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Weeds

Weeds – wild plants that live on agricultural land and reduce the value and (or) quality of production.

Weeds crops – cultivated plants that are weeds in the subsequent crops.

Useful weeds are plants that people often think of as weeds, but are grown in gardens and other cultivated areas.

Superweeds are weed plants that are resistant to the action of herbicides.

A field infested with weeds
A field infested with weeds

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Conceptual clarification of the term

The meaning of the term “weed” depends on subjective human perception. Calling certain plant species weeds is incorrect, since the same species can be a weed, a crop, a herb, an indicator plant, or the like. For example, sweet clover is used as a sideratum, wheatgrass for hay. A number of weed plants are used in veterinary medicine (Atropa belladonna, Valeriana officinalis, Gnaphalium uliginosum, Plantain (Plantago major), etc.[1]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 p.).

V.R. Williams referred to weed plants from the perspective of the farmer any plant that does not meet the goals of this crop. A.I. Maltsev argued that weed plant should be considered as wild or semi-cultivated plants, which not by the will of the farmer grow on the arable land, adapted to the arable conditions and grow together with cultivated plants.

According to GOST 16265-89 “Farming. Terms and Definitions”, weed plants are wild plants that live on agricultural land and reduce the value and quality of production.

A plant becomes a weed only when it is perceived as “bothersome, undesirable.” It does not matter whether the weed is a herbaceous or a woody species.

In forestry, the term “collateral growth” is often used because both negative and positive collateral effects can be expected.

In organic agriculture, the term “weed” means not only a noxious plant, but also an integral part of the ecosystem.

Plants are called weeds if:

  • they compete with cultivated plants for growth factors such as nutrients, light, and water, which affects economic returns;
  • there may be damage to crop quality;
  • have mass distribution (by sowing seeds, rapid or deep rooting, moving competitors), and thus threaten unvegetated land;
  • they disturb human aesthetics, for example in ornamental gardens, parks, lawns or areas without vegetation;
  • create a toxic effect that causes unsuitable farmlands (Colchicum in hay);
  • displace native plants (neophytes) from their habitats.

The term “weed” also applies to invasive plants, that is, those spread by human activity, outside their natural habitat. This includes man-made species for various economic purposes, such as Sosnowsky’s borage (Heracleum sosnowskyi), Mantegazzianum borage (Heracleum mantegazzianum), or introduced from other places, such as those in Europe include: Chaenomeles, sumac deerberry (Rhus typhina), glandular bramble (Impatiens glandulifera), and late bilberry (Prunus serotina).

The origin of weeds

Due to the systematic selection and cultivation over hundreds of years of some plant species with wide ecological plasticity, high productivity and a number of useful in terms of nutritional and economic properties, such plants as rye, oats, ryegrass, buckwheat and other species were introduced into cultivation.

Along with the emergence of the first cultivated plants also appeared weedy plants. Evidence of the origin of weeds simultaneously with the emergence of agriculture are the findings of archaeologists of the seeds in the plant remains of such weeds as white vermilion (Chenopodium album), rye brome (Bromus secalinus), oats (Avena fatua), common dipper (Agrostemma githago), common hedgerow (Echinochloa crus-galli), clementbush (Galium aparine) and others. These findings are dated to the Stone and Bronze Ages.

It has been suggested that weeds, in the sense of the term, rapidly growing plants that exploit the environment disturbed by man, arose in response to the Neolithic agricultural revolution some 12,000 years ago. However, evidence found at Ohalo II, an archaeological site in Israel, suggests that “proto-weeds” existed long before that – at least 23,000 years ago.[2]Ainit Snir; et al. (July 22, 2015). “The Origin of Cultivation and Proto-Weeds, Long Before Neolithic Farming.”

