WASHINGTON — The "Seeds of Change" come in a variety of packets apart from the show and the book. The Smithsonian's largest temporary show ever, it will have a bumper crop to follow:AmeriFlora in Columbus, Ohio, is producing an associate exhibition in cooperation with the National Museum of Natural History here, opening May 1992, and offering additional Midwestern material. The six-month exposition will combine major exhibits, with more than 50,000 live entertainers and a large-scale floral and garden competition. In addition, a collaborative form of the exhibition is scheduled at nearly a dozen Smithsonian Institution Traveling Exhibition Service (SITES) locales, among them the American Quarter Horse Heritage Center and Museum, Amarillo, Texas, (focusing on the horse's effect on Comanche Indians); the Brevard Museum, Cocoa, Fla. (an early Spanish explorers' site); and the Santa Barbara (Calif.) Museum of Natural History (highlighting the Chumash Indians). At Faust County Park (Chesterfield, Mo.), there will be a coproduction focusing on cotton, hemp, the Missouri mule, and tobacco - plants and animals vital to America's heartland. Also, a poster version of "Seeds" will go out around the world under the sponsorship of the United States Information Agency: 250 sets of 30 posters in diverse languages. Finally, SITES and the American Library Association have come up with a panel version of "Seeds of Change" that will appear in 60 libraries in 50 states.