Wednesday, July 1, 2015

Isolation Cages: For the purity of seed

For seed savers, assuring that seed-producing plants yield true-to-type offspring can mean some careful planning depending on the type of crop. If you’re interested in maintaining a supply of seed from a particular variety, you’ll need to make sure that the plants bloom and the flowers are pollinated for the plants to produce viable seed. That process is fairly straight-forward for self-fertile crops such as lettuce, tomatoes, peppers, eggplants, and beans, since open-pollinated varieties in these categories generally aren’t fussy and will produce savable seed without special treatment. Many other types of plants readily share pollen from other similar plants. This cross-pollination introduces genetic traits to the crop and can change the outcome of the next generation drastically. 

Cross-pollination can happen when pollen is transported between plants by wind, carried by birds and insects, or other means. Corn and beets are two plants that rely on wind to move their pollen, where vegetables such as cucumbers, squash, melons, and pumpkins need insects to carry their pollen for successful reproduction. When factors such as wind and insects are introduced into the breeding equation, they bring with them a multitude of variables that come with the potential of cross-pollination from outside sources, and that can result in undesirable outcomes. To ensure seed purity, it’s imperative to limit pollen contamination. Blooming plants require separation from other flowering plants that could provide contaminating pollen and result in corrupted seed. Such plants can be isolated from one another by distance, time, or physical barriers.

Caging Crops 
Isolating individual flowers on large-blossomed plants such as squashes, melons and cucumbers is a practical way to control the movement of pollen. Home gardeners who save seed from these plants can wrap or bag selected flowers and hand-pollinate them by moving pollen with small paintbrushes, cotton swabs or tiny spoons. This is a great way to exclude insects that might cross-pollinate your seed crop from a compatible plant in neighboring yards or within the garden. The individual bagging method isn’t as practical with insect-pollinated plants that have clusters of tiny flowers like brassicas. Instead, an entire stand of plants can be isolated from cross-pollinating bugs with a screened-in cage.


This spring our seed production farm grew a number of broccoli and turnip crops in isolation cages. Our farm crew constructed simple rectangular frames around the plantings and covered them with micro-fine screen or gauzy mesh. Built of lightweight materials such as wood, PVC, or wire, these structures are large enough to accommodate entire stands of mature plants. Stretching the screen material over the frame allows light, air, and moisture through, but keeps insect traffic out. The extra-long sides of the mesh are buried with a layer of soil to seal the enclosure, while a zippered doorway allows access. In the case of brassicas like broccoli and turnips, a stand of at least 10-12 plants is sufficient to produce an adequate pollen supply to yield viable seed.

Flies—Not All Bad
With the plants secure from outside insects, the job is only half done. The crop still needs help moving pollen to fertilize the flowers and produce seed; without assistance from insects, these caged plants have very little chance of success. Enter the worker bugs. The simple fix is to introduce a population of insects whose sole purpose is fertilizing the flowers. It may be surprising, but common houseflies or blue bottle flies do the pollinating work in the cages. Chosen over other insects (such as bees) for their availability, economy and effectiveness, the flies will forage for nectar in the flowers, moving pollen as they visit the blooms. While bees may be less offensive to many folks, flies are easily obtainable any time of the year, inexpensive, and have a swift life cycle that makes it easy to coordinate with the blooming of crops.
The No-Fly Zone
For home gardeners, it’s possible to produce two or more seed crops simultaneously with a simplified caging method. A common technique is to cage all but one crop in bloom. Allow a full day or more for insects to work on pollinating that exposed crop, and in the evening when the bugs aren’t active, rotate the cages to expose another stand. This approach uses the existing insect population without introducing fly pollinators.

Saving seed from your own garden is as rewarding as it gets for the sustainable grower. It’s important to know some basics before you dive into a seed production venture, but it’s definitely worth the effort. Of course, you’ll select open-pollinated varieties rather than hybrids to save the seed. (For a quick refresher on plants that can dependably produce true-to-type seed, see our blog post on Open-pollinated vs. Hybrid vs. Heirloom: Plant 'em all!) Over the course of a few seasons of choosing the most vigorous plants to bloom and produce seed, you can develop your own signature strain of your favorite veggies that are perfectly suited for your unique gardening conditions.  

 Author: Kat B.

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