so we can easily find things. We store everything in an unheated room which stays between 40f - 60f most of the year. It occasionally gets into the 70's on summer afternoons, but it doesn't seem to be a problem. Refrigeration would be even better for very long term storage, but we usually have a couple hundred pounds of assorted seed on hand in dozens of boxes, so it's really not practical for us. As long as the seed is kept dry and isn't exposed to excessive heat or light, tobacco seed should be fine where ever you store it for years to come.
I have find this interesting study, it is old but very useful.
The summary is that seed stored in very dry air (a few point of UR) stay viable for decades indipendently from temperature and seed stored in cool air (a few degree Celsius above zero) stay viable for decade indipendently from UR.
Obviously cool AND dry is better
If you take a handful of rice and toast in a pot until it is light brown, let cool it, and put in a closed glass jar with the seed, you can achiew a steady 3 - 5 % of UR ever after a lot of opening and closing or much years if you keep closed.
I know this because I store my seed in this way putting the jar in the fridge. Simple dry AND cool.
Thanks, skychaser, for the revelations of a real seed producer. And thank you, ciennepi, for that article.
Desiccant packs are fairly cheap (not as cheap as rice), but are typically available only in huge quantities from most outlets (like uline.com). If you search coin collecting sites (like this one: http://www.jpscorner.com/silica-gel-humidity-control.html) you can find small quantities, but at higher per-item pricing.
I finally managed to track down the study from India on seed storage at ambient temperature: HERE.
Flue cured tobacco seed var. CTRI Spl and White gold of crop season 1975-76 was stored at
ambient temperature in 500g capacity container with and without desiccant, anhydrous calcium chloride.
Seed stored with 5-6% and lower than 3% initial seed moisture with desiccant (dry storage) maintained
viability and the moisture content was below 4.5% for a period of 24 years (March 2000) whereas seed
stored without desiccant (control) contained seed moisture of 6-7% and lost viability,in 24 months. To
adopt this method for tobacco seed storage in large scale CTRI Spl (MR) of crop season 1982-83 was
stored in 50 kg drums with desiccant (250 and 500g) at ambient temperature. and the seed remained
viable for 17 years (March 2000), Seed stored without desiccant lost viability in 24 months and its seed
moisture was 6-7%. Stored seed was tested in nursery for seedling production and it was at par with
fresh seed. It is concluded that storage of tobacco seed at ambient temperature using desiccant.'is
reliable and inexpensive method as compared to traditional method of cold storage for long term
tobacco seed storage.
Grain elevators will refuse to receive wheat that is more than 12% moisture content. That's because any higher than that and it will start to rot, which causes heating and a potential for an explosion. I've seen pictures of grain bins that did just that...Explode. People were killed and the mess was incredible.