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Pollination in the veggie garden

If you are growing your own fruit and vegetables, you’ll know about the importance of pollination in the garden.
We often hear stories about massive pumpkin vines or zucchini plants with lots of flowers that fail to produce fruit. The problem is usually a lack of pollinating insects such as bees.
 
If you have lots of flowers and fruit on your veggie plants, no need to read on because the bees and insects are doing their job for you. If your plants aren't producing well or you’d like to increase your yields, attracting more pollinators to your garden may be the answer.
 
The transfer of pollen in and between flowers of the same species leads to fertilization, and successful seed and fruit production for plants.  Pollination ensures that your plants will produce full-bodied fruit and a full set of viable seeds.  
What is pollination?
 
Pollen is made by the male organs of a plant (stamens in flowers) and contains genetic information needed for plant reproduction. Pollination is the delivery of this pollen to the female organs of a plant (stigma in flowers). Pollen may be transferred to female organs on the same plant (self-pollination) or another plant of the same species (cross-pollination). As a result of pollination the plants produce seeds. Pollen can be dispersed by wind, water and animal pollinators such as insects, bats and birds.
Vegetable crops that produce a fruit require pollination in order to develop fruit.
Some vegetable plants produce a separate male and female flower - pumpkins, squash and cucumbers for instance. Pollination occurs when insects such as bees and hoverflies visit flowers, collecting nectar and pollen. Pollen is rubbed onto the insect and is then rubbed off onto the next flower the insect visits.
 
Types of pollination
 
Vegetables are pollinated in two basic ways: self pollination and cross pollination.
 
Self pollinators are plants that produce flowers that are usually fertilized by their own pollen, commonly when the male and female flower parts are contained within the same flower. Self-pollinated vegetables include bush and pole beans, lima beans, chicory, endive, lettuce, English and Southern peas, and tomatoes.
Self-pollinators (such as tomatoes and peas) have both male and female parts on the same flower. Wind or insects dislodge the pollen, which leads to fertilisation within the flower.
 
Cross pollinators are plants with flowers that require pollen from another flower (a male flower on the same plant–thus a form of self-pollination–or from another plant) to produce a fertilized seed. Cross pollinators commonly require the help of insects or the wind to achieve pollination.
 
Cross-pollination can accidentally occur when pollen from one vegetable variety fertilises a different variety of the same (or similar) species. For example if a bee pollinates a pumpkin flower with pollen from a butternut squash flower, the resulting fruit could be an inedible hybrid of the two, and its seeds will also produce a different fruit.
 
Insect-pollinated vegetables include: asparagus, broccoli, Brussels sprouts, cabbage, carrots, cauliflower, celeriac, celery, Chinese cabbage, collards, cucumbers, eggplant, gourds, kale, kohlrabi, muskmelons, mustard, okra, onions, parsley, parsnip, hot pepper, pumpkin, radish, rutabaga, spinach, squashes, turnips, and watermelon.
 
Wind-pollinated vegetables include: beets, chard, spinach and sweet corn.
When the wind blows on a sweetcorn plant, pollen from the male part of the plant falls onto the wispy immature heads of the corncobs. Planting sweetcorn in blocks of at least four increases the rate of wind pollination, ensuring that all the corn kernels on the cob will develop.
 
The pollinators
 
Species of bees, beetles, flies, wasps, thrips, butterflies and moths are all successful pollinators.
These insects make good pollinators because they share two important features:
  1. They fly, and so are capable of visiting many plants in a relatively short amount of time
  2. They are motivated to interact with pollen, as they either eat it or food items located nearby (e.g. nectar).
Bee pollination
 
The most sophisticated relationships between plants and insects are generally those involving bees. Bees collect pollen and nectar not only for themselves but also to feed their young. For this reason bees have developed a number of adaptations that make them particularly good pollen carriers. Bees have special hairs that are arranged to form pollen 'baskets' on their hind legs and the underside of their abdomen. These adaptations allow them to gather and carry large volumes of pollen. Bees are ideal pollinators because they visit many flowers while carrying lots of pollen, before returning to their nest. So the chance that a bee will transfer the pollen between flowers of the same species is very high.
 

