Department of Environment and Conservation
|The winning article
"Weeds: Why Are They Here?" appeared in the March/April 1998 issue and was
written by Andrea Brewer Shea. The focus of the article was to educate people about what
they may see and touch on a daily basis in their own backyard and to offer them a better
understanding of the larger natural world. Discovering more about what weeds are, and how
they may have arrived in Tennessee, is a part of that education.
Weeds:Why Are They Here?
Tennessee is proudly known as the Volunteer State. We have many species of "volunteer" plants inhabiting our nurtured lawns and gardens, our tended pastures and fields. However, these plants are not always the most desirable kind of "volunteers," and they are called "weeds;" aggressive, competitive, toxic, noxious, poisonous or deadly weeds. Millions of dollars are spent by land managers, farmers, and homeowners to rid Tennessee of these unwanted "volunteers" that can dominate a lawn or a garden within a short period of time. Where did they come from? Why are they here? What good are they? Just as the Tennessee Volunteers returned home from the War of 1812, why can't these "volunteers" go back home where they came from? "Any plant growing where it is not desired is a weed." That's the definition accepted by the Weed Society of America. Weeds have been associated with humans since civilization began, even during prehistoric times. Early travelers and immigrants to the New World brought herbs, crops and ornamentals from other continents that have been integrated into the American culture. For the most part, weeds depend on man for survival by creating a disturbed habitat in which they can thrive, and they have prospered at our expense. The intentions of these plant volunteers were admirable: to heal the sick, to provide food and shelter for man and animal, to produce a pleasant scent, to flavor food and beverage, and to cover the naked ground when nothing else would. They do have their value where they are not competitive and are respected members of the flora. If they won't go back home, perhaps we should enjoy them as they provide certain services to man and, in most cases, have beautiful features. Let them live a little, just long enough for us to use and appreciate. Then proceed to mow, pull, dig, cut, and spray; but do not feel guilty for no matter what you do, they will be back! Not all weeds are aliens. Some weeds native to Tennessee are considered pests. However, the "problem pest plants," the ones that are extremely invasive, have been introduced from other countries, particularly Europe and Asia. They thrive for many reasons. They produce enormous amounts of seeds. For example, one purslane plant could produce one million seeds. They have great seed vitality. Their seeds ripen after cultivation or mowing and their seeds ripen early in the spring before the crop seeds. They have developed specialized methods of seed dispersal and they have a disagreeable odor or taste and are avoided by livestock. Some have large creeping rootstocks or tubers. Most of us do not want to pollute, but when it comes to weeds, even the purists get out the Roundup. Man has continued to fight weeds throughout this century by inventing mechanical devices to duplicate the work of the hoe, producing specialized chemicals, and developing management practices such as prescribed burns. Weeds invade a variety of disturbed habitats including lawns, pastures, fields, roadsides, waste places, cut-over forests, ditches and railroad beds. Weeds are not picky; they are opportunistic. This article provides a display of some of the weeds we encounter or observe throughout the year, and describes their habitat. Volunteers in Grassy Lawns
Most lawn weeds are winter annuals whose seeds ripen throughout the summer, germinate in fall, grow a little in the winter, lie dormant until early spring, and then begin blooming by March. Mouse-ear chickweed, wild onion, henbit, ground ivy, deadnettle, hop clover, plantain, wild mustards, speedwell, heal-all, white clover, crabgrass, and dandelion are a few of the unwanted intruders.
Henbit - Lamium amplexicaule Henbit is a member of the mint family introduced from Europe. It has clasping leaves with lavender flowers, blooming from March through summer. The common name implies that the seeds of the plant are eaten by chickens. The plants are four to 12 inches tall.
Red dead nettle - Lamium purpureum Red dead nettle is similar to henbit but the leaves are heart shaped, purplish and are overlapping at the top. Deadnettle can grow in dense mats covering a large area of the lawn.
