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Composite Family (Compositaceae)

Composites As Weeds

Some members of the aster family have become regarded as important weeds. In many cases, these are aesthetic weeds, because they occur abundantly in places where people, for whatever reason, do not want to see these plants. For example, the common dandelion (Taraxacum officinale), originally from Europe but now widely distributed in North America and elsewhere, is often regarded to be a weed of lawns and landscapes. This is largely because many people only want to see certain species of grasses in their lawns, so that any dicotyledonous plants, such as dandelions, are considered to be weeds. As a result, many people put a great deal of time and effort into manually digging dandelions out of their lawns, or they may use a herbicide such as 2,4-D to rid themselves of these perceived weeds.

Interestingly, many other people consider the spectacular, yellow displays that dandelion flowers can develop in lawns and pastures in the springtime to be very pleasing. Dandelions are also favored by some people as a food, especially the fresh leaves that are collected in the early springtime. Clearly, the judgement of a plant as Late goldenrod (Solidago gigantea). © John Dudak/ Phototake NYC. Reproduced with permission. an aesthetic weed is substantially a matter of perspective and context.

However, a few species in the aster family are weeds for somewhat more important reasons. Some species are weeds because they are poisonous to livestock. For example, the ragwort or stinking-Willie (Senecio jacobea) has alkaloids in its foliage that are toxic to liver of cattle. The natural range of the ragweed is Eurasia, but it has become an important weed in pastures in parts of North America and elsewhere, possibly having been introduced as an ornamental plant. Recently, several insect species that are herbivores of ragweed in its native habitats have been introduced to some of its invasive range, and these are showing promise as agents of biological control of this important pest.

Some other species in the aster family are important weeds of pastures because they are very spiny, and livestock cannot eat them. These inedible plants can become abundant in pastures, displacing valuable forage species. Some examples of these sorts of weeds in North America include various thistles introduced from Europe, such as bull thistle (Cirsium vulgare), field thistle (C. arvense), nodding thistle (Carduus nutans), and Scotch thistle (Onopordum acanthium).

Some species in the aster family have anatomical mechanisms of attaching their seeds to the fur of mammals, for the purposes of dispersal. Animals with large numbers of these seeds in their fur can become very irritated by the matting, and they may scratch themselves so much that wounds develop, with a risk of infection. Examples of weeds that stick to animals, and to the clothing of humans, include the beggar-ticks (for example, Bidens frondosa), and several introduced species known as burdock (for example, the greater burdock, Arctium lappa). Interestingly, the finely hooked bristles of the globular fruits of burdock were the inspiration for the development of the well-known fastening material known as velcro.

The several species that are known as ragweed (Ambrosia artemesiifolia and A. trifida) are the major causes of hay-fever during the summer and early autumn. The ragweeds are wind pollinated, and to achieve this function they shed large quantities of tiny, spiny-surfaced pollen grains to the wind. Many people have an allergy to ragweed pollen, and they may suffer greatly from hay-fever caused by ragweeds.

Interestingly, at about the same time that ragweeds are shedding their abundant pollen to the air, some other, more conspicuous species in the aster family are also flowering prolifically. For example, pastures, fields, and other habitats may develop spectacular shows of yellow goldenrods (Solidago spp.) and white, blue, or purple asters (Aster spp.) at that time of year. Because people notice these brightly colored plants, but not the relatively small and drab ragweeds, the asters and goldenrods are commonly blamed for hay-fever. For this reason, fields of these attractive plants may be mowed or herbicided to deal with this perceived weed-management problem. However, the asters and goldenrods are insect-pollinated, and they do not shed their pollen to the wind. Therefore, these plants are not the cause of hay-fever—they are merely implicated by their association in time with the guilty but inconspicuous ragweed. Indeed, even people who suffer badly from hay-fever, often do not recognize the rather plain-green, unobtrusive-looking ragweeds as the cause of their allergy.

Resources

Books

Hvass, E. Plants That Serve and Feed Us. New York: Hippocrene Books, 1975.

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

Klein, R. M. The Green World. An Introduction to Plants and People. New York: Harper and Row, 1987.


Bill Freedman

KEY TERMS

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Achene

—A dry, indehiscent, one-seeded fruit, with the outer layer fused to the seed.

Bract

—A small, scale-like, modified leaf that is associated with a flower or inflorescence.

Floret

—This is a small flower, often with some reduced or missing parts. Florets are generally arranged within a dense cluster.

Head

—A dense cluster of flowers attached to a common receptacle. This is the characteristic arrangement of the flowers of members of the aster family.

Inflorescence

—A grouping or arrangement of florets or flowers into a composite structure.

Pappus

—A distinctive tissue of members of the aster family, attaching to the top of the achene, and resembling small scales or fine hairs, sometimes intricately branched to achieve aerodynamic buoyancy. The pappus is derived from modified tissues of the calyx.

Receptacle

—The enlarged tip of a peduncle where the parts of a flower are attached. Four distinct whorls of modified leaves, the sepals, petals, stamens, and carpels make up the parts of the flower.

Weed

—Any plant that is growing abundantly in a place where humans do not want it to be.

Additional topics

Science EncyclopediaScience & Philosophy: Cluster compound to ConcupiscenceComposite Family (Compositaceae) - Characteristics Of The Asteraceae, Horticultural Species, Agricultural Species Of Composites, Other Useful Species Of Composites