Seeds of some species of weed plants were found in the European tundra during the Ice Age, other species originated from natural species in frequently disturbed ecosystems (riverbanks, rocky slopes, dunes, marshes).[3]Jauzein, P., 2001. Biodiversité des champs cultivés: L’enrichissement floristique. Dossier de L’environnement de l’INRA 21, 43-64.[4]Baker, H.G., 1974. The evolution of weeds. Annual Review of Ecology and Systematics 5, 1-24. doi:10.1146/annurev.es.05.110174.000245.

Agriculture probably originated in the fertile zone of the Crescent[5]Ulrich Willerding: Zur Entwicklung von Ackerunkrautgesellschaften im Zeitraum vom Neolithikum bis in die Neuzeit in Der prähistorische Mensch und seine Umwelt. In: Forsch. u. Bericht Vor- und … Continue reading and, then, appeared in Central Europe around 5000 B.C. Weed plants spread along with the spread of agriculture. Already adapted to agricultural practices, they quickly adapted to the agroclimatic conditions of the new environment.

During archaeological research, it was found that the species composition of weeds in the period from 4000 B.C. (Neolithic to 1250 B.C.). (Neolithic) to 1250 BC. (Bronze Age) practically did not change and was represented by the following species in decreasing order of frequency: Quinoa (Atriplex), buckwheat (Fagopyrum), common warthog (Lapsana communis), sterile brome (Bromus sterilis), common elder (Galium), common sorrel (Rumex acetosa), common bluegrass (Poa trivialis), Persicaria maculosa and various vetch[6]Thomas Eggers: Werden und Wandel der Ackerunkraut-Vegetation. In: Otti Wilmanns, Reinhold Tüxen (Hrsg.): Werden und Vergehen von Pflanzengesellschaften. In: Berichte der Internationalen Symposien … Continue reading.

Fertile Crescent Zone
Fertile Crescent Zone

In Europe, agriculture spread both along the Mediterranean coast and up the Danube valley. These two directions are marked by differences in weed genotypes. For example, wild oat (Avena sterilis), represented in Northern Europe, and animated oat (Avena fatua), represented in Southeastern Europe.[7]Jauzein, P., 2001. Biodiversité des champs cultivés: L’enrichissement floristique. Dossier de L’environnement de l’INRA 21, 43-64.

Species that arrived in Europe before 1500 B.C. are called archaeophytes. Weeds introduced after 1500 B.C. – neophytes.

In Roman times, many species of weeds characteristic of cereal crops were imported by seeds from Mediterranean countries.

The discovery of America and the development of trade with other continents, caused an influx of new weed species into Europe, mainly from North America. The rate of appearance was about 3 species per year until the nineteenth century, and about 30 species per year in the twentieth century.[8]Jauzein, P., 2001. L’appauvrissement floristique des champs cultivés. Dossier de L’environnement de l’INRA 21, 65-78.[9]CLEMENT E.J, FOSTER M.C., 1994. Alien plants of the British Isles : a provisional catalogue of vascular plants (excluding grasses). Botanical Society of the Br. Isles, London, 590 p. For climatic reasons, they have spread predominantly in southeastern Europe. Toward the north of Europe their frequency decreases.

Among the species imported from the Americas are: amaranth tipped (Amaranthus retroflexus), common ragweed (Ambrosia artemisiifolia), common datura (Datura stramonium), etc.[10]P Jauzein, “Biodiversité des champs cultivés : l’enrichment floristique”, Dossier de l’environnement de l’INRA, no 21, 2001, p. 43-64

Chamomile fragrant was introduced to Europe from the Far East, Crested cress unequal-toothed (Senecio inaequidens) and Goat’s sagebrush (Oxalis pes-caprae) from Southern Africa.

In the Middle Ages most of the fields were cultivated according to the principle of three-field farming (winter cereals – summer crops – fallow). During the fallow period, cattle were driven into the fields. Cattle were even driven into the young sprouts of cereals, because there were significantly more weeds, and the damage to crops was negligible. At the same time it improved tillering. This period is characterized by the spread of weeds of pastures and meadows. Targeted weed control was accomplished by weeding.

