Hero Image

The Leaflet Archive

Pollination, what it is, how it works, and why we can't live without it

  • Plants and their related pollinators have co-evolved over 15 million years to produce the delicate and sometimes amazingly detailed balance of give and take; honey and pollen in exchange for sexual services rendered; the transfer of pollen from stamen to stigma. Pollinators are the awe-inspiring intersection between the plant world and the animal world.

    About 25 percent of plants are able to exchange pollen by wind. The rest rely on pollinators to do the job. Fruit and seeds are the result - the procreation of the plant world. While only 15% of our crops are pollinated by honeybees, that still accounts for about $15 billion worth of crops each year in the U.S. and represents one out of every three mouthfuls of food most people eat. 

    How pollination works

    As the reproductive part of the plant, flowers are the means by which most of the plants in our gardens attract pollinators. Pollinators —typically wind, bees, hummingbirds, butterflies, moths, flies, bats, beetles, and, rarely, water —carry pollen from one flower to another. The manner by which pollination occurs varies from plant to plant, with most plants having specific pollinators. To attract their pollinators, flowers develop bewitchingly colored petals, elicit pleasant scents/compelling odors or produce inviting nectars. Butterflies and birds are drawn to flowers with red and/or yellow petals while bees, which cannot see red, prefer blue and ultraviolet. Moths and bats focus on white or pale tinted petals.

    To further entice their pollinators, flowers adapt their shape to the needs of their pollinator. The size and shape of the flower's parts and their alignment are critical to assure pollination when the right pollinator visits. A long nectar spur protects the pollen from robbers and yet allows a welcome reward for the desired pollinator. For example, the tube-like shape of the fuchsia or snapdragon works perfectly for its pollinator, the hummingbird. Non-hovering insects and birds need perches or landing platforms as part of the flower, i.e., large, flat flowers allow poor fliers, like beetles, to land on top. Flowers with "landing pads" (e.g., the iris) enable insects, like bees, to walk into the base of the petals to collect the pollen. Plants that use wind for cross-pollination generally have flowers that appear early in the spring, before or as the plant's leaves are emerging. Wind pollination is the prominent method in grasses, most conifers and many deciduous trees.

    Some pollinators are host specific. This means that the insect relies exclusively on one plant species and the plant relies exclusively on the insect for pollination. The adult insect benefits from the nectar of the flower then carries pollen to another flower. In the yucca-yucca moth relationship, the yucca moth is the pollinator. It also lays its eggs in the yucca flowers and the larvae live in the developing ovary and eat yucca seeds. Figs and fig wasps are another example of a mutualistic relationship.


    Plants: built for pollination

    Pollination is a well-planned accident! Pollinators are attracted to flowers due to the sticky pollen or sweet nectar at the base of the petals, which supplies the nutrients they need. While feeding, breeding, or hiding in a flower, insects rub against the stamens and get pollen all over their face, legs, or mouthparts so that when they move on to the next flower, they carry some of the pollen onto the stigma of that plant. As described below, pollination subsequently leads to the creation of seeds that eventually grow into new plants. Germination of the seed occurs with increasing daylight in spring, warming temperatures, proper timing, and quantity of moisture, and even with fire, abrasion, and also after passing through the digestive track of birds or animals.

    Flowers have male parts, stamens, that produce pollen, a sticky powder, and a female part called the pistil. The top of the pistil is called the stigma, and is often sticky. Seeds are produced at the base of the pistil, in the ovule. To be pollinated, pollen must be moved from a stamen or male part to the stigma of the female part. After this occurs, sperm are produced within the pollen grain and a pollen tube carrying the sperm grows toward the ovary at the base of the flower. Fertilization occurs when the pollen tube grows into the ovary and sperm are united with an egg. When the flower wilts, the ovule enlarges and a seed and/or fruit develops.

    When pollen from a plant's stamen is transferred to that same plant's stigma, the process is called self-pollination. Cross-pollination, which produces stronger plants, occurs when pollen from a plant's stamen is transferred to the stigma of another plant of the same species. To be effectively pollinated, the plants must be of the same species. For example, only pollen from a rose can pollinate another rose. Pollen from a daisy or an apple tree would not result in new rose seeds and ultimately rose plants.

    Some plants, such as mosses and ferns, which do not produce flowers, reproduce by spores. Cone-bearing plants (e.g., pine or spruce trees) reproduce when pollen produced by a male cone is passed by wind to a female cone of the same species. The seeds then develop in the female cone.

    Like all living things, a flowering plant's goal is to procreate by setting seed. However, once the petals become brown and unattractive, it's a good time to deadhead or trim off the spent flower heads. This encourages the plant to produce another round of blooms instead of putting its vital energy into setting seeds. The act of deadheading works particularly well for annuals which have short lives and are driven to set seed, as they do not need to store energy for another year's growth. Once cold weather sets in, leave the dead flower heads so that birds and other wildlife can forage on the seeds through the winter.