An heirloom plant, heirloom variety, or (especially in the UK) heirloom vegetable is a cultivar that was commonly grown during earlier periods in human history, but which is not used in modern large-scale agriculture. Many heirloom vegetables have kept their traits through open pollination, while fruit varieties such as apples have been propagated over the centuries through grafts and cuttings. The trend of growing heirloom plants in gardens has been growing in popularity in North America and Europe over the last decade.The definition and use of the word heirloom to describe plants is fiercely debated.
One school of thought places an age or date point on the cultivars. For instance, one school says the cultivar must be over 100 years old, others 50 years, and others prefer the date of 1945 which marks the end of World War II and roughly the beginning of widespread hybrid use by growers and seed companies or industrial agriculture. Many gardeners consider 1951 to be the latest year a plant can have originated and still be called an heirloom, since that year marked the widespread introduction of the first hybrid varieties. It was in the 1970s that hybrid seeds began to proliferate in the commercial seed trade. Some heirloom plants are much older, some being apparently pre-historic.Another way of defining heirloom cultivars is to use the definition of the word "heirloom" in its truest sense. Under this interpretation, a true heirloom is a cultivar that has been nurtured, selected, and handed down from one family member to another for many generations. Additionally, there is another category of cultivars that could be classified as "commercial heirlooms," cultivars that were introduced many generations ago and were of such merit that they have been saved, maintained and handed down - even if the seed company has gone out of business or otherwise dropped the line. Additionally, many old commercial releases have actually been family heirlooms that a seed company obtained and introduced. Regardless of a person's specific interpretation, most authorities agree that heirlooms, by definition, must be open-pollinated. They may also be open pollinated varieties that were bred and stabilized using classic breeding practices.