Landrace Gardening: Don’t Worry About Plant Purity


This week I’m discussing three inter-related topics. Landrace gardeners do not worry much about plant purity because we believe that mixing varieties can promote hybrid vigor and eliminate inbreeding depression. We consider cross pollination to be desirable because it can create more vigorous plants. It may be prudent to grow different populations of the same species as distinct landraces if there are big differences in how they are used.

Heirlooms typically grow poorly in my garden. I believe this is because heirloom seed is often highly inbred. The term “inbreeding depression” describes a general lack of vigor which arises when a cultivar pollinates itself over and over again. Heirlooms typically have little genetic diversity because they have been inbred for up to 50 or 100 generations.

The large seed companies also offer highly inbred seed, but they partially solve the inbreeding depression problem by making hybrids. Then they market the seed as having “hybrid vigor”, which is characterized by the plants growing more robustly than the inbred parents.

The term “open pollinated” is another synonym for highly inbred, because a variety can only “breed true” if it has been significantly inbred. The intense inbreeding makes it possible to offer nearly identical seed year after year.

peasThe plants in a landrace population avoid inbreeding depression because they are genetically diverse and are allowed to cross pollinate. The crossing between different family groups generates naturally occurring hybrid vigor in landraces. The new genetic combinations that arise with each new hybrid allow landrace gardeners to practice survival of the fittest selection. In my sweet corn crop I grow as many as ten thousand new genetically unique plants per year. An industrialized farm might only grow one genotype per year so the crop ends up being essentially a field of clones. So much can go wrong when a farm is only growing one cultivar. So much can go right when a farm is growing thousands of different varieties.

Some crops like tomatoes might only have a natural cross pollination rate of 5%. Other crops like spinach have a 100% cross pollination rate. Either way the principle of survival of the fittest plant selection can be used. I get faster results with higher cross pollination rates. It only takes a few crosses to create huge diversity among the second generation of offspring. At the top of the post I included a photo of the different types of peas that were generated by one manual cross.

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