Problems with poor pollination

 
If your vegetable plants are not yielding fruit it could be due to poor pollination. Poor pollination can occur for a number of reasons:
  • Late frost – frosts can damage flowers and ruin your crop. If the frost was mild you can save the blooms by spraying them with icy cold water first thing in the morning. This slows down the rate at which the flowers warm up and allows them to thaw out gently.
  • Poor weather – a prolonged cold spell and heavy rain can result in fewer insects to pollinate your crops. Pollinate the blooms by hand until the warmer weather arrives.
  • No access to insects – open the door of your greenhouse on sunny days and let the insects in to pollinate your plants. Alternatively, pollinate by hand (see below).
  • Dry atmosphere – a dry atmosphere can cause poor pollination or malformation of the fruit. Leave a bucket of water in your greenhouse or regularly mist your crops to increase humidity.

Encouraging insect pollination

 
You can encourage insects to visit your food garden by planting a wide range of flowers. Whilst gathering nectar and pollen from the flowers they will also pollinate your fruit and veggies, increasing your yields.
 
Flowers which are particularly good at attracting pollinators to your food garden include asters, forget-me-nots, lavender, calendula, nasturtiums, catmint, salvia (especially the blue flowering variety), comfrey, geranium, lupin, borage, and sunflower.
Some herbs are also very bee-friendly. They have a natural affinity with vegetables and many are said to deter insect pests in the garden. Try planting extra basil, thyme, sage, rocket, borage, chives, garlic chives and coriander.  Allow them to flower and go to seed.
Grow lots of borage between your veggies.The bees love the blue colour and you can eat the leaves and flower petals.
Pollinating by hand

Hand pollination is not normally necessary if there are plenty of insects around. However certain vegetables (such as eggplant and kiwi fruit) can be difficult to pollinate, so hand pollination may be necessary. It may also be necessary if you are growing your vegetables under a cover or netting.

Pollinating by hand also avoids cross-pollination which can be useful if you want to save seeds.

The method you use to pollinate your crop will depend on the type of flower you are pollinating. Plants in the squash family such as pumpkin, zucchini and cucumber, have male or female flowers. Female flowers have an immature fruit just behind the flower and male flowers have a long stem with no swelling at the base. Simply pick an open male flower and strip off the petals to expose its stamens and pollen then rub them against the stigma of a female flower until you can see the pollen has rubbed onto it. You should have enough pollen on the male flower to pollinate a few female flowers. This hand pollination should be done when the temperature is above 13 degrees C, otherwise the pollen may be damaged.
The male flower containing pollen Rubbing the pollen onto the female flower
You can dislodge the pollen in self-pollinating flowers by shaking the plant gently. A more reliable method is to use a soft paintbrush. Gently brush the inside of each flower. You will see the pollen transfer onto your brush; if you transfer pollen between the flowers you will mimic the natural movements of insects.

When to avoid pollination

Some vegetables are not grown for the fruit they produce. Rather, they are grown for the plant as a whole (such as lettuce), a bulb (onions), or oversized roots such as beetroot.
You should avoid letting these plants bolt (produce flowers and seeds). Once plants have flowered they tend to produce fewer leaves and concentrate their energy on seed production. This can make the leaves taste tough and bitter or reduce the size of the root or bulb you are growing.
If you do see flowers on these plants, remove them immediately. 

Pollination particulars

Below are descriptions of vegetables that rely on pollination, the pollination method they use, and possible problems and solutions.

Squash/pumpkins/zucchinis etc

Squash plants have male and female blossoms on each plant. The females will have a small squash behind the flower; the males will just have the long thin stem. If pollen is not transferred from the male flower to the female, the little squash will not grow completely, and will eventually rot from the tip. To get the best squash, pollination should occur right after the flowers open.
Squash are pollinated by bees. At the beginning of the season, pollination may not occur if the bees have not yet found the flowers. In this case, you can hand pollinate the squash.

Cucumbers

The pollination process for cucumbers is the same as for squash, except all the “parts” are much smaller. It’s much harder to hand pollinate cucumbers.
If your plant has produced too many cucumbers, all of them may not pollinate. In this case, pick some of the small fruits and toss them onto the compost heap.