In the 18th century there was an improved tri-weeding (winter cereals – summer crops – root crops), which contributed to the spread of weeds typical of the crops of root crops and row crops.

Changes in agricultural practices lead to a decrease in diversity and abundance of weeds, which begins at the end of the 18th century and then increases after 1950. This loss of biodiversity is caused by improvements in agricultural practices: seed sorting, mechanical weeding of crops, increased complexity of crop rotations, and then, after 1945, the use of herbicides.

Since the 1950s, the addition of mechanized weeding, new crop rotation systems, and better control of the environment (liming, fertilization, drainage) have led to soil homogenization.

For example, weeds such as field toadstool (Filago arvensis) and field bilberry (Nigella arvensis) have not been observed in France since 1920. In 2001, 300 species were found to be declining in population and 100 were endangered. The average number of weed species in Europe decreased by 20% between 1945 and 2000 and by 42% between 1970 and 2000 in France[11]Jauzein, P., 2001. L’appauvrissement floristique des champs cultivés. Dossier de L’environnement de l’INRA 21, 65-78. [12]Fried, G., Petit, S., Dessaint, F., Reboud, X., 2008b. Arable weed decline in Northern France: Crop edges as refugia for weed conservation? Biological Conservation 6-11. … Continue reading [13]Richner Holderegger Linder Walter 2015, “Reviewing change in the arable flora of Europe: a meta-analysis”, Weed Research, vol. 55, no. 1, 2015-02-01 1er février 2015, p. 1-13 (ISSN … Continue reading.

Since 1980, there has been an increase in the number of weed species, probably due to the development of organic farming and the decreased use of herbicides. Predominantly these species include species that prefer rich soils, neophytes, or monocotyledons. The numbers of rare and endangered weed species are not increasing[14]Richner Holderegger Linder Walter 2015, “Reviewing change in the arable flora of Europe: a meta-analysis”, Weed Research, vol. 55, no. 1, 2015-02-01 1er février 2015, p. 1-13 (ISSN … Continue reading.

The problem of weeds is as old as agriculture itself. The Bible mentions them as God’s punishment for sin.

Herbology

Herbology is the field of weed science, weed biology and ecology, control methods, and related environmental impacts.  It has developed in recent decades.

Covers the following issues:

  • competitiveness of weeds in relation to arable crops;
  • harmfulness of weeds;
  • positive effects of weeds;
  • determination of thresholds of harmfulness of weed vegetation;
  • evaluation of biological effectiveness of herbicides and their mixtures depending on weather conditions, soil conditions and degree of infestation;
  • chemical, physical, mechanical, biological and agrotechnical methods of weed control;
  • testing the varietal sensitivity of agricultural plants to herbicides;
  • immunization of weeds;
  • environmental consequences of herbicide use;
  • follow-up treatment with herbicides;
  • residues of herbicides in the environment and plants;
  • development of an integrated method of weed control;
  • making phytosociological maps;
  • weed zoning.

Harm caused by weeds

Weeds cause considerable damage to agricultural production. In 1996 it was estimated that weeds reduce yields by 20-40%.[efn_note]Erich-Christian Oerke, Ulrike Steiner: Ertragsverluste und Pflanzenschutz. Die Anbausituation für die wirtschaftlich wichtigsten Kulturpflanzen. In: Schriftenreihe der Deutschen Phytomedizinischen Gesellschaft. Band 6. Ulmer Verlag, 1996.[/efn_note] Globally, by 10%.

According to the CINAO, in Russia more than 60% of the area under cereal crops is moderately to severely weeded. The use of agronomic methods of weed control is decreasing due to the reduction of the share of pure fallow (9,2%) and row crops (23,7%) in crop rotations, as well as the transition to no-tillage cultivation. 

The negative impact of weeds on cultivated plants is the direct deterioration of the living conditions of plants.