Dandelion - Taraxacum officinale Dandelion is a very familiar perennial growing from a long taproot. It was introduced from Eurasia and has basal leaves that can be up to one foot long. The fragrant yellow flowerheads bloom from February to September (usually throughout the year) and later form a white fluffy ball, with parachute-type seeds. The many uses for this plant include beer and wine made from the flowers. The leaves, raw or cooked, can be eaten as a green. A coffee-type beverage can made from the dried roots, and the milky juice in the stem can be used as a medicine. The flowers are important to bees for nectar.
Ground ivy - Glecoma hederacea Ground ivy is a perennial herb in the mint family from Eurasia. It blooms from March to July. Other common names for ground ivy are "alehoof," referring to the leaves used in brewing ale long before hops were used, and "gill-over-the ground," with gill derived from the French "guiller" - to ferment. The green and dried leaves are poisonous to cattle.
Wild onion, wild garlic - Allium sp., Allium vineale Field garlic was introduced from Europe and is one of the most noxious weeds. It has been recorded as a problem for more than 100 years. The bulbs have a strong odor and taste and can ruin the taste of milk and butter when digested by cows. Wild garlic and can decrease the value of flour when harvested with wheat.
White clover - Trifolium repens White clover, a winter and spring annual, was introduced from Europe as a forage crop. It spreads by seed and has a creeping stem. The flowers, blooming from May to October, are important to bees for nectar. White clover was brought to North America by colonists as a component of "English Grass" along with Kentucky bluegrass.
Common blue violet - Viola sororia var. sororia Native to U.S., the common blue violet's purple flowers bloom from March to June. It can be a dominant plant in lawns. The leaves are high in Vitamin A and C and can be used in salads or as a green. The English settlers used violets in cosmetics and sweet waters.
Wood sorrel - Oxalis stricta Wood sorrel is a perennial herb native to the U.S. with clover-shaped leaves and green upright capsules (fruit). The leaves and fruit have a pleasant sour taste; the capsules are sometimes called "pickles." It has been used as a folk remedy for treatment of cancer. Heal-all - Prunella vulgaris Introduced from the Old World, heal-all has a dense terminal spike of purple flowers that bloom from May to September. The plants are used medicinally to treat throat ailments.
Venus's looking-glass - Triodanis perfoliata var. perfoliata (Specularia perfoliata) Native to U. S., Venus' looking-glass has blue-violet flowers set singly in the axils of clasping leaves. Venus' looking-glass blooms from May to August, grows in dry woods but can occur in cultivated flower gardens.
Carolina geranium (or cranesbill) - Geranium carolinianum Native to U. S., the Carolina geranium has small pink flowers with palmately lobed leaves and a distinctive elongated beaked capsule. The genus name is derived from the Greek geranos, a crane, relating to the bill-like fruit.
Volunteers of Wooded Edges, Gardens, or Ditches
Japanese honeysuckle - Lonicera japonica Japanese honeysuckle is a woody vine introduced from Asia in the 1800's as an ornamental vine. The sweet smelling white and yellow flowers bloom from May through July. It is reported to grow up to 30 feet in one year and will engulf a woodland and strangle all vegetation. It outcompetes the native flora. Birds are fond of the blackberries and thus spread the seeds over large areas.
Pokeweed - Phytolacca americana Pokeweed is a native perennial that can grow up to six feet tall. Although the plant is poisonous, especially the roots and berries, the springtime leaves can be boiled and eaten as a green. The berries were used by the settlers in the New World to make one of the first natural inks. Birds feed on the berries and sometimes act drunk afterwards.
Smartweed - Polygonum sp. In the buckwheat family, many smartweed species are native to the U.S. and grow in moist areas. The flowers are pink, rose and white, blooming from June through September. The hard seeds (achenes) are important food for wildlife and can be used by man for a flour. Seeds have been recovered from prehistoric Indian sites in the Eastern U.S. These plants contain an acrid juice that cause the mucous membranes to "smart." American smartweeds were taken to England in the 18th century to use as ornamentals. Several species from Asia, including the ornamental Polygonum cuspidatum, are aggressive and invasive and are very difficult to eradicate. Poison ivy - Toxicodendron radicans Poison ivy is a woody climbing vine supported by aerial roots, native to the U.S. All parts of the plant produce a nonvolatile oil causing a severe reaction in the form of blisters on human skin. Learn to identify the leaves!
