A comprehensive guide to pollination.
Pollination is truly fascinating. It is a very delicate process in which everything must be right. It cannot be too hot, too cold or too cloudy. There must be birds and insects buzzing about and the right genetic material needs to land on the right flower and match. Because many plants are wind-pollinated, there needs to be a light breeze but not a rampaging storm because it could damage the stamen and stigma. Other things that are vulnerable to going wrong are the female flower not opening at the right time and missing the pollen from the male flower. Avocados are notorious for this. It’s amazing that anything does get pollinated!
The first plants, such as conifers, were wind-pollinated but the success rate was not very high, so over time flowers evolved. They relied on insects, birds and animals to pollinate them and the success rate was much higher. Some plants are able to pollinate themselves while others need separate male and female plants. The odds seem stacked against success but somehow it does happen.
There are two types of plants: monoecious, or those containing the male and female parts on the same flower and/or plant, for example tomatoes and pumpkins; and dioecious, where you need a male plant and a female plant to produce fruit, such as kiwifruit and most apples and plums.
Pollination occurs when the pollen from the stamen (male sexual organ) comes into contact with the stigma (female sexual organ) and both are compatible. Bees, birds and a number of other insects visit many flowers to collect nectar. They brush past the anthers and the pollen sticks to their bodies and is transferred. They fly to the next flower and the pollen is brushed off. The stigma is chemically able to recognise that the pollen is from the same species and is able to accept or reject it.
Some species are able to self-pollinate and that means the wind or insects dislodge the pollen and it falls on the stigma, so fertilisation occurs. Tomatoes are really clever as they have evolved to be able to pollinate themselves. Before the petals open, the anthers are pushed up against the stigma and, when the stem moves in the wind, the pollen is transferred. Cross-pollination is where the flowers must receive pollen from the same species but from a different plant for fruit to set. Pollen is transferred by wind, birds and insects.
Gardeners will often complain that certain vegetables or fruit are difficult to pollinate. Pumpkins can be really tricky to get fruit to develop. How many times have you joyfully thought little baby pumpkins were developing only to find they had fallen off because they weren’t fertilised. Pumpkins produce the male flowers first and there are always more of them than females. To encourage more female flowers, one trick is to pinch out the growing tip and encourage lateral growth and, hopefully, more female flowers.
Avocados are another tricky species. Their pollination requirements are complicated and even the experts are not sure whether they are wind or insect pollinated. It’s all about timing. First, you need to know whether you have type A or B varieties. Then plant an A-type cultivar in the first planting area. A-type avocados open first as female flowers then open as male flowers the next day. Plant a B-type avocado in the second planting area, as these flower first as male then flower as female. Hopefully, by crossing all your options, you will get some pollination occurring and delicious homegrown avocados.
Tips for improving pollination:
It’s impossible to control the weather but there are some things you can do to improve pollination. The more biodiversity you have in your garden, the better chance of your fruit and vegetables being pollinated. To attract insects and birds into your patch, you need to have a variety of flowering plants and a water source. It’s communicated around the neighbourhood that there is fresh, clean water at your place.
It’s a good idea to plant species that are flowering at the same time as your fruit and vegetables and will attract pollinators. For example, bees love blue flowers and are attracted to French lavender (Lavandula dentata) and any of the tall dark-blue salvias. But they also love Iceland poppies (Papaver nudicaule). Another trick is to plant species nearby, such as bottlebrush (Callistemon sp.) or grevillea, that have long flowering periods to attract the birds and insects.
It’s also recommended that you do not use strong chemicals in the garden, especially the systemic types. These are absorbed into the plants’ vascular systems and can last for days or weeks. These chemicals don’t discriminate between the good insects and the bad ones. Insects such as bees eat the nectar and sometimes pollen that has this poison in it and it kills them.
One way to increase the odds of pollination is to do it yourself. For plants like pumpkins and squash, pick and open the male flower, remove the petals and then rub the stamens and pollen against the stigma of the female flower until you can see some pollen has been transferred.
Or you can gently shake plants such as tomatoes, peas and eggplants. This will move the pollen from the male to the female flowers. Some people also like to use a soft paintbrush to gently brush the inside of each flower, transferring the pollen. This mimics the flight of the insects.
Pollination is truly a special and amazing process. It depends on many factors coming together at the right time. I don’t think we realise when we see insects buzzing around our gardens just what a special job they are doing. Birds and insects are an integral part of our world environment.