How to avoid cross pollination

Avoid and block cross-pollination

While it’s difficult to keep microscopic pollen from flying around, there are a few things that will reduce cross-pollination.

Increase distance to other varieties

Plant crops at a distance when there's a risk of cross pollinationPollen may travel far, but in many cases having several hundred yards or meters distance will already significantly reduce pollination.

  • This explains why for certain crops to be certified as non-GMO (genetically modified organism), distance is a critical parameter. The grower has to prove that the nearest GMO field of the same species is at least several miles or kilometers away. Distance greatly reduces cross-pollination. Minimum distances are sometimes regulated by certification or government agencies.
  • Seed growers are especially careful to avoid cross-pollination so that offspring remain true to the original variety. They often have fields that are at great distances from one another.

The greatest possible distance isn’t even in miles or kilometers: it’s time! Sometimes it’s best to only grow a single variety, and the following year to grow another. That way, you’re sure that the two won’t cross-pollinate. Seeds are usually viable for several years, so it won’t compromise your ability to grow.

Keep pollinating insects away

Use nets to lock pollinators outA net with fine mesh wrapped around a blooming tree will keep insects from bringing foreign pollen in. You only need the net during the blooming period. It also acts as an obstacle to wind-borne pollen.

You might also want to grow your special plant variety in a greenhouse that is closed to outside insects instead. This should block cross-pollination entirely.

Set up obstacles

Hedges slow wind that carries pollenEven simply setting up a garden hedge or wall will reduce the amount of pollen carried over by air and insects to prevent cross-pollination.

  • Continuous hedges cut pollination distance by half.
  • This means you can reduce the buffer distance by half by growing a thick hedge between two plant species or varieties you don’t want cross-pollinating.

Grow varieties that bear little pollen

Some varieties are known to be excellent pollinators while others produce little pollen, poor pollinators. Steer away from excellent pollinators if you fear your trees would get damage from over-pollination.

Apple & pear varieties that bear no pollen

For apples and pears, for instance, varieties known to be “triploid” won’t bear any pollen at all.

Plant low-pollen and no-pollen varietiesExamples of triploid apple varieties:

  • Baldwin
  • Belle de Boskoop,
  • Blenheim Orange,
  • Bramley’s Seedling,
  • Calville Blanc D’Hiver,
  • Cat’s head,
  • Gravenstein,
  • Holstein,
  • Jonagold,
  • Mutsu,
  • Ribston Pippin,
  • Stayman winesap,
  • and Winesap

For pears, triploid varieties include:

  • Belle Angevine,
  • Beurré Alexandre Lucas,
  • Catillac,
  • Merton Pride,
  • and Vicar of Winkfield

Gardens with poor pollinators will not be attractive to pollinating insects. In general, such trees and vegetables will tend to have seedless fruits.

  • If you’re severely allergic to bees, consider planting only plants that are poor pollinators. This will reduce the risk of having unintended bees buzzing around. Not to mention, less hay-fever triggering plants.

Cut and remove undesirable flowers

This solution is often implemented when trying to produce a very specific hybrid.

Remove flowers to keep cross-pollination from happeningFor example, most of today’s corn (maize) are hybrid seeds. These are a cross-breed between two different corn parents. One must be the mother, and the other a father: if their roles were reversed, the hybrid offspring would not be the one sought. In this case, since corn has both female and male flowers, the following is done:

  • Male flowers on the “mother” plant are removed before they can mature.
  • This ensures that only male pollen from the correct “father” plant reaches the female flowers.
  • Rows of “father” stalks are destroyed after pollination and fruit set so there’s no point in removing flowers from them.

It’s time-consuming work but the hybrid seed sells well: it produces a more vigorous plant than either mother or father plant!

Refresher: what is cross pollination
Opposites attract: how to increase cross-pollination

Image credits (edits Gaspard Lorthiois):
CC BY 2.0: Apple and Pear Australia Ltd, Katina Rogers
CC BY-SA 2.0: Alexander van Loon