A type of seed that has been passed down through a number of generations; a type of seed that is traded between individual gardeners rather than through a seed catalog.
"Cohill points to the Moon and Stars watermelon as an example of the importance of saving heirloom seeds: The dark-green, spotted variety was feared lost until the Seed Savers Exchange finally pinned down a man who was growing it. Because of the seeds obtained from him, the melon has been brought back to the point that it's a commercial success, Cohill said."
Mary Beth Breckenridge, "Heirloom crops, flowers are often timeless treasures," The Charlotte Observer, October 18, 2001
A Wisconsin woman is looking for a white pole bean handed down in her family for 87 years until the seed was destroyed in a fire. An Oregon resident wants a bird egg bean dating back to at least 1918. A Missouri man offers to share seed from the blue banana squash.
And Kent Whealy is trying to bring together those who have seeds and those who want them. The 33-year-old rural Princeton, Mo., man grows a lot of the seeds himself on a 5,000-square-foot plot of land devoted to crops from heirloom seeds the kind you don't find in seed catalogs.
Barbara McMahon, "American Style: The Kind You Don't Find in the Catalog," The Associated Press, October 4, 1979
As the title of the above citation suggests, heirloom seeds are used to grow heirloom crops and heirloom flowers (also heirloom plants, heirloom trees, heirloom shrubs, and heirloom grains). The Seed Savers Exchange mentioned above was formed in 1975 to aid in the preservation and trading of heirloom seeds, so it's possible that the term is at least that old (I don't know if they used the "heirloom" adjective back then). However, the earliest print citation I could find is from 1979.
I found an older reference to the use of the word heirloom as an adjective for seeds, plants and bulbs. It is on page 30 of the book entitled, Pioneer American Gardening, compiled by Elvnia Slosson, copyright 1951, Published by Coward-McCann, Inc., New York.
The quotation is:
Seeds, bulbs, and plants are looked upon as heirlooms in Vermont, as respected and cherished as fine old glass or china. Mrs. Willard Eaton of Fair Haven has masses of poet's narcissus, a weigelia bush, a section of flowering almond, and an eglantine rose all inheritances from her grandmother's garden that was laid out in 1852.