Mysteries of Pollination

A poorly pollinated pumpkin.

As home gardeners are becoming more aware of how important bees are to food production, it’s also raising questions of how the whole system works. This gives me the opportunity to review my horticulture and botany fundamentals, draw some sad looking pictures and help answer some of the mysteries. Understanding the concepts of pollination isn't crucial to vegetable gardening, but it will help you to understand why some crops sometimes don’t produce as expected.

We need to remember what parts of each plant we are interested in. If we’re after the leaves of our chard and spinach or the roots of carrot, beets and rutabaga, then the pollination doesn’t matter at all – usually we’re more concerned that the plant will bolt and go to flower, period. Bolting is when a cooler weathered plant gets too much heat, realizes it will be meeting it’s end soon and it send up a stalk, the leaves get tough and the stalk flowers and goes to seed. This means that we’ve lost our crop for the season. Anyway, what I’m getting at is if the part that you eat isn’t the product of a flower of some sort then you don’t have to worry about pollination. With fruits and vegetables we grow for the fruit or seeds (squash, corn and tomatoes), pollination is almost always needed. But, there’s a bit more to it.

The Male and Female Flowers of a squash plant:

Self-Pollinating Flower:

Excuse my poor drawings but I think you've got the picture. Here are pictures of the real thing:

Male Flower

Female Flower

Pollen is produced in the anthers, which is the male part of the flower and it must be moved to the female part which is called the pistil or stigma. The base of the pistil is the ovary and this is the part that develops into the seed or fruit that is eaten—tomatoes, squash, cucumbers, corn, eggplant, etc. It is the pollen that needs to move from the anthers to the pistil and this happens in one of three different ways:

1. The pollination of tomatoes, beans and peas is pretty straightforward because they are self-pollinators and they do not need any help from insects or wind. These plants are self-fertile and don’t require pollen from another plant. This can involve pollen from blossoms on the same plant, or it could involve pollen transferred from the male portion of a blossom to the female portion of the same blossom, sometimes without the blossom ever opening.
2. Corn is pollinated a bit differently. Its pollen is carried by the wind from the tassels overhead down to the silks on the ears. If this process doesn’t happen the result will be small ears with empty rows and missing kernels. Corn should be planted in blocks so that the silks have a good opportunity to be pollinated. If the corn is planted in a single row, its chances of getting pollinated properly go down significantly. I planted my corn in a row but I’m planning to hand-pollinate each ear. I’ll share more on that when my tassels start to develop more.
3. Cucurbits (squash, pumpkins, melons and cucumbers) are insect-pollinated – bees and bumble bees are the number 1 helpers here. These are the types of plants that have the male and female flower parts in separate flowers but on the same plant. Insects transfer the pollen from male flowers to female flowers while collecting nectar and pollen. This means that you will need to have male and female flowers open at the same time to get fruit production. It’s easy to tell the difference between the male and female flowers – all that you need to do is look at the base of the flower, if it has a thick and bulging stem then it is a female and the male flowers have a thin stem. Believe me - you’ll be able to tell the difference.

Four bees on one squash flower.

Bees can often be seen on plants that are wind- and self-pollinated, while they aren’t necessarily helping the pollination process, they aren’t hurting it either, they are simply collecting pollen and nectar. You need to remember that the pollinating insects are so very important to the garden. This is something that you need to consider when choosing and applying insecticides. If you are not managing an organic garden, and even if you are, you should choose insecticides that are least toxic to bees and apply them late in the day when bees are not actively working in the garden. Remember that some organic solutions are just as toxic to bees and frogs, etc. Some pesticides are not harmful to mature bees but when the contaminated pollen and nectar are taken back to the hive, it kills the young bees and/or brood.

As for pollination problems that exist outside of the realm of insects and wind, sometimes vegetables that are self- and open-pollinated can suffer from lack of fertilization due to high temperatures, not enough sun, and dry conditions. In these situations, pollen (which is a protein) does not behave normally and causes a lack of fruit development or deformities in fruit. Poorly shaped fruit is usually the result from incomplete pollination.

I know that some gardeners, I have been one of them, have concerns about crosspollination between different vegetables. In most cases, I’ve learned that this is an unnecessary worry. Different varieties of open-pollinated fruits and vegetables may cross but different fruits and vegetables won’t. The zucchini, summer squash, pumpkins, acorn squash and ornamental gourds are all closely related and can cross-pollinate if planted closely together. Also, different varieties of corn can easily cross and create speckled ears with a variety of colors or may even make them inedible. However, in most cases, crosspollination is really only of concern to gardeners who save their seeds. If the cross pollination is a legitimate issue for you, you should purchase fresh seed every year. For self-pollinating fruits and vegetables, this isn't an issue - or so I've heard.

I hope this clears up some of your questions.

Happy pollinating,

Daphne