Pollination is a critical process that most plants need to set seeds and produce fruit. Understanding pollination ecology is important for both commercial growers and home gardeners; however, this publication focuses on the home gardener. The following information will help gardeners optimize pollination conditions to ensure maximum seed and fruit quantity and quality. If more information is needed, see the References and Further Reading sections at the end of the publication.
What is pollination? Pollination is the transfer of pollen from the male part of the plant to the female part of the plant of the same species. Figure 1 shows a honey bee pollinating an apple blossom. The anthers, the male part of the flower, produce pollen. The pollen must be transferred to the upper female part of the flower, the stigma. Here the pollen germinates and grows down the style (the elongated portion of a pistil that connects the ovary with the stigma of a plant) (see Figure 2) to where fertilization occurs and the seeds develop in the ovaries. A fruit is a ripened ovary. Usually transfer of pollen occurs with the help of animals (insects, birds, and bats), but gravity, wind, and water can also transport pollen (Willmer 2011).
How important are bees? Directly or indirectly responsible for a third of our food, bees pollinate the majority of the types of food we eat. So protecting bees and understanding how human actions can enhance or hinder their well-being is critical.
There are many types of flowers, from very simple to very complex, and each has evolved ways to ensure successful plant reproduction. Some plants are self-fertile and require no external assistance for pollen transfer. Wheat, for example, is self-fertile, and pollination occurs within the flower before it opens.
Other flowers are self-fertile but still dependent upon wind, water, or animals to help the transfer of pollen to the stigma. Depending on the plant species, the pollen may need to come from another plant or another variety of the same species to set seed. The transfer of pollen from one plant to another plant is called cross-pollination. Cross-pollination helps to increase the fitness and survival of plants by increasing genetic diversity.
Evergreen trees, called conifers, are wind pollinated; these trees include pine, fir, and hemlock. In the Pacific Northwest large amounts of “yellow-green dust” fill the air in the spring. This is pollen from the conifers or evergreen trees, and it is an “expensive” way to pollinate, from the plant’s point of view. Plants that rely on wind must produce massive amounts of pollen to ensure an adequate amount is available for successful pollination.
A far more efficient means of pollination is to attract an animal that will travel from flower to flower and transfer the pollen between plants. Of the more than 250,000 flowering plant species in the world, most (over 75%) rely on animals for pollination. Most of these animals are insects, although birds, and even some bats are important pollinators ( National Research Council 2007).
Between 130,000 and 300,000 species of animals regularly visit flowers and pollinate while in the process of feeding. These animals visit flowers for food; the pollen itself provides protein and the nectar provides sugar (carbohydrates).