Weeds damage agriculture by:

  • creating a deficit of moisture and nutrients for cultivated plants;
  • shading of crops;
  • mechanical effects on cultivated plants;
  • suppressing the growth of cultivated plants through toxic effects (allelopathy, can also have a positive effect);
  • the spread of diseases and pests;
  • complication of production activities;
  • damage to livestock production;
  • deterioration of product quality;
  • lower yields.

In the absence of a proper system of weed control, conditions are created for their accumulation, which aggravates the harm in subsequent periods.

Weeds damage not only agricultural land, but also industrial, transport and other economic areas and facilities, opening hard surfaces of roads and airfields, destroying buildings and structures, blocking waterways, etc.

Dandelion growing near the wall
Dandelion growing near the wall

Positive effects of weed plants

Weed vegetation can have a neutral or positive effect on the functioning of agrocenoses. They provide food for invertebrates, birds, and microorganisms, some of which destroy agricultural pests. For example, loosestrife (Convolvulus), when properly used, helps maintain a symbiosis of beneficial mycorrhizas during the winter.

Proper use of weeds can protect the soil from erosion, and some species are capable of fixing atmospheric nitrogen. They can be useful as fertilizers, forage, food, dyes, or medicines (chamomile (Matricaria), plantain (Plantago), field horsetail (Equisetum arvense)). Ground cover weeds – as mulch in fruit and berry plantings and vineyards. For example, the poppy (Chenopodium) and quinoa (Atriplex), considered weeds in modern farming, were grown as food sources before the crops imported by Columbus from the Americas.[15]Zimdahl, R. L., 2007. Fundamentals of weed science. Elsevier , Amsterdam, The Netherlands. doi:10.1016/0378-4290(95)90065-9 [16]Wilson, J. D., Morris, A. J., Arroyo, B. E., Clark, S. C., Bradbury, R. B., 1999. A review of the abundance and diversity of invertebrate and plant foods of granivorous birds in northern Europe in … Continue reading [17]Franke, A. C., PLotz, L. A., van der Burg, W. J., van Overbeek, L., 2009. The role of soil weed seeds for agroecosystem functioning. Weed Research 49, 131-141. doi:10.1111/j. 1365-3180.2009.00692.x These plants were used as food during mass starvation in Russia, including during the Great Patriotic War of 1941-1945.

Weed vegetation can be used as indicator plants that can very accurately characterize the condition of the soil, such as lack or excess of organic matter of plant or animal origin, compaction, degradation, excess nitrates, etc.).

Chinese naturalists have long observed the characteristics of weeds. They noticed that the occurrence of certain plants in a particular place could be an indicator of the presence of underground deposits of zinc, selenium, nickel or copper. Combining the knowledge of mineralogy and botany, a section was created: geobotany.[18]Robert Temple, Le génie de la Chine, Trois mille ans de découvertes et d’inventions, traduit par Joëlle Faye, Alain Impens, Marlyse Schweizer et al, Picquier, Arles, 2007, cité par Clifford … Continue reading.

Some species of weeds have ornamental value, such as poppies (Papaver), chamomile (Matricaria), cornflowers (Centaurea) and others or used as honey plant, attract pollinators.

In the United Kingdom, an attempt is being made to culturally use the weed plant Field Sparrow (Buglossoides arvensis) for the production of omega-3 fatty acids.[19]Nowoczesna uprawa 7/2011. APRA Sp. z o.o., 2011, s. 56. ISSN 1896-9046.

Poppy, chamomile, and cornflower
Poppy, chamomile, and cornflower are considered a weed in agriculture, but have ornamental value

Biological and ecological characteristics of weeds

Over a long period of time, various species of weeds have acquired morphological and biological features very similar to the cultivated plants in whose crops they most often grow, which helps their spread.

The main features of weed plants are:

  • less demanding to agroclimatic conditions;
  • high seed production;
  • the ability to reproduce vegetatively;
  • the ability of seeds to spread over long distances in different ways; 
  • the ability to retain seed germination for a long period;
  • long germination period of weed seeds;
  • high survival rate in a variety of habitats.