Lamb's-quarters - Chenopodium sp. A summer annual with minute greenish flowers, lamb's- quarters can produce up to 70,000 seeds per plant in one year. Chenopodium album, introduced from Eurasia, is one of the world's most troublesome weeds, invading gardens and disturbed sites. Several species of Chenopodium are native to the U.S. and have been used for food since prehistoric times. The tender shoots can be cooked as a green and the seeds can be ground into a flour.
Pigweed - Amaranthus sp. Pigweed is another prolific seed producer similar in habit and aggressiveness to lamb's-quarters. Native to the Eastern U.S. and Central and South America, it was grown as a major crop by the Aztecs and is still used today as a food source. However, many species such as spiny amaranthus are noxious weeds.
Tall morning glory - Ipomoea purpurea Tall morning glory is an invasive vine from Tropical America. It has funnel-shaped purple, blue or white flowers blooming from June through September. Hernando Cortes collected morning glory seeds from the Aztecs and took them back to Spain for the monastery gardens. The whole plant can be used as a laxative.
Horsenettle or nightshade- Solanum carolinense Horsenettle's genus name comes from the Latin "solamen" meaning "comfort, solace" referring to its narcotic properties. Livestock and poultry have been poisoned by eating the plants. There are many native and European species of horsenettle. They can reduce the quality of grazing pastures due to the spines densely covering the plant. Volunteers of Pastures and Cultivated Fields
Queen Anne's lace - Daucus carota This beautiful plant in the parsley family, Queen Anne's lace is from Eurasia and is a variety of the cultivated carrot. The flowers are white, usually with one purple floret in the center of the head. Dried up flowers form a cup-shape and are called "bird's nests." The deep taproot is edible. The plant can cause dermatitis and give milk a disagreeable odor when eaten by cattle.
Buttercup - Ranunculus sp. The bright yellow, shiny buttercup's flowers make this one of the most showy plants in the early spring. Buttercups grow in moist pastures and fields; the Latin name "Rana" means frog and refers to the aquatic habits of some species. Many species are native, but the most common is Ranunculus acris, introduced from Europe. It is widespread in pastures, in part due to its acrid taste discouraging browsing by animals.
Thistle - Cirsium sp. and Carduus nutans There are many types of thistles, some species are native to North America and some are from Eurasia. They are among the most troublesome weeds. The seeds are airborne bristly plumes and the rootstocks can be divided by cultivation, with each part of the root producing new plants. Thistles are covered with sharp spines which can cause mechanical damage to mouths of livestock, especially in hay. Thistles are a big problem along roadway corridors due to the difficulty in controlling the production and spread of seeds.
Mayweed - Anthemis cotula Introduced from Europe, mayweed is also called stinking chamomile because of the pungent odor of the fern-like leaves. The daisy flowers bloom from May to October, with the dome-shaped yellow centers persisting after the white petals have fallen. This plant causes severe dermatitis in some people and is troublesome in pastures where it is shunned by livestock because of the acrid taste. It is known to repel insects.
Johnson grass - Sorghum halapense Johnson grass, introduced by William Johnson in the 1840's from the Mediterranean, is one of the worst weeds infesting pastures in the Southern U.S. Herbicides that are strong enough to kill it will also kill other crops around it. In severe cases, once infested, croplands, pastures and orchards must be abandoned. It was brought here for a hay crop but can be poisonous in early stages of growth. The frozen leaves of Johnson grass can be fatal to cattle. Cut hay or silage should be cured for six weeks.
