Your zucchini or squash plants are blooming profusely and you are looking forward to a bumper crop. But despite lots of flowers, the harvest may be meager. The culprit is usually insufficient pollination, which not only affects members of the cucurbit family but also strawberries, tomatoes, corn, and other common garden crops.
With the worldwide decline of pollinators, often the necessary insect populations just aren’t there (although there are many things you can do to attract bees and other pollinators). When natural pollination fails, you might have to resort to hand pollination.
What Is Hand Pollination?
Plants are either pollinated by wind, insects, or birds, and hand pollination mimics that natural process.
The way a plant is pollinated depends on its type of flower. In plants with perfect or complete flowers both male (stamen) and female (pistil) reproductive organs are contained in the same flower. Plants with imperfect flowers have both male and female flowers with either a pistil or a stamen but not both. Hand pollination transports the pollen from male flowers to the stigma of female flowers.
What Is a Flower's Stigma?
A female part of the flower. It is the sticky bulb that you see in the center of flowers and is the part where the pollen lands and starts the fertilization process.
There are three main purposes of hand pollination. Plants are pollinated manually to improve the crop yield when there aren't enough pollinators. Gardeners and farmers who save seed often pollinate plants by hand to prevent cross-pollination. Summer and winter squash varieties are often cross-pollinated by bees when grown less than half a mile from each other. To prevent this, after the pollen from the male flowers is manually transported to the stigma of female flowers, the female flowers are covered up with tape so pollinating insects cannot reach them. This practice ensures the developing plant and seeds are guaranteed to be true to type. Plant breeders also use hand pollination to create hybrids.
For most home gardeners, the purpose of hand pollination is to make up for the lack of natural pollination. Here are some techniques to improve the harvest in your garden.
Why Some Plants Need Hand Pollinating
The use of broad-spectrum pesticides and habitat loss has resulted in a widespread decline in pollinator populations. In some cases, however, the pollinating insects, which are often highly specialized, are simply not present when crops are grown outside their natural habitat. This applies to about 20 commercially grown crops, including vanilla beans, passion fruit, and date palm. To produce fruit, these crops need to be hand pollinated.
Hand pollination is also required for fruit-bearing indoor plants such as Meyer lemon trees and fig trees, as well as other potted fruit trees grown out of season when there are no pollinators around to do their job.
When to Hand Pollinate
Northern growers with short growing seasons face a greater challenge when pollinators aren't available at the right time. Once it becomes obvious a crop is not producing, it may be too late to remedy the problem for the current year. Keeping a close eye on the plants and watching for those first tiny fruits to develop will help. Increase your number of pollinator plants in the next gardening year and familiarize yourself with hand pollination techniques so you can jump in early if needed., Planting pollinator-friendly flowers is a good garden practice for every gardener and if you live in a climate with a long growing season, you also have the option of replanting a crop when the first harvest is insufficient.
How to Hand Pollinate
While the techniques of hand pollination vary, they have one thing in common: hand pollination is most effective in the morning hours when the humidity is high, which helps to activate the pollen.
Hand-Pollinating Tomatoes and Other Plants With Perfect Flowers
Strawberries, tomatoes, tomatillos, peppers, eggplant, okra, peas, and beans have perfect flowers. Transporting the pollen from the stamen to the pistil does not require insects, only some air movement or vibration. Theoretically these plants self-pollinate but you can also help them along by gently tapping the flowers or using a small brush or a cotton swab to move the pollen from the stamen to the pistil. If you have different tomato varieties, make sure to clean the brush after each plant, or use a fresh cotton swab, to avoid cross-pollination.
With strawberries, pollination through wind alone can lead to smaller or malformed berries so insects are needed for complete pollination. If there are no insects to do the job, hand pollination is the method to improve the strawberry yield. You can either transfer pollen from the stamens on the outside of the flower to the pistils in the center with your finger, or use a fine, small brush, or a cotton swap.
How to Hand Pollinate Squash
Home gardens often end up with something unidentifiable in the squash patch. This vegetable is particularly vulnerable to cross pollination and the result is usually odd-looking and inedible. Hand pollinating squash can also give you an earlier harvest than waiting for pollinators. To hand pollinate squash, follow these steps:
- Identify the male and female flowers. Male flowers have a plain stem below the flower and female flowers have the tiny rudimentary squash below the petals.
- Gently peel back the petal from the male flower to uncover the male anther that carries the bright orange pollen.
- Use a small paint brush or a cotton swab to pick up the pollen and apply it onto the stigma of a female flower of the same squash plant. You can use the same male flower to pollinate several female flowers.
How to Hand Pollinate Corn
Corn, which has male and female flowers on the same plant, relies on wind to carry the pollen grains from the tassels (the male organ) to the silks (the female organ) with each silk developing into a kernal that makes up the edible ear. Your homegrown corn benefits from hand pollination. For best results, however, this needs to be done within ten days after the silks emerge.
Cut off a whole tassel at the base and hold it over the silks. Shake it directly over the silks and brush it a little to break loose the pollen so it is distributed over all the silks.
Strawberry Pollination Basics. NC State University Cooperative Extension.