Cross-Pollination

Is it true that melons and squash can cross-pollinate? If I plant a lemon tree too close to an orange tree, will the oranges be sour? You’ve heard of cross-pollination, but what does it really mean and do you have to worry about it? Let’s start by reviewing pollination.

Honey bee on salvia (Stuart Russell)

Pollination and fertilization

Pollination refers to the act of pollen, the male genetic information (gametophyte), moving from the anther to the (female) stigma. From there, the pollen grain grows a pollen tube, which makes its way down the style to the ovary. Two sperm cells (gametes) then move through this tube to fertilize female gametes. One male gamete fuses with a female gamete to produce an embryo, while the other fuses with a different type of cell, called a polar body, to form the endosperm, which will feed the developing embryo. [You can think of these two much like the yolk and white of an egg, respectively.] Since two fertilizations are actually occurring, it is called double-fertilization, but I digress.

Parts of a flower (Mariana Ruiz) Public Domain

Types of pollination

Pollination can occur one of two ways: self-pollination or cross-pollination. Self-pollinating flowers pollinate themselves. Cross-pollination, or allogamy, refers to the way pollen moves from one plant to another of the same species. Wind and insects, such as honey bees, are the main perpetrators of cross-pollination. Natural cross-pollination can only occur within a species (we will not discuss genetic manipulation at the nano surgery level).

To give you a clearer idea, consider this: Horses breed with horses. Donkeys breed with donkeys. When a female horse breeds with a male donkey, their offspring, a mule, is nearly always infertile. The same is true in the plant world.

This means that zucchini plants can cross-pollinate with pumpkins and other summer squash varieties, but not with melons or cucumbers. This is because squash and pumpkin are both members of the Cucurbita pepo species. In the same way, cucumbers (Cucumis sativus) cannot cross-pollinate with muskmelons (Cucumis melo). Their genetic information doesn’t match up properly. 

When cross-pollination within a species does occur, the offspring (seeds) are often useless, but it has no affect on the current season’s fruit or vegetable. The only exception to that rule is sweet corn. When varieties of sweet corn cross-pollinate, the current season’s crop will exhibit characteristics of both species. In nearly all other cases, it is the DNA found in seeds that is altered. If you save seeds for next year’s crop (and I urge you to do so), you will grow plants with characteristics of both parent plants. This is how we get many new cultivars with desirable traits or unique properties. The only way to prevent cross-pollination is to keep crops 100 yards or more apart, which probably isn't realistic in your home garden. You can reduce the chance of cross-pollination by keeping plants as far away from each other as possible.

The science of genetics owes its start to cross-pollination among common pea plants 
​and a central European monk, named Gregor Mendel, back in the mid-1800’s.
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As for those sour oranges, they were probably unripe, irrigated improperly, or victims of the Asian citrus psyllid, not the result of cross-pollination.