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That is, these are sources for seeds you can save from year to year, provided in the cases of corn and squash, that you haven't got other varieties right close by. (Peas, beans, and tomatoes are the easiest to save, because they mostly self-pollinate and thus don't trade genetic info with nearby alternate species.) Generally, pick several of your best, earliest fruits to save seed from. Separate seeds from pulp, rinse in a strainer, and lay out on a dry paper towel, labeling it in permanent marker with the varietal name. Once dry, put into a small paper bag or a baggy with a dry paper towel or one of those silica packs from vitamins or a pill bottle. Keep in a dry place with steady temps (mine do fine in my house, honestly, but some people say you need to refrigerate seeds).
Now, who's got these?
Best for vegetables is Baker Creek Heirloom Seeds, started by an Amish high school boy, now in his late 20s. The Mennonites left Europe and settled in groups all over the world--Russia, El Salvador, Brazil, Thailand, Mexico. This has given Jere Gettle enormous access to a world-wide stock of heirloom vegetables. Occasionally his seedsmen (seed-people?) send a "sport" or two (genetic variant) among the plants, so start several of each seed and if you don't like the "sport", pull it before it cross-pollinates. (Same goes for all these, especially the heirloom potatoes you can grow from seed available at New World co; these often have "rogue" plants and you need to walk your rows and "rogue" them every few weeks, pulling, bagging and disposing of the bad ones, weak ones, sick ones.)
Three other places I often see Must-Haves for many veggies are John Scheeper's Kitchen Garden Seeds, Nichols Garden Nursery (which is, along with its regular stock, carrying seeds newly selected by heirloom plant breeders), Adaptive Seeds (many interesting heirlooms), and Territorial Seed Company. I tried Pinetree Garden Seeds last year and had poor germination and weak plants; however, this may have been because I had fungus gnats around from my VC bins; I am getting some germination from their last year's strawberry seeds held over to plant this year. (NOTE: Getting good germination now from last year's Pinetree Seeds for alpine strawberries.) I also like Underwood Gardens and Seeds of Change for their varieties and good photos.
For perennial flowers, you want to shop for seeds at Swallowtail Garden Seeds. Their strawberries, lily seeds, and perennial flowers just JUMP out of the ground.
I just bought seed from TheStrawberryStore. My seeds arrived TWO DAYS LATER, and I started them, along with seeds from Pinetree, Stokes, Gurneys, et al. Yes, the newer ones were this year's, the others were last year's. But the first to germinate were definitely The StrawberryStore. Some 10-year-old ones from Swallowtail Garden Seeds have also germinate!
OhioHeirloomSeeds is a nice operation and very generous with seed packs at low cost. Very nice people running this little company. Seeds arrived quickly and germinated well.
Bountiful Gardens is the handiwork of John Jeavons and his colleagues at "GrowBiointensive" out in California. Wide selection of grains (slightly different varieties than Baker Creek's). Quick delivery.
For beans, forget the Vermont Bean Seed Company and look in Canada, at Heritage Harvest Seed. I've never seen quite this much variety.
For sweet potatoes, try Sandhill Preservation Farm or Duck Creek Farms (WHAT variety these two places offer!) Sandhills also has other things--poultry and heirloom seed.
For potatoes, New World Seeds and Tubers is most interesting, having mostly varieties from Mexico and the Andes that GROW FROM SEED! One of their two principals is having major health problems but they are back online for now. I am also getting some potatoes from Tucker Farms. Another place is called Moose Tubers, but I didn't see quite all the varieties I wanted there (although they have many). Not recommending the latter two, as I haven't tried them yet. New World comes recommended by Carol Deppe in The Resilient Gardener, which means they are fairly swamped right now.
For turnips (so easy to grow, so easy to store long-term, and so good in chicken soup with potatoes and carrots), the best of all sources is Gourmet Seed International. Amazing variety, and their idea of a "packet" is a HUGE amount of seed! To store turnips, clean them up, cut the tops off right at the root, cut back the non-bulb root, dry off, and wrap in barely dampened paper towels (I conjecture the bleach in these helps with the preservation). Put in the bottom shelf or drawer of your fridge. Check every now and again to remove any that have rotted; a few will, but most will be fine for up to a year.
The best two places I have EVER bought plants are Petals from the Past (great fig trees, best of the online sellers) and Sam Kedem's nursery (specializing in hardy roses). I have also often used Gurney's for plants and seeds, Jung's and Schumway's for seeds, Stokes for seeds (absolutely the best DIRECTIONS for starting seeds; their catalog is a TREASURE for this reason), and Henry Fields for plants and seeds. All were fine, but you do get deliberately dormant plants and you do need to FOLLOW THEIR DIRECTIONS. When I don't find time to do things right, I lose plants from Gurney's and Henry Field's. When I pay attention, their things grow fine. Miller Nursery, on the other hand, has rarely sent me anything that survived or broke dormancy.
And that, after 35 years of "how much of my own food can I grow?" summers, is what I know about seed companies.