Herbicides are substances used to kill unwanted plants, often used in modern gardens in lieu of or alongside other weed control techniques. The use of herbicides has benefits in terms of saving labor and time, but can also cause some problems by killing non-target plants or other organisms, contaminating the environment, and in some cases may have toxic effects on humans and other animals.
Types of herbicides
Herbicides are classified in several ways, including effect, selectivity, persistence, application, and action. They are also further divided into herbicides that are acceptable in organic growing methods and non-organic growing methods.
- Pre-emergent herbicides work by interfering with the germination of seeds. Pre-emergents must be well-timed according to the germination time of the weed being controlled, but when timed correctly they provide the best solution to the problem, as a preventative rather than a cure.
- (see Pre-emergent Herbicides for details on particular pre-emergents)
- Contact herbicides work by killing any part of the plant they come in contact with. In most cases these will not kill the entire plant and will need to be reapplied periodically, eventually draining the plant's energy stores and killing it after several applications.
- Systemic herbicides work by killing the entire plant over a short period of time, and are conducted throughout the plant through the vascular system. Many weeds are resistant to these herbicides however, so repeat applications are often needed.
- Drench herbicides are applied to the soil, rather than or along with application to the plant. Most of these herbicides are long-lasting, and therefore not appropriate for use in places where plants are to be grown.
Selective herbicides kill specific target plants while leaving the desired plants relatively unharmed. Herbicides used to control lawn weeds are in most cases selective herbicides.
Non-selective herbicides kill any plant they come in contact with (though some plants are resistant to various herbicides).
Different herbicides have various "persistence", meaning some stay active over a long period of time, while others become inactive shortly after application.
Herbicides can be applied in one of a few ways. Some are sprayed as a liquid, some applied as granules, some applied through irrigation systems or fumigation (though rarely done in gardens), and some are painted directly onto plant parts. Most herbicides can be applied using more than one method, depending on what they are being used for.
Their classification by mechanism of action (MOA) indicates the first enzyme, protein, or biochemical step affected in the plant following application. The main mechanisms of action are:
- Dessicators work by removing the water from plant cells, causing cell death.
- Acids and bases work similarly to dessicators, by chemically "burning" plant cells. These are substances that are extremely acidic or alkaline.
- Nutritional controls work by shifting the balance of nutrients, either providing too much of a particular nutrient, or restricting the availability of other nutrients. This most commonly involves slatering the soil pH, but in some cases certain nutrients can be used to control specific species of plants.
- ACCase inhibitors are compounds that kill grasses. Acetyl coenzyme A carboxylase (ACCase) is part of the first step of lipid synthesis. Thus, ACCase inhibitors affect cell membrane production in the meristems of the grass plant. The ACCases of grasses are sensitive to these herbicides, whereas the ACCases of dicot plants are not.
- ALS inhibitors: the acetolactate synthase (ALS) enzyme (also known as acetohydroxyacid synthase, or AHAS) is the first step in the synthesis of the branched-chain amino acids (valine, leucine, and isoleucine). These herbicides slowly starve affected plants of these amino acid]]s which eventually leads to inhibition of DNA synthesis. They affect grasses and dicots alike. The ALS inhibitor family includes sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinyl oxybenzoates (POBs), and sulfonylamino carbonyl triazolinones (SCTs).
- EPSPS inhibitors: The enolpyruvylshikimate 3-phosphate synthase enzyme EPSPS is used in the synthesis of the amino acids tryptophan, phenylalanine and tyrosine. They affect grasses and dicots alike. Glyphosate is a systemic EPSPS inhibitor but inactivated by soil contact.
- Synthetic auxin inaugurated the era of organic herbicides. They were discovered in the 1940s after a long study of the plant growth regulator auxin. Synthetic auxins mimic this plant hormone. They have several points of action on the cell membrane, and are effective in the control of dicot plants. 2,4-D is a synthetic auxin herbicide.
- Photosystem II inhibitors reduce electron flow from water to NADPH2+ at the photochemical step in photosynthesis. They bind to the Qb site on the D2 protein, and prevent quinone from binding to this site. Therefore, this group of compounds cause electrons to accumulate on chlorophyll molecules. As a consequence, oxidation reactions in excess of those normally tolerated by the cell occur, and the plant dies. The triazine herbicides (including atrazine) are PSII inhibitors.
Acceptability for organic growing
By organic is meant a herbicide that can be used in a farming enterprise that has been classified as organic. Organic herbicides are expensive and may not be affordable for commercial production. They are much less effective than synthetic herbicides but of course do not inject unnatural chemicals into the environment.
Organic herbicides include:
- Vinegar  - effective for 5-20% solutions of acetic acid with higher concentrations most effective but mainly destroys surface growth and so respraying to treat regrowth is needed. Resistant plants generally succumb when weakened by respraying.
