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Unless you live somewhere that’s perpetually sunny and warm (like my home state of Florida), you are likely welcoming the advent of Spring.  After months of nothing but a brown, barren landscape, it is delightful to see the world in various shades of green again. Punctuating the green, many trees and plants have started to flower.  

Bradford pears blooming all along the roadside

In my area, you can’t look in any direction without seeing trees exploding in an abundance of white blooms.  These Bradford Pear trees are among the first trees to bloom each Spring. As beautiful as the trees may be, it’s best to avoid being downwind of them.  Why? They smell horrible. The scent of Bradford Pear tree blooms has been compared to the smell of dead fish. I’d venture to guess that most of us expect flowers to smell sweet or, barring that, to lack any fragrance at all.  But why would flowers stink? If you’ve spent much time studying nature, you’ve likely discovered that there are few (if any) mistakes in nature. If flowers stink, there must be a reason.

What is the Purpose of a Flower?

A flower contains the reproductive organs of the plant
A flower contains the reproductive organs of the plant

The primary purpose of a flower is for plant reproduction. Flowers contain both male and female reproductive organs. Deep inside a flower is an ovary which contains ovules (or eggs)—the female gamete (sex cell) of the flower.  Pollen—produced by the male part of the flower—contains the male gamete. Plant reproduction occurs by pollination. During pollination, pollen, produced by the anther, is transferred to the sticky stigma.  Once there, the male gamete within the pollen migrates down to fertilize an ovule within the flower ovary. The fertilized ovule will grow and divide, eventually forming a seed. That seed contains everything necessary to form a new plant.

 If you look closely at a flower, you can often see pollen grains on the flower stamens
If you look closely at a flower, you can often see pollen grains on the flower stamens

There are two types of pollination: self-pollination and cross-pollination. Self-pollination occurs when pollen from the anther of a flower is transferred to the stigma of the same flower, or to another flower on that same plant.  Plants that employ this method of pollination include orchids, sunflowers, tomatoes, peaches, and peas. (The fact that peas are able to self-pollinate made them ideal for the experiments that Gregor Mendel performed that led to our understanding of genetics and heredity).  

Other plants (including grasses, dandelions, daffodils, and tulips) use cross-pollination and only permit fertilization of flowers by pollen from a different plant of the same species.  How does the pollen get from one plant to another? Some plants rely on the wind to transfer their pollen. If you’ve ever gone outside and seen a coating of pollen on your car or deck, you can thank a plant that relies on wind for cross-pollination. Many plants—including ⅓ of the world’s food crops—rely on pollinators.

Pollinators

While most of us associate bees with pollination, other insects and even small animals are important pollinators.  These creatures aren’t altruistic and don’t pollinate just to help flowers out; they receive something in return. Bees, butterflies, moths, birds, and even bats visit flowers to collect nectar and/or pollen for food.  As they stop at each flower, they inadvertently pick up pollen grains which they then transfer to another plant as they continue their quest.

Because plants depend on these pollinators for their survival, they use many methods to advertise their flowers.  Some flowers use vibrant colors, others use fragrance, and many use a combination of both. The methods plants use to attract pollinators depends on the type of pollinators they need.  Plants that rely on bees or butterflies for pollination typically have bright, fragrant flowers with “nectar guides” that point insects to the center of their flowers. Plants that depend on moths or bats for their pollination often have white flowers which are easier to see at night when their pollinators are most active.  Some flowers even lure pollinators with the promise of a mate as the following video demonstrates.

And now, to answer the question posed at the beginning:  why would a flower stink?

How to Attract a Fly

You know that old saying, “You can catch more flies with honey than with vinegar”? While that may be true, if you really want to attract a fly you offer something stinky.

While most of us consider flies a nuisance, they do serve a purpose.  They act to control other pests and are important decomposers that help recycle nutrients back into the environment. In fact, without flies we wouldn’t have chocolate (GASP). Why? Flies are the unsung heroes of pollination. In terms of rank, flies are second only to bees in their importance for plant pollination since more than 100 cultivated crops depend largely on some of the 160,000 species of flies.

And how does a flower attract flies?  It stinks.

Flies are attracted to the foul smell of rotting food.  Plants that depend on flies for pollination have flowers that produce the type of smell that flies can’t resist.  The scent of the Bradford Pear blossoms attract fly pollinators. Compared to bees, flies are much more tolerant of the lower temperatures found in early Spring.  Since Bradford Pears are among the first trees to bloom each growing season, it makes sense that they attract the pollinators that are most active at this time of year. Thankfully, their blooming season is short (only a week or two) so we don’t have to put up with their smell for long.

What Other Plants Depend on Flies?

Other plants have putrid-smelling flowers to attract their fly pollinators.  These include Jack-in-the-pulpit, paw paw, and the skunk cabbage (never would’ve guessed). And of course, no post about fly pollination is complete without mentioning the corpse flower.



Flies are also used to enable the reproduction of certain fungi.  While fungi don’t reproduce via pollination, they do employ spores.  During the damp, warm days of summer, unusual fungi begin to emerge in my flowerbeds.  These stink horn fungi are capped with a stinky slime that attracts flies. As the flies converge on the fungi, they unwillingly pick up the fungal spores and distribute them as they go about their day.

So in summary, while I won’t try to convince you to welcome the scent of Bradford Pear blossoms, I’m hoping that I convinced you that perhaps flies aren’t so bad after all. After all, can you even imagine a world without CHOCOLATE?

Further Information

Flower Dissections:

Flowers are easy (and fun) to dissect in order to identify their assorted parts. I like giving students a chance to dissect several different types of flowers so they can compare and contrast the ways distinct flowers are organized. You can use dissection tools, but you don’t have to; scissors, toothpicks, and a butter knife can get the job done. You can use flowers from your yard (even the flowers of so-called “weeds”). I typically scout the floral section of my grocery store for flowers marked down for clearance. Sometimes, you can even get flowers for free if you mention you’re using them for a science experiment. The following video demonstrates how easy it is to perform flower dissections. He not only walks you through the process, but even takes his time describing the role of each flower component. (He has a cool accent, too).

This video reviews the many ways flowers exchange food (or other benefits) for pollination.

Fly Pollination Facts

Meet the Pollinators: Flies

All About Pollinators: Flies

Flies: Pollinators on Two Wings

The Curse of the Bradford Pear: These Pretty Trees Can Be a Menace to People and the Environment

What purpose does a flower serve? How does pollination occur? What different ways do plants use flowers to attract pollinators? Why on earth would a flower stink? Lots of cool videos and a suggested science experiment are included. #science #flowers #plantscience #pollination #flowerdissection

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