Jimson weed - Datura stramonium Jimson weed is a tree-like summer annual in the nightshade family, also called Devil's trumpet. It is native to Asia. The large trumpet-shaped white flowers bloom from June to September. The leaves are poisonous and have mind-altering potential. The prickly fruits, called Devil's apple, were a dangerous ingredient of love potions in folk medicine. This species is becoming very popular as an ornamental plant in the south.
Field bindweed - Convovulus arvensis A noxious vine, field bindweed, in the morning glory family, is native to Eurasia and is difficult to eradicate because of the extensive root system, up to 10 feet deep. The rootstocks are toxic to swine. The large white or pink flowers bloom from June to October.
Volunteers of Roadsides and Waste Places
Chickory - Cichorium intybus Boasting a showy blue flower, chickory is an attractive perennial herb from the Mediterranean. It is related to endive and was introduced to the U.S. in 1785 as a salad green. Chickory can be aggressive. The roots are used as a popular coffee additive.
Mullein - Verbascum thaspus Mullein's large woolly leaves and tall spike of yellow flowers are conspicuous in the summer, growing up to seven feet tall. Introduced from Eurasia, the plants have been used by man for many purposes. The Romans used the stalks for torches and the leaf hairs are used as candlewicks. Mullein leaves were used for a yellow hair dye as early as the 4th century. Soap was made from the leaves. Colonists and Indians lined shoes with the leaves in the winter and a tea was made from the leaves for colds.
Ox-eye daisy - Chrysanthemum leucanthemum The ox-eye daisy is a perennial plant, native to Europe. It is a noxious weed in Tennessee. The showy white and yellow daisy flowers are attractive, thus, the species is very popular as a garden wildflower. However, it is a problem to farmers in pastures because of the pungent, bitter taste in the milk of browsing cattle. The ox-eye daisy seeds can contaminate grain seed and the amount of contamination is controlled by the Department of Agriculture. Plants are spread throughout the state on major roadways. Asiatic dayflower - Commelina communis Asiatic dayflower was introduced from Asia and is a reclining summer annual with deep blue showy flowers that bloom for only one day. It grows commonly in ditches along roadsides.
Teasel - Dipsacus fullonum A biennial herb introduced from Europe, teasel has cylindrical, spiny fruit and lavender flowerheads with large bracts curving upward, persisting through the winter. The plants were originally grown by wool manufacturers for the dried flowerheads that were used on spindles to "tease" the cloth. All parts of the plant are covered with short spines similar to thistle. Teasel aggressively invades roadsides and waste places. The fruit is popular for dried flower arrangements.
Crown vetch - Coronilla varia Crown vetch, a species in the pea family, grows in thick masses along Interstate roads throughout the Eastern and Midwestern U.S. It was introduced from Europe to stabilize soil and has since become naturalized. The pink flowers bloom from June through August. The plants have no wildlife value and can spread aggressively into pastures. Kudzu - Pueraria lobata Kudzu is an excessively aggressive vine introduced from Japan after it was exhibited in 1876 at the Philadelphia Centennial Exposition. It was grown in the southern U.S. for livestock fodder and pasture. In 1935, the Soil Conservation Service began using it for severe soil erosion caused by overfarming. It soon became known as "King Kudzu." By the 1950's, kudzu had become a curse, with plants growing 100 feet a year. The beautiful lavender flowers, blooming from August through September, have a strong grape-like fragrance that fills the air. The large tubers and seeds are eaten in Japan.
Goldenrod - Solidago sp. Most of the goldenrod species are native to the U.S. The yellow flowering plants blanket fields and roadsides throughout the fall season. The showy, large cluster of flowers of the different species take on many shapes and forms - pyramidal, elongated or cylindrical, flat-topped, bell-shaped, or arching on one side of the stem. It has been proven that goldenrod does not cause hay fever, but does, indeed, have some medicinal value. The plants can be a nuisance to farmers in pastures and hay meadows.
(Andrea Brewer Shea is endangered species coordinator with the TDEC's Division of Natural Heritage.)
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