Heirloom Expert: Avoid Cross-Pollination

These tricks and resources can help you keep your heirloom crops from cross-pollinating.

Flower bed and hoe

Heirlooms can be planted right next to each other without worry of this season’s fruit being affected.

Photo by Fotolia/asferico

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Is there a list anywhere that has the spacing requirements between plants so they will not cross breed? I know tomatoes need 15 to 20 feet from each other, but what about squash, beans, melons and other garden plants? I have started an heirloom garden club and we would like to know how to keep our garden an heirloom garden.

I assume you're saving seeds, since that’s the only reason to worry about cross-pollination. Heirlooms can be planted right next to each other without worry of this season’s fruit being affected; it’s the harvested seeds which might not grow true.

Susan Ashworth’s Seed to Seed is a good primer as is our own Jere and Emilee Gettle’s The Heirloom Life Gardener.

There are lots of resources online which will give you the distances between similar species including the USDA. In fact, everyone who grows seeds for a living has a different distance for isolation and different ways to keep their seed pure.

With the distance between corn cultivars being 2 miles, and 5 miles for spinach, it’s impractical for a gardener or small farm to use distance as the only way to isolate crops. But there are other ways to isolate crops so they don’t cross-pollinate. Growing one variety of each type of plant is one way, but that’s not very exciting. Grow varieties in their own separate screened-in cages, cover individual flowers with bags or time planting so that different cultivars don’t flower at the same time.

I can only speak for myself when discussing tomatoes. Since they are self-pollinating, I don’t worry about crossing. It happens sometimes as an insect forces its way up into the flower, but most of the tomato seed I save looks just like their parents.

Letting crops cross also might create a new and improved variety. Anyone who has a compost pile loves the surprises which sprout up from the warm organic material. The plant might be unidentifiable, but could also be wonderful.

Once cross-pollination issues have been solved, the seed saving is pretty easy.

Open-pollinated varieties, like most heirlooms will produce seeds similar to their parents. Hybrid seeds will not. That’s why most seed saving is done with open-pollinated varieties. The only way to know the difference is to read the seed packet or plant label.

When I save seeds from any plant, I study the variety to see when and how it delivers seed. In most cases a plant flowers as a way to produce seeds and each one does it a little differently. When the plant is about to discard seeds, that’s when the gardener swoops in to collect them.

Part of the seed-saving experience is selecting the best varieties. For tomatoes we save seed from the first fruit to perpetuate earliness. For something like lettuce, the last plants to develop seeds is chosen. We want the plant which bolts last in order to select for that trait. There are lots of other factors to consider like production and vigor, and it’s fun to see which plants will produce the best seed.

I store my seeds in paper envelopes which are in turn put into airtight mason jars. The jars have a little bit of silica gel in the bottom to keep the environment dry. They are stored in a cool dark place until ready to be planted.

You’ll be surprised at how high the germination rate is. Most of my seeds are above 90 percent.

There’s a simple way to test seeds to see if they are still viable. Take 10 seeds and wrap them in a moist paper towel; put this into a plastic bag. Store the bag in a warm place and check to see how many seeds sprout after several days. If more than 50 percent germinated, keep the seeds; if it’s less, you’re better to start with a new packet of seeds.

Saving your own seed is a worthwhile endeavor. All it requires is a passion for learning from season to season. 


Doug is the Pittsburgh Post-Gazette’s Backyard Gardener and co-host of The Organic Gardeners radio program on KDKA.