- Steam - has been applied commercially but now considered uneconomic and inadequate  . Kills surface growth but not underground growth and so respraying to treat regrowth of perennials is needed.
- 2,4-D, a broadleaf herbicide in the phenoxy group used in turf and in no-till field crop production. Now mainly used in a blend with other herbicides that act as synergists, it is the most widely used herbicide in the world, third most commonly used in the United States. It is an example of synthetic auxin.
- Atrazine, a triazine herbicide used in corn and sorghum for control of broadleaf weeds and grasses. Still used because of its low cost and because it works as a synergist when used with other herbicides, it is a photosystem II inhibitor.
- Clopyralid, is a broadleaf herbicide in the pyridine group, used mainly in turf, rangeland, and for control of noxious thistles. Notorious for its ability to persist in compost. It is another example of synthetic auxin.
- Dicamba, a persistent broadleaf herbicide active in the soil, used on turf and field corn. It is another example of synthetic auxin.
- Glyphosate, a systemic nonselective (it kills any type of plant) herbicide used in no-till burndown and for weed control in crops that are genetically modified to resist its effects. It is an example of an EPSPs inhibitor.
- Imazapyr, is a non-selective herbicide used for the control of a broad range of weeds including terrestrial annual and perennial grasses and broadleaved herbs, woody species, and riparian and emergent aquatic species.
- Imazapic, is a selective herbicide for both the pre- and post-emergent control of some annual and perennial grasses and some broadleaf weeds. Imazapic kills plants by inhibiting the production of branched chain amino acids (valine, leucine, and isoleucine), which are necessary for protein synthesis and cell growth.
- Metoalachlor, a pre-emergent herbicide widely used for control of annual grasses in corn and sorghum; it has largely replaced atrazine for these uses.
- Paraquat, a nonselective contact herbicide used for no-till burndown and in aerial destruction of marijuana and coca plantings. More acutely toxic to people than any other herbicide in widespread commercial use.
- Picloram, a pyridine herbicide mainly used to control unwanted trees in pastures and edges of fields. It is another synthetic auxin.
Prior to the widespread use of chemical herbicides, cultural controls, such as altering soil pH, salinity, or fertility levels, were used to control weeds. Mechanical control (including tillage) was also (and still is) used to control weeds.
The first widely used herbicide was 2,4-dichlorophenoxyacetic acid, often abbreviated 2,4-D. It was developed by a British team during World War II and first saw widespread production and use in the late 1940s. It is easy and inexpensive to manufacture, and kills many broadleaf plants while leaving grasses largely unaffected (although high doses of 2,4-D at crucial growth periods can harm grass crops such as maize or cereals). 2,4-D's low cost has led to continued usage today and it remains one of the most commonly used herbicides in the world. Like other acid herbicides, current formulations utilize either an amine salt (usually trimethyl amine) or one of many esters of the base compound. These are easier to handle than the acid.
2,4-D exhibits relatively poor selectivity, meaning that it causes stress to non-target plants. It is also less effective against some broadleaf weeds, including many vinous plants, and sedges. A herbicide is termed selective if it affects only certain types of plants, and nonselective if it inhibits most any type of plant. Other herbicides have been more recently developed to achieve desired selectivities.
The 1970s saw the introduction of Atrazine, which has the dubious distinction of being the herbicide of greatest concern for groundwater contamination. Atrazine does not break down readily (within a few weeks) after being applied. Instead it is carried deep into the soil by rainfall causing the aforementioned contamination. Atrazine is said to have high carryover, a very undesirable property for herbicides.
Glyphosate, frequently sold under the brand name Roundup, was introduced in the late 1980s for non-selective weed control. It is now a major herbicide in selective weed control in growing crop plants due to the development of crop plants that are resistant to it. The pairing of the herbicide with the resistant seed contributed to the consolidation of the seed and chemistry industry in the late 1990s.
Many modern chemical herbicides for agriculture are specifically formulated to decompose within a short period after application. This is desirable as it allows crops which may be affected by the herbicide to be grown on the land in future seasons. However, herbicides with low residual activity (i.e. decompose quickly) often do not provide season-long weed control.
Herbicides have been alleged to cause a variety of health effects ranging from skin rashes to death. The pathway of attack can arise from improper applicatrion resulting in direct contact with field workers, inhalation of aerial sprays, food consumption and from contact with residual soil contamination. Herbicides can also be transported via surface runoff to contaminate distant surface waters and hence another pathway of ingestion through extraction of those surface waters for drinking. Some herbicides decompose rapidly in soils and other types have more persistent characteristics with longer environmental half-lives. Other alleged health effects can include chest pain, headaches, nausea and fatigue. Most herbicides (primarily the non-organic) must be extensively tested prior to labeling by the Environmental Protection Agency. However, because of the large number of herbicides in use, many are concerned about potential health effects. Some of the substances in use are alleged to be mutagenic, carcinogenic or teratogenic.