Marin Master Gardeners
University of California
Marin Master Gardeners

Pollination

By Martha Proctor, Katie Martin, and Dave Phelps

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.

Flower size and shape is critical to attracting the right pollinators
Flower size and shape is critical to attracting the right pollinators

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.

Bees, birds, beetles, and more: say hello to the pollinators

Sixty five percent of flowering plants are pollinated by insects. Some plants are pollinated by other animals, including birds and bats. Some plants, e.g., oaks, poplars, walnuts, birch, gymnosperms and grasses are wind pollinated. And some plants are self-pollinated, in which case, cross pollination providing genetic diversity does not occur.

Beetles - have a highly developed sense of smell. Flowers they are attracted to are often white or dull in color and have strong odors. Western skunk cabbage is pollinated by small beetles in the family Staphylinidae.

Bees - Bees are the most important group of pollinators. One third of the food supply of the United States depends on bee pollination. For example, honeybees pollinate 90% of California’s almond crop. We need these guys! They feed on nectar and simultaneously carry away a load of pollen from flower to flower. Scientists have shown that bees can learn to recognize colors, odors, and shapes. The bee can see ultraviolet, but cannot see red (it appears as black to them).

California native bees are highly diverse, including thousands of species of bumble bees, sweat bees, carpenter bees, and many others. Native bees are often solitary, stingless, and sometimes quite small. They are generally more efficient individually at pollinating than honey bees and this helps make up for their lack in numbers. Many need holes or bare earth to rear their young. For this reason, an area free of tilling and heavy mulch is desired in the garden. Bee “houses” can also be made or purchased that supply a variety of diameter holes to help promote their survival.

The diverse native bees and the imported honey bees desperately need our help by providing healthy environments that offer clean water, an absence of pesticides, a safe habitat, and a diverse palette of nectar-producing plants throughout the season.

Moths and butterflies drink nectar from flowers with their long slender proboscis. The pollen from one flower is deposited on the proboscis and transferred to the female part of the next flower. Butterflies can see colors, but their sense of smell is not well developed. They are daytime fliers and like to rest on the flower to feed. In contrast, moths are twilight- or night fliers. Colors are not easily distinguishable at night and many flowers are not open at night. Moths are attracted to flowers with pale colors and strong odors. They do not need a place on the flower to land as they move quickly from flower to flower.

Butterflies and moths need our help. Giving them a spot with some mud for them to drink from can help lure them to the garden. Moths go after nocturnal trumpet-shaped fragrant flowers, usually white or cream colored. Plants for the caterpillars to feed on and eggs to be hatched on such as milkweed are also necessary for their survival.

Birds and hummingbirds - Throughout most of the United States, hummingbirds are the only birds that pollinate flowers. Since birds have a poor sense of smell, they are not attracted to flowers with strong scents. (In the Southwest white winged doves pollinate the giant Saguaro cactus.)

Hummingbirds are attracted to tubular, brightly colored flowers, especially red and orange. They hover while they feed on nectar and brush the top of their heads against the pollen bearing stamens before moving to the next flower. Hummingbirds feed about every 10 minutes all day and consume up to 2/3 of their body weight by extracting nectar at the rate of about 13 licks per second and can service 20 flowers per minute. This amount of sugar is necessary to keep their wings beating at up to 55 beats per second and allowing them to fly at speeds up to 50 mph, to hover, or even fly backwards or upside down. Attracting them to your garden is easy and can be especially rewarding.

Bats - Found in Marin, the Pipevine swallowtail butterfly only feeds on California pipevine (Artistolochia californica). The sprawling vine with flowers shaped like a calabash pipe are pollinated by fungus gnats which are attracted to the flower odor.

Wasps - Asclepias curassavica, milkweed, is pollinated by large wasps. The nectar of milkweed is a food source for Monarch butterflies. The butterflies lay their eggs on milkweed and the newly hatched larvae feed on the leaves.

Wind - Many trees, such as box elder, oak, poplar, walnut and birch, are wind pollinated. The pollen of plants that are wind pollinated is very fine and produced in great quantities. It must be light enough to be carried by the wind and copious enough to survive the great losses that occur. All gymnosperms, such as pines, spruce and fir, spread their pollen by the wind. Corn and other grasses (oats, wheat, and barley, for example) are also wind pollinated. Corn has an especially interesting means of pollination and fertilization. The top of the corn ear has the male staminate flowers in a tassel. The female flowers are along the ear of the corn. Pollination takes place at the top, the pollen grain travels down the silk to fertilize the female flower. And voilá, one kernel of corn is formed. Corn is an important crop plant. More than half of the world crop is raised in the United States!

How to attract pollinators to your garden

The rule with plants as it is with any ecosystem is diversity. Tubular flowers (such as California fuchsia, penstemon, honeysuckle, lobelias, and the sages) plus flowers that have many small florets including composites (such as sunflowers, coneflowers and daisy-like plants), umbels (such as dill or fennel) and other inflorescences that likewise have many small flowers close together, such as yarrow. Other plants such as currants, ceanothus, manzanita, gum plants, buckwheat, and even trees such as the buckeyes are fantastic plants for pollinators. Basically, many small florets that can be harvested without much effort and a diversity that offers overlapping blooming periods is best. Some non-natives such as clovers, mints, lavenders, or thymes can be beautiful as well as helpful to the pollinators. Another added benefit of these types of flowers is that they attract many other beneficial insects that can prey or parasitize pests in our gardens and create a more balanced ecosystem.


Edited by Marie Narlock and Anne Wick

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