Heirloom vegetables are the star attraction at farmers markets and on organic produce shelves in supermarkets. Open pollinated (OP) vegetables are garden favorites from which many gardeners save seeds for next year's crop. So what's the difference? And what qualifies a cultivar (a cultivated variety) to be called an heirloom?
Everyone agrees that all heirloom cultivars are, by definition, open pollinated. That means heirloom seeds will produce plants that are identical to the parent plants. Traditionally, what is called an heirloom is a cultivar that has been handed down through many generations, often within one family or in a relatively small geographic area.
Heirlooms have excellent taste, are easy to grow, and often have other worthwhile qualities.
Some gardeners insist that to qualify as an heirloom, a cultivar must be at least 100 years old. Others say 50 years, some say 25. There is no agreement on the period of time an heirloom must have existed to qualify for the name.
Introduction of Hybrids
There is growing support for the suggestion that an heirloom must have been in existence before the period around 1951. That is when hybrids---crosses between heirloom cultivars or open-pollinated varieties---were first introduced by the seed trade on a large scale to home gardeners. Hybrid seeds were not widely used in commercial agriculture until the 1970s.
Hybrid and Genetically Modified Seeds
Seeds saved from a hybrid plant will not grow "true"---that is, the plants will not be the same as the parent plant. Instead, they may resemble either one of the hybrid plant's parents, or an even earlier trait. Neither hybrids nor plants created as a genetically modified organism (GMO) can be considered heirlooms.
Open Pollinated Seeds
All heirlooms are open pollinated (OP), but not all OPs are heirlooms. Many excellent open pollinated cultivars have been created since the 1950s, but they are not generally considered to be heirlooms. Once a cultivar is stable---it breeds "true"---it can be called an open pollinated cultivar.