Melons 

Melons are also pollinated like squash and they also have very small flowers.  

Peas, okra, and beans
 
These do not need pollination. Runner beans are one exception; they require pollination.

Tomatoes and capsicums

Tomatoes and capsicums pollinate through a very gentle shaking. Usually the wind is enough to perform this vital task. If you are planting indoors or in a greenhouse or hoophouse, tapping the flowers gently will do this shaking. One or two light taps on each cluster of blossoms is sufficient. Do this every few days to pollinate all of the flowers.  

Eggplants

Most varieties of eggplants are self-pollinating. If your eggplants aren’t growing, you can try the tomato tapping method. If these methods don’t work, try knocking some pollen from a male flower onto your finger and deposit it on the female flowers.

Sweetcorn 

Sweetcorn relies on the wind for pollination. Corn pollen is produced in the tassel at the top of each stalk. When the wind shakes the plant, pollen falls from the tassels onto the silks, which are the plants’ female flowers.  Each silk that is pollinated results in a kernel of corn developing on that ear. To provide this maximum wind pollination in an area, corn should be planted in blocks rather that in long straight rows.

Pollen from one plant rarely fertilizes the silks of the same plant. Its been reported that 97% or more of the kernels produced by each plant are pollinated by other plants in the plot. Because wind can carry pollen up to a mile or more, pollen from one variety can pollinate anther variety unless there is great distance between the plants or unless your variety choices are ones that do not pollinate at the same time. 

Each corn tassel contains from 2 to 5 million pollen grains which translates to 2,000 to 5,000 pollen grains produced for each silk of the ear shoot. Poor seed set is more often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed). Under good conditions, all silks will emerge and be ready for pollination within 3 to 5 days and this usually provides adequate time for all silks to be pollinated before pollen shed ceases.

Most sweet corn is in pollination mode for about 10 days. When the tassels at the tops of the plants show dangling anthers, and the ear tips show hairy tufts of silk, the pollination process is underway. Here’s how it works: a pollen grain falls on a sticky strand of silk and imbeds itself. For the next 12 to 24 hours, the pollen grows a tube down the length of the silk to a waiting ovary. If all goes well, a corn kernel is born. Excellent pollination produces ears that are filled with wall-to-wall kernels; poor pollination leads to ears with lots of missing kernels.

Tomatoes

Tomatoes are self-pollinating. They have what's called "perfect" flowers. What this means is that they have flowers that have both the stamen and the stigma (male and female parts) on the same blossom. So, the pollen from the stamen falls onto its own stigma and we have pollination and eventually, fruit. Often this is process has been completed before the flowers are fully open - but not always. Beans, eggplant and peppers are self-pollinating, too.

But you can actually help your tomato plant set more fruit by showing it some tactile love. You just take the flowering branches and give them a gentile shake. The pollen will drop from the stamen of the flower onto the pistil.  
Left alone, the wind would shake the blossoms, as would the fluttering of bees' wings. But, gardeners everywhere swear that they harvest higher fruit yields with this simple technique.
These tomato flowers suffered blossom drop and failed to pollinate.The reason: heat shrivelled up the flowers before they could pollinate.
Some interesting facts about pollination
  • We depend on pollinators for 1/3 of the food we eat.
    http://www.berkeley.edu/news/media/releases/2006/10/25_pollinator.shtml
  • Worldwide, roughly 1,000 plants grown for food, beverages, fibers, spices, and medicines need to be pollinated by animals in order to produce the goods on which we depend.
  • Foods and beverages produced with the help of pollinators include: apples, blueberries, chocolate, coffee, melons, peaches, potatoes, pumpkins, vanilla, almonds, and tequila.
  • After bees, flies—especially hoverflies (aka flower or syrphid flies)—are among the most important pollinators of agricultural crops. For instance, chocolate lovers have midges to thank for their vice: These flies are the sole pollinators of the cocoa tree
  • Bats pollinate more than 300 species of fruit, including mangoes, bananas and guavas. Some bats have very long tongues, similar to the moth’s’ proboscis. They can also hover, like hummingbirds.
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