Cross-pollination is when pollen from one plant variety fertilizes flowers of another variety, usually within the same species.
Learn more about cross-pollinating different fruit, vegetable and flower species in the garden. How to promote or hinder cross-pollination is also explained, with the benefits of each.
What is cross-pollination?
Most plants that bear flowers reproduce through sexual reproduction. This means each seed comes from an ovule of the mother plant that was fertilized with pollen from a male plant. These special reproductive cells, called gamete, are produced and released in flowers.
Cross-pollination is what happens when each gamete comes from different varieties within a plant species.
Cross-pollination is also called cross-fertilization.
- For example, male pollen from a ‘Gala’ apple tree variety will reach a flower on a neighboring ‘Golden Delicious’ apple tree variety and fertilize it.
This almost always happens between two different varieties of the same species.
- In rare occasions, two plants that are more distantly related might also successfully crosspollinate. This is called “hybridization”.
What is not cross-pollination?
Self-pollination is different from cross-pollination
Many species are capable of producing both male and female gametes on the same plant, sometimes even on the same flower. When these are able to bear fruit without any outside pollen, it means they’re self-pollinating plants (or self-fertilizing).
- Self-pollination is often considered the opposite of cross-pollination.
- Two trees of the same variety planted together is self-pollination, not cross-pollination.
Hybridization, or hybrid plants
Sometimes two different species within a family can bear fruit together and will have offspring. This is called hybridization or hybridizing, and the offspring is called a hybrid.
- For example, butternut and red kuri will result in fertile fruit. The new fruit, if grown from seed, will resemble a mix of both. It will be a new squash hybrid.
- Most types of citrus are hybrid.
In a way, hybridization is a successful cross-pollination between two different species.
- Hybrid plants may be sterile, but not necessarily.
- Sometimes traits (like new flower color or fruit size) are lost after a few generations. In other cases they’re stable and a new species has been created or bred.
Is cross-pollination necessary?
Cross-pollination is sometimes mandatory for some species to bear fruit.
In most cases, cross-pollination increases the harvest in number and fruit size.
Lastly, in some cases cross-pollination is completely unnecessary, or should even be avoided.
When is cross-pollination necessary?
Many fruit tree varieties require cross-pollination to bear the most fruit, but some absolutely require it. Indeed, on their own or if only planted with the same variety, they would never bear any fruit.
Fruit tree varieties that absolutely need cross-pollination to bear fruit :
- certain apple tree varieties – Spigold apple, Jonagold apple, Roxbury russet apple, Mutsu apple, and others which are triploid or polypoid.
- certain pear tree varieties – Catillac, Merton Pride, and more polypoid varieties
- all types of blueberry and bilberry
- certain citrus varieties – mandarin orange for example
Cross-pollination is also necessary if you’re breeding plants in the hope of discovering a new plant variety. It’s faster than hoping for stable mutations to appear from propagated cuttings or natural growth.
- Most ornamental flower varieties grown and sold in nurseries are patented plant varieties produced through cross-pollination.
Cross pollination possible to increase the harvest
Almost all varieties and species will bear a larger harvest if they are cross-pollinated with other varieties.
This is especially true for most self-pollinating varieties. Although they will bear fruit without any outside pollen, there are still advantages of cross-fertilizing them:
- larger fruits
- more numerous fruits
- fruits less likely to drop when stressed (heat stress or water stress)
- fruits aren’t “seedless” anymore though
Cross-pollination not needed
Most self-pollinating varieties will bear fruit perfectly without any pollen from other varieties: their own pollen is enough.
- Sweet orange, ‘Temple’, ‘Lee’ and ‘Fallglo’ orange varieties and many other citrus
- self-pollinating apple, pear and other varieties
- Vanilla (an orchid actually) is self-pollinating
For some plants, the fruit appears even if the flower isn’t pollinated or fertilized. This is called parthenocarpy.
Of course, cross-pollination isn’t needed for all plants not grown for fruit or seed:
- leaf vegetables and root vegetables
- flowers for the garden and flower shrubs (check if you want berries from them though)
- fragrant vines and indoor plants
When to avoid cross-pollination
Sometimes it’s actually a bad idea to cross-pollinate because the harvest would increase too much. Fruits would stay small and branches may break off. Additionally, trees that bear too many fruits will age and perish within a few years. Over-pollination exhausts the mother plant.
