Weeds have been a part of civilization for as far back as recorded history. Many ancient documents speak of humans battling weeds in the crops they grew; where there has been agriculture, weeds have also been there. The spread of invasive noxious weeds on public lands in the West has been described by Steven A. Dewey of Utah State University as a "biological wildfire raging out of control." Wildfires and noxious weed infestation both may begin in a small area and spread slowly. The fire may spread quickly if embers are dispersed by the wind, creating "spot" fires which can grow and join each other and the main fire; weed seeds may be dispersed by wind, water or animals and create spot weed patches which grow and merge with the main infestation within a pasture, agricultural crop, recreation area, golf course, or backyard.

Wildfires and noxious weeds both dramatically change plant communities and may affect wildlife, livestock, crop production, soil erosion, water quality, or recreation; ecosystems extensively invaded by nonindigenous noxious weeds may be permanently altered.

Fire fighting is so similar to fighting weeds that modern wildfire management provides useful ideas on ways to improve noxious weed control programs. According to Dewey, wildfire management includes prevention (education and regulation), early detection and rapid response, containment and control, and site rehabilitation. Experience in developing and implementing effective public education programs in fire prevention could serve as a source of ideas to help increase the public's awareness, concern, and involvement in noxious weed prevention.

There are many definitions of a weed; the most commonly accepted definition is that a weed is a plant growing where it is not wanted. Almost any plant can be a weed if it is not wanted; however, plants that are commonly described as weeds can be desirable if they are growing on a hillside and controlling erosion.

Weeds are undesirable and considered to be pests just as insects and disease organisms are considered to be pests. They can be found everywhere that humans live or work, infesting lawns, clogging waterways, filling up ponds, invading flower beds, invading golf courses, and growing in pastures and forests. Weeds compete with crops for materials essential to growth, including fertilizer, water, sunlight and space.

Weeds may also serve as a host for insects and disease organisms, cause impurities in agricultural products, and be poisonous to livestock. Cows that graze in pastures where wild onion or wild garlic grow will produce milk that has an offensive odor and taste, which lowers the quality and value of the milk. Similarly, wool that contains burs or other parts of weed plants is less valuable because of the added expense of removal.

Tansy ragwort (Senecio jacobaea), cocklebur (Xanthium strumarium), nightshade (Solanum nigrum), and bracken ferns (Pteridium aquilinum) are examples of plants that can be poisonous to livestock. Tansy ragwort grows in the northwestern United States and can be deadly? if a horse ingests this plant, its liver ceases to function normally death can occur in a short time. The removal of these weeds from hay fields and pastures is time-consuming and expensive.

Weeds cause problems for humans: many people suffer from allergic reactions to pollen produced from noxious weeds during certain times of the year, such as ragweed (Ambrosia artemisiifolia) and goldenrod (Solidago spp.). In addition, weeds lower the aesthetic value of landscaped areas, including gardens, lawns, and golf courses.

Individual states determine which weeds are noxious, that is, the most difficult to control and create the biggest problem; obviously, a weed may be declared noxious in one state and not in another. Weeds are also classified according to their life span, annual, biennial, or perennial. The Federal Noxious Weed Act, Public Law 93, was enacted January 3, 1975 to establish a Federal program to control the spread of noxious weeds. The Secretary of Agriculture was given the authority to designate plants as noxious and the transport of all such weeds in interstate or foreign commerce was prohibited. The Secretary was also given authority to inspect, seize and destroy products, and to quarantine areas, if necessary to control, eradicate, or prevent or retard the spread of such weeds.

Particular plants become troublesome pests because they grow and reproduce efficiently and out-compete agricultural crops ; many weeds produce a large number of seeds, as many as 250,000 from a single plant. Many species have adapted efficient methods of seed dispersal: the milkweed (Asclepiadaceae syriaca), thistle (Cirsium odoratum), and dandelion (Taraxacum officinale) have long featherlike structures and may be carried by the wind for long distances; other seeds, such as cocklebur and beggar's lice (Galium aparine), have projections on their seed coats that may catch in the hair of animals or the clothing of humans to be carried away.

Seeds from the fruit of weeds may stick to the feet or bodies of birds and be carried to new locations; other weed seeds may be ingested by animals and pass through their digestive tracts far from the parent plant; some weed seeds can be carried by farm machinery from one field to the next or travel on wagons or crates; and, some seeds may be carried by water in streams or dispersed onto fields from open canals during flood irrigation. Weed seeds generally remain viable over long periods of time, inactive in dormancy until the germination process is triggered by soil moisture, soil temperature, or light.

Weeds may reproduce by more than one method, both sexually and asexually. For example, the noxious weed yellow nutsedge (Cyperus esculentus) propagates by seed and rhizomes, underground stems that grow out from the plant, forming tubers and new shoots. In developing a method of weed control, a dual form of reproduction makes the task more difficult.

