Pollination: a quick guide for food growers
by Tom Moggach
Bee on a courgette flower credit: Mr eNil
Given it’s a vital part of plant life, it’s bizarre that pollination is so often poorly understood. After all, it’s crucial to our gardens, plots and harvests and indeed the world’s food supply as a whole.
Do you know which of your plants require pollination and how you can help the process? I thought not. Gardening books rarely describe pollination in any detail, and the scant information on offer is often vague and confusing.
Perhaps this haziness is no surprise. After all, pollination is a subtle process and seldom visible. Moreover, plants that we harvest for their leaves, stems and roots, such as herbs, leafy greens and salads, carrots, beets, radishes and potatoes don’t require pollination to produce a crop. (It only becomes an issue if you want to save their seed).
The plants that form fruit, however, do need to be pollinated to produce their harvest. And here we mean ‘fruit’ in the strict botanical sense of the word – the fertilised ovary or ovaries of a flower which contain seeds. In everyday language, we sometimes describe things as vegetables when technically they’re fruits – tomatoes, peas and courgettes are classic examples. All of these, and many more, absolutely require pollination.
The physical process involves the transfer of pollen from the anthers (part of the stamen – the male part of the flower) to the carpel (the female part) where they fertilise ovules or ‘eggs’. In some instances, insects may be needed to move the pollen; in other cases, it’s the wind.
Many plants rely heavily on insects. Classic examples are courgettes and squashes, which have separate male and female flowers. Insects are needed to move between the two, transporting pollen on their bodies. It’s also important to grow more than one plant to boost the numbers of flowers open at any one time.
(In extremis you could pollinate the flowers by hand, by, for example, picking a male flower, removing the petals then dabbing the pollen onto the stigmas of the females.)
Most varieties of apples and pears require a pollination partner (another tree of a different variety) to provide the pollen they need to produce a crop and even trees thought to be self-fertile will fruit better. “It’s nature’s way of preventing inbreeding,” explains Hamid Habibi of Keepers Nursery. Insects are needed to move the pollen between the flowers of the different trees.
A third group of plants have their male and female sexual organs inside the same flower (termed a ‘perfect flower’) and are capable of pollinating themselves. Common examples are tomatoes, peas, beans and soft fruit such as raspberries. Insects are often needed to dislodge the pollen from the anthers so that it comes into contact with the female stigma. (You can sometimes mimic this process by lightly shaking the plants. This is common practice when growing tomatoes in a greenhouse, for instance.)
Honey bees grab the headlines as the key agents in this process and they can pollinate as many as 2,000 flowers per day. But the reality is more complex. Wild bee species such as bumblebees perform the same role, as do some flies, beetles, butterflies and moths.
For the city gardener, the best way to help things along is to attract as many beneficial insects as possible on to your plot. Grow as many flowering plants as you can, ideally so that something is in bloom and providing a food source almost all year round. Leaving a few patches to grow wild always helps too.
If you don’t grow organically, steer clear of using insecticides if at all possible. If essential, deploy them in the evening when bees are not flying. When growing under cover, in a greenhouse for example, open the doors on sunny days to allow insect access.
A smaller number of plants such as sweetcorn, cobnuts and grasses rely on the wind to dislodge pollen. For this reason, sweetcorn should be planted close together in a block, to maximise the chances of pollen reaching the female stigmas.
Various issues can affect pollination. This year, for example, has been tricky for many types of fruit because heavy rain during their flowering period meant that fewer insects were flying. Lack of moisture or very high temperatures can also have a negative effect.
If your yields are smaller than expected, poor pollination is not always the problem though. Lack of sun and low temperatures, for example, can slow growth right down. Our feeble weather this summer has a lot to answer for.
By Tom Moggach, author of ‘The Urban Kitchen Gardener: Growing and Cooking in the City‘ (Kyle Books)