This is a precaution you should take with:
- Pear tree, citrus and apple varieties that are vulnerable to over-pollination
- If over-pollination does occur, you can cancel bad consequences by thinning out fruit
The other main reason to avoid cross-pollination is to maintain a pure variety. This is especially important if you’re using your own seeds year after year. Squash, bean, carrot, are good examples of this: if you grow two varieties close to each other, it’s impossible to predict what their seeds will grow to become. It’s also relevant if you want to protect your seeds from genetically modified organisms (GMO) crops that are planted nearby.
How to increase cross-pollination
It’s easy to ensure and increase cross-pollination for your target species.
Plant several different varieties of the same species around your target plant
- Select at least one, ideally two, varieties that can cross-pollinate your target plant.
- Check that they share the same blooming dates and will survive in similar conditions like hardiness.
- Plant them as close as you can together. Usually pollen flows most easily within a few yards or meters. You can even graft two varieties together!
- Avoid obstacles like hedges or closed fences which may interfere.
- Select varieties that are excellent pollinators
- Note that younger trees bear few flowers, if any. You might need to wait three or four years for maximum cross-pollination.
More inventive solutions will benefit the community at large:
- Organize your neighborhood to plant different varieties in each garden. As a result, pollen and pollinators will cross over and bring pollen from one variety to the other. And each variety’s harvest can be shared around, hmmm!
- Many fruit trees have ornamental varieties that can be planted in public areas, like ornamental apple and ornamental pear. These will successfully cross-pollinate fruit varieties, too.
Attract cross-pollinators to the garden
- Most types of insects and butterflies in the garden are helpful in spreading pollen around. Protect butterflies and bumblebees from chemicals!
- Set up a beehive in the garden or neighborhood!
- Attract beneficial animals
- In larger orchards, work with beekeepers – they’ll be happy to rent beehives out to you to pollinate your orchards.
Note that bees love working along rows. Make sure your hive is set for the bees to easily “enter” into rows instead of having to “cross across” them.
Cross-pollinate by hand
This is what’s done to get prize vegetables like pumpkin and other vegetables! Male pollen from a massive father plant is used to pollinate the female flower of the mother plant to produce offspring that share the qualities of both.
For other squash, such as butternut or kuri squash, you may want to hand-pollinate flowers as well. Indeed, these gourds tend to have male and female flowers at different times, making self-fertilization difficult. If you’ve only got a few plants, hand pollination is the way to go.
In the vast world of iris, hand pollination is a good way to ensure proper parentage.
Some flowers have exclusive pollinators. When the plant is grown where it isn’t native, hand pollination might be needed if the original insect isn’t there.
Use drones to pollinate
In larger orchards, drones can be used to pollinate trees. Drones equipped with special containers and nozzles spray pollen directly on top of flowering trees. Gravity and wind do the rest.
- Drone pollination makes it easy to cross-pollinate with a specific variety. All that has to be done is purchase that variety’s pollen specifically.
- Pollination with drones is getting price competitive in areas where bees are rare.
How to hinder 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
Pollen 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 regulated by certification or government agencies.
Keep pollinating insects away
A 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
Even 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 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.
- For example, the wood of most pear tree varieties is brittle. If you can’t take the time to thin fruits every year in season, it may be best to have varieties that don’t pollinate very well. That way, branches won’t break due to heavy fruiting.
Gardens with poor pollinators will not be attractive to pollinating insects.
- 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.
Smart tip about cross-pollination
Sometimes choosing the right varieties to pair can get tricky. When you buy a new plant, simply ask the horticulture store for help. Staff training and counsel will be invaluable!
Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Apple tree cross-pollination by Hans Braxmeier under Pixabay license
Two blooming apple trees for cross-fertilization by Hans Braxmeier under Pixabay license
Cross-pollinating insect home by mborgm under Pixabay license
Iris cross-pollination by Yonat Sharon under © CC BY 2.0