Preventing Cross Pollination in Seed Plants
Seed growers should accept that many plants left to go to seed look really ugly and weedy. These plants also take up a lot of room, and the tall ones tend to blow over in a strong wind. So, if you can put aside your need for a garden that looks like it belongs in a gardening magazine, and still want to save seeds from plants like brassicas, lettuce, and spinach, read on…
Genetic diversity is simply the degree of variability in the genetic makeup of a plant or animal. A species of plant with a high genetic diversity will tend to produce offspring that may or may not resemble others from the same seed stock. But, when the environment they are living in changes, at least some of these seeds will survive and grow well. Species with low genetic diversity could become extinct if the conditions they are growing in change.
Self-pollinating plants like tomatoes and beans have a low genetic diversity, and growing three or four plants is often enough from which to save the seeds. But plants that need insects or wind for pollination are more genetically diverse. Many more plants need to be grown to save seeds of these types. It is very important to grow as many plants of one variety as you can manage, so that it will be able to adapt to changing growing conditions. For example, if you are growing carrots and you choose the five best plants to save seed from, and continue to do this for a few years, you might find that as environmental conditions change these carrots don’t grow very well anymore. But if you had saved seeds from 20 or 30 plants of the same variety, this wouldn’t be a problem – the poorly adapted plants don’t grow, but the plants that express genes for the new conditions do. Home gardeners should try to grow as many plants as possible for seed crops. Two or three radish plants simply aren’t enough unless you intend to save seeds just for your own use. If you are trying to develop a land race (plants adapted to your soils and climate) or produce seeds commercially, you would need to plant between 20 and 200 plants of each variety of insect or wind pollinated types, depending on the species.
Sometimes even pure seeds can produce plants that are not typical of the variety. If they are very different, they should be pulled out before they bloom. This practice is called roguing. This helps eliminate seed plants with unwanted characteristics, like spinach that bolts immediately. Of course, the flowers can just be cut off until the plant is big enough to harvest if you just want it for food.
Most gardeners and farmers like to experiment with new varieties, and rarely grow only one type of corn, tomato or brassica. If you want to save seeds, however, you should know how to make sure one variety doesn’t cross-pollinate another variety. If this should occur, you will likely have a hybrid plant when you grow out the seeds.
Even if pollen from a different plant fertilizes the flowers, its fruit or pods will have the same growth habits and taste as the variety you planted. But the following year, your saved seeds could produce plants that are very different. This is because the fruit from the ones you planted have the same genetic material as the plant itself. But the cross-pollinated seeds within the fruit don’t.
Self-pollinating plants can fertilize themselves, unlike plants that need more than one plant to produce fertile seeds. It is relatively easy to prevent crossing in self-pollinators, like peppers, tomatoes, beans, peas, and several types of grains. It is unlikely that a heritage variety of wheat, for example, will cross-pollinate. But many self-pollinating types of plants can be fertilized when bees are working on the flowers. To prevent this, you can isolate some of the flower buds or the whole plant with a mesh bag or cage to prevent insects from transporting pollen from other varieties. Because these plants self-pollinate anyway, you will still get fruit and seeds from them.
If you want to save a large quantity of seeds from a self-pollinating variety, you can plant one variety in a block of say 30 plants, and just harvest fruit for seeds from the middle. Or you can plant rows of different varieties with a row of plants that pollinators prefer in between each row.
Plants Needing Wind or Insects for Pollination
Other types of plants cross-pollinate easily. For example, one variety of corn will cross-pollinate another variety, and the same with spinach, broccoli, and many other types. For these, you can’t have more than one variety of a species blooming at the same time unless you take some precautions. You can isolate varieties by time, distance, alternate-day caging, and hand pollination. A brief summary of these techniques follows:
Time Isolation. If you want to save seeds from two varieties of a species, you can grow early and late varieties, or start one variety early, and then plant another variety as soon as the first variety begins to produce flowers. This should work fine if your neighbours aren’t growing the same species, for example corn, which could easily cross with yours if they are in tassel at the same time.
Distance Isolation. Another way to isolate your seed-saving plants is to grow them far from other types. Each type of plant has different isolation distances, from 8 m (25 feet) for lettuce, to 8 km (5 miles) for spinach and Swiss chard. For the home gardener living in town, saving pure, named varieties of seed from spinach in the outdoors would be very difficult if you used distance isolation alone.
Alternate Day Caging. If you want to grow two varieties of a type of plant that needs pollinating insects at the same time, you might try alternate day caging. You will need a cage or cages to cover all of the plants in a row of one variety before they start to bloom. You can make cages by constructing a wood frame and covering it with tulle or spun polyester cloth (row cover).
As soon as this variety starts to flower, you place the cage or cages over it. The other variety, assuming it is blooming as well, you leave open. That evening (after pollinators have gone to bed), take the cage off of the first variety and put it on the other one; the next evening switch them again. You will have to keep doing this until all plants have set seed. This won’t work if your neighbours have the same type of plant blooming, because bees and other pollinators can fly long distances.
Hand Pollination. For plants that need insects for pollination, you could try hand pollinating them to prevent crossing. First you protect ready-to-open buds of both male and female flowers, preferably on different plants of the same variety. The next day you pick the male flower and transfer pollen from it to the stigma of the female flower. After the pollen has been transferred, you would have to protect the female flower from receiving other pollen. This technique works well for plants like squash and melons.
Ashworth, Suzanne. 2002. Seed to Seed, 2nd Edition. Seed Savers Exchange, Decorah, Iowa, USA
Vegetable Seed Saving Handbook. http://howtosaveseeds.com/isolate.php
Village Garden Web. http://faq.gardenweb.com/faq/lists/tomato/2005025852004159.html
Cross-pollination: When pollen from one variety of a plant species is transferred to another variety by wind or insects. This results in a hybrid plant when the seeds are grown.
Genetic Diversity: The degree of variability in the genetic makeup of a plant or animal.
Hybrid: A variety or species resulting when two different plants reproduce.
Roguing: In agriculture, to remove plants of a particular variety that are not true to type.