Heirloom varieties date from before World War II. Established varieties are more recent (developed post-war) but have remained popular amongst gardeners for decades. Both have survived presumably because they are good. Plants valued as food or for their beautiful and complex scent are particularly often grown by gardeners. Perhaps their main feature is that they were selected by observation (empirically) rather than by scientific measurement. But let’s not get starry-eyed about old varieties; many weren’t very good and have been lost.
Most heirloom varieties date from a hundred years or so ago; only a very few are medieval or even pre-history. Many, like modern varieties, result from purposeful crossings, but are unlike modern varieties in that they were selected by gardeners, market gardeners and smallholders. Some heirloom varieties may not be native, for example, here in the UK we grow Ratte potatoes from France and Blauhilde climbing French beans from Germany. The Aquadulce group of broad beans derive from a medieval introduction from Spain yet are easily the most popular over-wintering variety grown in the UK; Real Seeds sells several heirloom varieties it has recently obtained from overseas.
Evidence for the popularity of heirloom and established varieties amongst gardeners is that there are many small seed companies thriving on sales of only these varieties. They often suit our needs better than ‘new-fangled’ varieties. This may be because, unlike commerce, gardening still uses pre-War practices. We still use mostly manual labour to grow our plants and harvest their fruits! We also still put priority on good taste. And heirloom varieties originally had to grow unprotected by modern pesticides – and most gardeners still value this ability.
Yet conditions have changed. Pre-WWII, stable manure was widely used, the climate cooler and CO2 (the main food of plants!) in the air was half its current amount. New pests and diseases have arrived or come to the fore. And we have changed, adopting/ bringing new habits and tastes. So our retention of heirloom varieties is also somewhat odd.
This webpage explains the important advantages and disadvantages of heirloom varieties