Have you ever wondered why we usually only see the male blossoms of zucchini at farmers’ markets? Or why some flowers turn into cucumbers, while others fall off the vine? Or why a corn on the cob will have missing kernels on its end?
The answer to all this is pollination—or lack thereof.
And I’m here today with a little lesson on the sex life of squash.
What does it mean to be self-pollinating?
As a gardener, I’ve always been fascinated by how all the different plants in my yard flower and then fruit.
We commonly associate the term “fruit” with the fleshy, seedy, sweet, or sour parts of plants like bananas (which are, in fact, herbs and berries in the wild and wonderful world of botany), apples, lemons, and mandarins. But scientifically speaking, a fruit is the structure on a plant that disseminates seeds, including squash, cucumbers, beans, peas, peppers, eggplants, and tomatoes.
These vegetables are known as self-pollinating plants; that is, they reproduce via the transfer of pollen from the anther (male part) to the stigma (female part) of the same flower, or another flower on the same plant.
Self-pollinating plants do not have to receive pollen from other plants in order to produce fruit and set seed. (This is why they’re sometimes referred to as self-fruitful or self-fertile.)
Some plants, like tomatoes, grow with both male and female parts on each flower (known as “perfect,” or complete flowers).
Tomatoes can be pollinated simply by growing outside in the breeze, or—for greenhouse-grown plants—sitting near a fan or having the vines lightly rattled to help some of the pollen drop from the anther to the stigma.
Other plants, like corn, have separate male and female parts on the same plant that have to be pollinated by wind. Each corn stalk has (male) tassels and (female) silks, which are fertilized when the wind shakes some of the pollen off the tassels and onto the silks.
When this happens, babies happen—in the form of every fertilized silk turning into a corn kernel.
Missing kernels on an ear of corn means the silks weren’t fully pollinated, or there wasn’t enough pollen to go around (since a stalk may have three or more ears waiting to be pollinated).
Another type of self-pollinating plant is summer squash and winter squash (and all other members of the Cucurbitaceae family), which have separate male and female flowers on each plant.
Because of their “imperfect” (or incomplete) flowers, squash can’t produce fruit without a bee, butterfly, hummingbird, or other pollinator passing pollen from the anther to the stigma to stimulate fruit development. In other words, the male and female parts have to make physical contact in order to reproduce.
Flower sex isn’t all about the birds and the bees, however. Successful fertilization also depends on temperature, sunlight, hormones, and plant maturity. (See, your plant has to be in the right mood for it, too.)
Daily temperatures that are too high can affect the quality of the pollen (even turning it sterile). Heavy rain or overhead watering can also reduce the amount of pollen available (which is one good reason to opt for drip irrigation lines or soaker hoses to water your plants more efficiently).
How a squash plant produces fruit
“Why does my butternut squash start to rot before it grows any bigger?”
Or, “How come I have lots of flowers on my zucchini plant but never get any fruit?”
These are the questions that plague gardeners every summer, and they’re often blamed on irrigation, pests, or diseases, when in fact they could very well be blamed on the bees (or the insecticides that keep them from coming around).
You see, a squash plant has both male and female blossoms on its vines (in plant lingo, this is called a monoecious plant) that show up at different times.
In heirloom varieties of squash, the males usually appear first, growing in abundance on the end of long, thin stems. Females usually appear first in hybrid varieties of squash, but sometimes this is all dependent on the weather.
Long days and warm nights tend to favor male flowers, while shorter days and cooler nights favor female flowers. So you often see male flowers in the beginning of the season, a balanced mix in mid-summer, and gradually more female flowers as the season comes to an end.
Male flowers tend to be rather large and showy, flaunting their stuff before the females arrive. As in other facets of life—ahem—the males vastly outnumber the females. If squash could take out personals ads, there would be lots of them from single males looking for females!
Female flowers usually appear a week or two after the males, growing low to the ground and close to the vine.
They form with what look like miniature squash between the flower and the vine. This is the ovary, as the female always bears the baby. The ovary is essentially an immature squash awaiting pollination (fertilization) by the male flower. Without it, that baby will never grow beyond the size you see.
If the ovary is not pollinated when the female flower opens in the morning, the flower will close that evening, start to wither, and eventually fall off the vine in a few days. That means no squash will come, even if it looked promising at first.
If a bee (or other pollinator) does come around that morning, you have to hope that it does a good job of spreading the pollen around so you get some squash.
Bees land inside open male flowers to collect nectar and with all their activity, they also happen to gather pollen on their bodies.
As they buzz around the plant, keeping busy and doing what bees do, they may land inside an open female flower and unintentionally transfer pollen to it.
