Plant Family Focus: Asteraceae

Posted on 9/10/2020
by Malena Brotherson & Kristen Yost, Environmental Education Interns
Get to know the Asteraceae family, the largest flowering plant family.

Late summer brings variable temperatures, cicada calls, and brightly colored roadsides spotted with wildflowers. A large number of those brightly colored wildflowers may very well be in the Asteraceae family, also known as the aster or sunflower family. Asteraceae is the largest flowering plant family, with about 24,000 identified species that can be found on every continent other than Antarctica. This family includes a large variety of species which includes annuals, perennials, vines, shrubs, and trees. The most recognizable flowers in this family are the true sunflowers (Helianthus). However, many plants such as goldenrod (Solidago), blazing star (Liatris), and coneflowers (Echinacea & Rudbeckia) are a part of this family as well.

Left: Sunflower (Helianthus) at Hitchcock Nature Center, Photo by Kristen Yost.
Middle: Goldenrod (Solidago), Photo by Kristen Yost.
Right: Sulphur Butterfly on a Purple Coneflower (Echinacea purpurea), Photo by Malena Brotherson

The Asteraceae family is unique because of their composite flowers. What looks like one single flower is actually many small flowers attached to a specialized part of the stalk called the receptacle. Each of the large petals are individual flowers called ray flowers, also known as ray florets. Ray flowers have petals that are fused together and hang to one side. In the middle of the flower are disk flowers, which can also be called disk florets. There can be hundreds of disk flowers attached to one receptacle. The disk flower typically have five fused petals and because the Asteraceae family is so diverse, species in this family can have either disk, ray, or both types of flowers present. For example, true sunflowers (Helianthus) contain both types of flowers while ironweed (Vernonia) and blazing-star (Liatris) contain only the small disk flowers.

Left: A labeled half of an Asteraceae plant.
Middle: A single ray flower is on the left of the picture and a single disk flower is on the right of this picture.
Right: A single disk flower; photo taken through a microscope.
Photos by Malena Brotherson
Left: Helianthus species with many ray and disk flowers from Hitchcock Nature Center.
Middle: Ironweed (Vernonia) and right: Prairie Blazing-Star (Liatris pycnostachya) are both examples of Astereaceae plants that contain only small disk flowers from Arrowhead Park.
Photos by Malena Brotherson

Another distinguishing characteristic of the Asteraceae family is that each flower produces only one seed. These dry, indehiscent (do not split open) pods with one seed are called achenes, and because they don’t split open, they rely on decomposition and animals to release the seeds over time. Some flowers on Asteraceae plants are sterile, meaning they do not have the reproductive parts to produce seeds. One example of this can be seen in members of the true sunflowers (Helianthus). These sunflowers have sterile ray flowers, so only the disk flowers produce seeds.

Many Asteraceae also have bracts which are modified leaves underneath the flower and above the regular leaves. They look similar to the layers of artichokes. Bracts can be smaller or larger than the flower and can be any color. Bracts help protect and give shape to the flower or attract pollinators if brightly colored.

Left: Two achenes attached to the dried disk flowers from a Pale Purple Coneflower (Echinacea pallida); Photo by Malena Brotherson
Right: An achene from an Asteraceae opened up to show the single seed inside; Photo via science.jburroughs.org
Left: The underside of an Asteraceae plant. You can see the petals from the ray flowers, green bracts, and two opposite leaves
Right: The head of a Pale Purple Coneflower (Echinacea pallida) cut in half. If you look closely, you can see the achenes attached to the receptacle of the flower. Between the achenes are dried bracts that stick out and create the hard, spiky head of the coneflower.
Photos by Malena Brotherson

The Asteraceae family provides many ecosystem services with its vast diversity. Many insects and birds rely on Asteraceaes as a food source. As previously mentioned, many Asteraceaes attract pollinators and provide nectar for them. The plants also provide leaves and seeds to feed wildlife. Not only do wildlife rely on members of the Asteraceae family for food, but humans do as well. We get cooking oils, sweetening agents, herbal tea, and vegetables like lettuce and artichokes from Asteraceae plants. Asteraceaes also help with erosion control by holding soil with their roots and growing in disturbed areas that many other plants would find too harsh to grow in. This provides the necessary ground cover to keep banks and other areas stable. There is no limit to what Asteraceaes can do with their vast diversity and number of species.

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Ladybug on an Asteraceae Flower; Photo by Kristen Yost

 

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