What’s in a Family? The Asteraceae

by Frederick B. Essig

  Did you know that the sunflower is not a flower? Nor is an aster, a marigold, a dahlia, a daisy or a zinnia. These are indeed members of the largest of all plant families, the Sunflower Family, but what we call flowers in this family are actually compound flowers — compact clusters of very small flowers arranged to look like a single flower. The Asteraceae was formerly known as the Compositae in recognition of this unique floral structure.

The compound flower heads typically consist of two differently-shaped flowers. The tiny flowers that occupy the center of a typical sunflower or daisy are called disk flowers. Sepals are typically lacking or turned into bristles (collectively called a pappus). Petals are united into small, trumpet-shaped tubes with five petal lobes. The five stamens cling like a collar around the style, which raises a two-part stigma above them. Each flower bears a single seed within a hard fruit (the achene). In these same flower heads, the flowers around the edge, called ray flowers, are modified to each look like a single long petal. In the subfamily that includes lettuce, dandelion, chicory, and Youngia, however, all flowers are ray flowers, but in Liatris, Senecio, Cirsium (thistle), Cynura (artichoke) and many others, the heads consist only of disk flowers.

With nearly 1000 genera and 20,000 species, the Asteraceae is on the cutting edge of flowering plant evolution. They are highly competitive and prolific. Widespread, they’ve moved into just about every conceivable niche. Most are annual or perennial herbs, sometimes shrubs and occasionally tree-like forms. The rare silverswords of Hawaii grow like agaves or yuccas in the barren rubble of volcanic craters. The weird, tree-like Dendrosenecios of the high slopes of African mountains are adapted to survive daily alternations of freezing temperatures and sunny warmth. Within genus Senecio and its close relatives we find not only conventional herbaceous forms, but also vines, cactus-like succulent stems and bulbous succulent leaves. Species of Bidens, Mikania and others have invaded swamps and other aquatic habitats.

There are numerous useful members of this group. Edible members include the most common of all salad materials, lettuce, and the more selectively appreciated artichoke. I never have understood the joy of rummaging for bits of edible mush hidden within this largely inedible thistle bud — but to each his own. Lettuce is relatively tasteless and inoffensive as vegetables go, and a useful catchment for more interesting foods like nuts, cheese, fruits, dressing and croutons. Sunflower seeds might be thrown into that salad, and sunflower oil could very well be in the dressing for the salad. The leaves of Stevia, also in this family, provide a natural sweetener to use in a glass of tea or lemonade to go with your salad.

The Sunflower Family is also home to chamomile, echinacea and chrysanthemum teas that we use to calm ourselves, prevent colds or provide antioxidants. It is home to a weed that is a favorite of children: the dandelion. Who hasn’t blown the umbrella-equipped seeds (actually single-seed dry fruits called achenes) into the air? The dandelion is a close relative of lettuce, and its greens are also edible. In a pinch, rubber can be made from latex found in the roots.

Another close relative, also a lawn weed here in Florida, is Youngia japonica. It contains compounds useful against certain types of respiratory disease, and possibly leukemia.

Another far more annoying weed, with no evident redeeming value, is beggar’s tick, Bidens alba. The small fruits of this and other members of the genus are covered with tiny Velcro-like hooks, and stick to socks, pants and animal fur. One species of Bidens is more attractive; B. laevis, a native aquatic species, produces masses of yellow flowers.

The genus Artemisia provides dozens of medicinally-important compounds that ward off internal worms, malaria, blood flukes (schistosomes), diabetes, toothache, indigestion, dermatitis, kidney disease, mouth sores and bad breath. Of course you can’t use any of these if you’re allergic to ragweed pollen, which is a close relative.

Horticulturally, this family is irreplaceable. In addition to the bedding staples listed in the first paragraph, we can add the favorite flower of autumn, the chrysanthemum, along with African daisies, cosmos, coneflowers, Liatris, Rudbeckia, Ageratum, Gaillardia, Gerbera daisies and dozens more. What would our gardens be without this cheery lot?

Dr. Essig, USF Associate Professor Emeritus, writes blogs about botany at  http://botanyprofessor.blogspot.com.

© 2014 Frederick B. Essig. Originally published in Florida Gardening, Apr/May 2014. Reprinted by permission. All rights reserved.