Of the fifteen of the most serious weeds in the United States, thirteen were imported. For example, Kudzu (Pueraria lobata) was introduced from Japan in the late 1800's to control erosion in large areas of the southern U.S. The kudzu vine grows quickly in thin soil and the dense foliage is a palatable, high protein forage for cattle; furthermore, since kudzu is a legume and dies back each year at frost, it adds organic matter and nitrogen to the soil. Unfortunately, kudzu grows so quickly (up to one foot per day!) in a year's time, the aggressive vine can cover the ground or forest, blocking sunlight and killing trees; with its deep root system, it is difficult to control. Due to its prolific nature and lack of natural insect or disease controls, kudzu quickly became a pest in the southeastern United States; kudzu has the ability to out-compete and eliminate native plant species and upset the natural diversity of plant and animal communities.

As methods of controlling weed pests improve, growing crops becomes more efficient and profitable. Billions of dollars have been invested in research and a variety of methods are available to control weeds, including mechanical, chemical, biological, cultural control.

People learned that simply pulling weeds by hand from around plants is time-consuming and disrupts the roots of desirable plants; when farmers learned to plant crops in long rows, hoes and plows were invented to cut off, uprooted, or cover up weeds. This method was used in the United States until the 1960s when chemical herbicides became readily available.

Mechanical cultivation to control weeds has four disadvantages:
1. The roots may suffer some collateral damage,
2. Equipment is expensive to purchase, operate and
maintain,
3. Tillage can cause problems with erosion, and,
4. Cultivation results in soil moisture loss and may increase irrigation costs.

Centuries ago, farmers controlled weeds with salt; unfortunately, compounds such as salt are residual in the soil and prevent plant growth when concentrations are too high. With the invention of 2-4D around World War II, herbicides were developed rapidly and became widely used by the 1960s. Modern herbicides must meet rigid environmental standards set by the Environmental Protection Agency.


How do herbicides work ? The active ingredient of a specific herbicide may interfere with plant photosynthesis, inhibit plant enzyme production and cell growth, block cell division ("mitotic poisons") and plant growth, prevent the formation of carotenoids and other photosynthetic plant pigments, or disrupt auxins levels and plant growth.

Herbicide Selectivity ? Nonselective herbicides are chemicals which kill all vegetation; selective herbicides kill only a particular plant type. Post-emergent chemicals are applied to growing crops absorbed after contact with the leaves, stems, or roots. Pre-emergent chemicals are applied to the soil surface before seeds germinate. Plants have different responses to these systemic herbicides; some weeds are unaffected by certain herbicides because they can produce an enzyme to neutralize the chemical poison. If crop plants can be genetic engineered to produce enzymes to neutralize herbicides, weeds can be more efficiently controlled. Research and development of new herbicides and herbicide-resistant crop varieties is expensive and requires years of extensive testing.

Biologic agents include animals, insects, and disease organisms. Goats may be kept in areas where sheep and cattle graze to eat the coarser, brushlike plants and make more room for grasses, geese have been used in cotton, mint, and strawberry fields to control grass, and fish and snails are used to control aquatic weeds in lakes and aquaculture ponds.

Importation and release of phytophagous (plant-eating) insects for control of weeds in Hawaii began in 1902 with introductions against lantana (Lantana montevidensis), a pest primarily of lowland pasture. During a span of more than 50 years, at least 27 species of lantana-feeding insects were released, 15 of which were successfully established: one species eats the flowers and seeds, another eats the leaves, yet another girdles the stem of the bush. In all, 71 species of phytophagous insects and one species of fungus had been purposely introduced and released up to 1985 to aid in the control of 21 weeds. The largest number of insect species introduced belonged to the Lepidoptera (31 species of butterflies and moths), followed by the Coleoptera (24 species of beetles), and Diptera (8 species of flies). To date, 43 of the insects and the fungus have become established.

Disease organisms can also be used to control weeds, including bacteria, viruses, and fungi. Researchers in Minnesota examined fifteen genera of fungi frequently isolated from purple loosestrife for their potential as biological control agents. The objective was to develop a mycoherbicide that would be applied alone or in combination with other agents, such as insects, to reduce purple loosestrife populations. The most virulent fungus tested was Septoria lythrina.

The use of biological control is relatively slow and the introduction of insects, animals, or diseases can be expensive. In the near future, genetic engineering may provide pathogens that attack only specific species of weeds; however, producers must be vigilant that agents introduced for biocontrol do not become pests themselves.

Equipment, such as cultivators and plows, can spread weeds from field to field, especially if the weed has rhizomes which cling to the bottom. The rotation of crops can disrupt weed life cycles and provide control options, especially if the weed is relatively easy to manage in an alternative crop. Black plastic mulch can be used to achieve almost total weed control.

The use of cultural methods of weed control in conjunction with mechanical methods, herbicides, and biological controls is called integrated pest management, or IPM. By using the best of all these methods, weed control can be effective and efficient, while protecting the environment. In the future, scientists will find even better ways to control weeds that are friendly to the environment.