Plant Populations and Isolation Distances

A version of this article I wrote appeared in the August 2009 issue of the Seeds of Diversity magazine. It is a summary of a couple of key points from the March 17,  2009 principles of seed production course held by the Eastern Canadian Organic Seed Growers Network.

John Navazio taught the course. He is the Seed Research and Education Specialist for Organic Seed Alliance and Washington State University. The two aspects of seed production he focused on were


Seed growers need a large enough population of plants to maintain genetic breadth and resiliency. Populations that are too small will gradually result in varieties that no longer perform as well.

In addition, seed growers also need to keep plant populations far enough away from each other to make sure different varieties of a same species do not contaminate each others’ genetic makeup.

Both population size and isolation distance are a function of how a crop pollinates itself – whether it is predominantly self pollinated (selfers) or cross pollinated (crossers).


Selfer flowers have petals that completely cover the plant’s reproductive parts, making it difficult for insects to get to the pollen and carry it elsewhere. Selfers have evolved  to not suffer from inbreeding depression. They do not need large population or isolation distances.

For the highest quality seed, you should save seed from no fewer than 12 to 16 selfer plants, preferably 50 to 80. Since selfers can still cross-pollinate, most need 80 –150 ft of isolation distance.

The main selfer crops are: Solanacea crops (tomatoes, peppers, eggplants), Fabacea crops (beans and peas), lettuce, and many of the staple grains (wheat, oats, barley, rye)


Almost all other crops are crossers. Crossers have evolved to encourage fertilization from different plants, and most suffer from inbreeding depression.

Crossers need much larger populations and isolations. John Navasio recommends saving seed from at least 64 to 80 plants of a crosser variety, preferably 200 plants. Insect pollinated crossers should be separated by 1-2 miles. Wind pollinated crossers should be separated by up to 5 miles.

It is a much bigger undertaking to produce seed from crossers than selfers.


Cucurbit (squash, cucumbers, and melons) fall somewhere between the two groups.  Cucurbits are technically crossers. They need the same large isolation distances. However, Cucurbits are very resistant to inbreeding, so only need population sizes similar to those of selfers.


John Navasio highlighted that the exact population size and isolation distance you use depend on your seed production goals. If you grow mainly for your own use, you can afford to have fewer plants planted in closer proximity to other varieties of the same species. If you intend to maintain and improve a variety, John stood steadfast by his recommendations. Especially for population size – John has found that the best varieties he has seen were developed from large populations.


ECOSGN is hosting a Seed Workshop in Ottawa on April 10-11.

The Eastern Canadian Organic Seed Growers Network (ECOSGN) has been meeting since March 2008. It’s goal is to foster a community of seed growers and seed sellers who can protect and enhance an economically viable and ecologically sustainable organic seed supply for Eastern Canada. ECOSGN strives to do this through education, shared resources, research and a united political voice.

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