Vegetable crops that produce a fruit require pollination in order to develop fruit. Pollination occurs when pollen from a flower’s male sexual organ (stamen) comes into contact with a flower’s female sexual organ (stigma). 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 fertilization within the flower.
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. Fruit will develop if male pollen has been transferred into a female flower of the same species.
Other types of pollination
Corn is pollinated by wind. Pollen from the male part of the plant falls onto the wispy immature heads of the corncobs. Planting corn in blocks of at least four increases the rate of wind pollination, ensuring that all the corn kernels on the cob will develop.
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 will encourage insects to visit your garden or allotment by planting a wide range of flowers. Whilst gathering nectar and pollen from the flowers they will also pollinate your crops, increasing your yields. Flowers which are particularly good at attracting insects to your plot include comfrey, geranium, lupin, borage, buddleia, lavender and sunflower.
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.
Pollinating by hand also avoids cross-pollination which can be useful if you want to save seeds. Cross-pollination occurs when pollen from one vegetable variety fertilizes 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.
The method you use to pollinate your crop should 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 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 beets.
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. Check the conditions in which the plant is growing and avoid growing the same variety in the same position again. Lettuce and spinach appreciate partial shade during the hottest parts of the day and do not tolerate drought.