That’s right—your next meal wholly depends on these little creatures finding your squash flowers amongst thousands of other flowers in the neighborhood and unwittingly fertilizing them.
If there isn’t enough pollen to pollinate the female flower, the ovary on that flower won’t be fertilized. Since it sometimes takes many bees to pollinate your female flowers and turn them into squash, this is all the more reason to try to attract as many bees as possible to your garden with other beneficial, nectar-rich plants.
Why squash bees are important for squash pollination
Though any bee can pollinate your squash crop, there are actually specific bees that do a particularly good job at it: the so-called “squash bees,” a group of native solitary bees from two different genera (Peponapis and Xenoglossa).
Before Europeans brought honeybees to the New World, squash bees were busy helping in the domestication and production of squashes and gourds by Indigenous peoples throughout the Americas. They’re still out there, working diligently, but don’t get all the fanfare of honeybees.
Look in your garden a few hours after sunrise, and you might find male squash bees darting from flower to flower in search of mates. If they get lucky, they’ll meet female squash bees foraging at the flowers of squash, pumpkins, and gourds, their sole pollen hosts.
Assuming there’s enough of them, squash bees do such a thorough job of pollinating all available squash flowers that later visits by honeybees are deemed superfluous.
When bee activity is low, some gardeners will take the extra step of hand pollinating their flowers to help them along.
Why should you pollinate squash by hand?
When a female squash flower isn’t fertilized, the small fruit attached to it remains stunted. The flower shrivels and falls off. Within a few days, the fruit itself starts to shrivel and turn brown at the blossom end.
A lot of people think they’ve done something wrong when it’s simply a problem with pollination—a problem that’s completely preventable.
Even if you have a healthy bee population in your garden, you might try hand-pollinating a few blossoms just to increase your overall yield. You might be surprised at just how many flowers don’t end up getting pollinated naturally!
How to hand-pollinate squash
First, get yourself familiar with squash blossom anatomy.
On a male flower, the stamen is the reproductive structure in the center, consisting of an anther (the pollen producer) supported by a thin filament. This phallic anther is what you see covered in yellow pollen grains.
On a female flower, the pistil is the ovule-producing structure, consisting of an ovary (immature fruit) that supports a long style, topped by a stigma (a sticky orange structure in the center). This stickiness is what helps the pollen adhere to it.
Remember that male flowers grow on tall, skinny stems, and female flowers grow close to the vine with an immature fruit at the base.
During hand pollination, you’re simply mimicking the bees by collecting pollen from the anther and depositing it onto the stigma. Yes, it’s as easy as it sounds!
The ideal time to pollinate squash is in the morning as soon as your squash blossoms open (and temperatures are mild). They tend to close up by early evening, so you might spend several days pollinating by hand if you want to get to them all.
Hand-pollination method #1: Pick the male flower.
- Identify a male flower on the squash plant and make sure it’s fully open, or the pollen won’t be ripe. (You’ll know pollen isn’t ripe when you rub the anther with your fingertip and no grains come off.)
- Pick the male flower; you’ll be using it as your “tool” to fertilize the female flowers.
- Peel back (or strip off) the flower petals to reveal the anther. Gently rub the anther onto the entire surface of the stigma (of your female flower) until it’s sufficiently pollinated. Be sure to work quickly, as pollen only remains viable for a few minutes after it’s taken from the anther.
- Repeat with as many male flowers as needed to pollinate all the female flowers.
Hand-pollination method #2: Use a paintbrush.
- Using a soft, small paintbrush, lightly brush the anther of a male flower until the bristles are covered in pollen.
- Brush the stigma of a female flower a few times (as if you’re painting it).
- Gather more pollen from other male flowers with your paintbrush and continue “painting” stigmas until all the female flowers are pollinated.
Some people use a Q-tip or other cotton swab to hand-pollinate, but from personal experience, I’ve found that a lot of the pollen sticks to the fibers (resulting in not as much pollen transferring to the stigma).
Once pollination is successful—you’ll know in two to three days and it’s almost a sure bet with hand pollination—the ovary begins to swell and mature into a seed-bearing fruit. And within a few weeks, you can harvest that squash!
If pollination did not take, cut off the rotting fruit. It will never develop into anything, and leaving it on the vine is an energy drain on the plant (not to mention a landing pad for pests).
As for the male flowers? Well, if they’re not harvested for food or picked for hand-pollination, they die off soon after they open. So don’t let them go to waste!
What to do next with your squash harvest:
This post updated from an article that originally appeared on July 16, 2014.