Agrophytocenoses of agricultural lands

Agrophytocenosis is a plant community consisting of crops and unwanted vegetation that forms on cultivated land.

The abundance of uncultivated plants in agricultural fields varies greatly and is determined by a number of conditions:

  • the natural conditions of a particular habitat;
  • the seed stocks of the different species of weeds in the soil;
  • the floristic composition of the surrounding areas and lands;
  • agrotechnics of crops cultivation;
  • species and variety composition of cultivated plants;
  • level of cultivation culture and other factors.

Within one agrophytocenosis plants with different ecological and biological characteristics have a diverse mutual influence on each other. The nature, severity and direction of plant interactions in the field strongly depend on specific agroclimatic conditions. Therefore, a specific agrophytocenosis, different from the agrophytocenoses of neighboring fields and territories, is formed on a single field or on each of the dissimilar plots.

Some of the weeds over a long period of evolution have become so adapted to growing among cultivated plants that they do not occur outside the crops. For example, purple cockle (Agrostemma) is a weed of grain crops, small-fruited rye (Camelina microcarpa), which is found in flax crops, etc.

Classification of weeds

In Russian farming practice (and some Eastern European countries) use the classification according to the biological characteristics of weeds: method of nutrition, life span and method of reproduction.

Parasites:

  • Stem parasites
  • Root parasites

Semiparasites:

  • Stem semiparasites
  • Root semiparasites

Non-parasites:

  1. Annuals and biennials (juveniles):
    • Ephemerals
    • Early spring crops
    • Late spring crops
    • Winter
    • Wintering
    • Biennials
  2. Perennials
    • Rhizomatous
    • Root shoots
    • Creeping
    • Fibrous-rooted (brush-rooted)
    • Taproot
    • Tuberous
    • Bulbous

In addition to the above classification, there are also specialized, hard-to-remove and quarantine weeds.

Types of weeds

In Russian farming practice, it is customary to divide weeds into species according to various characteristics:

  1. According to the method of spreading:
    • anthropochores – get to other fields in the process of human economic activity (with seed, manure, harvesters, etc.).
    • apophytes – come to arable land from surrounding natural areas or are renewed from seed stocks preserved in the soil after plowing a specific area. For example, couch grass (Elytrigia repens), field thistle (Cirsium arvense), field horsetail (Equisetum arvense), etc.
  2. According to habitat conditions:
    • Weedy-field or segetal (from Latin segetalis – growing among the loaves) – weed species that prefer permanently cultivated land and are well adapted to the crops. For example, wild radish (Raphanus raphanistrum), field mustard (Sinapis arvensis), common boll (Agrostemma githago), oats (Avena fatua), cornflower blue (Centaurea cyanus), chamomile non-scented (Tripleurospermum inodorum), rye brome (Bromus secalinus), tatar buckwheat (Fagopyrum tataricum), creeper’s thistle (Fallopia convolvulus), hedgehog (Echinochloa), field thistle (Sonchus arvensis), bar-thistle (Euphorbia virgata) and many others. After cessation of tillage, weeds of this group completely fall out of the forming grass stand.
    • Trash or ruderal weeds (from Latin ruderis – construction debris, rubble) inhabit mostly near residential and household buildings, at household and industrial waste dumps, along roadsides and roadsides, etc. For example, the black hemlock (Hyoscyamus niger), the lily of the valley (Lepidium ruderale), the stink bug (Datura stramonium), the white chemise (Chenopodium album), the common wormwood (Artemisia vulgaris), etc.
    • Meadow, or grassland, or weeds of natural lands – weeds clog natural forage lands. For example, poisonous eel (Cicuta virosa), tall aconite (Aconitum septentrionale), tall larkspur (Delphinium elatum), Lobel’s hemlock (Veratrum lobelianum), hemlock (Conium maculatum), Stellaria graminea, Cynoglossum officinale, Ranunculus repens, etc.[20]Nikitin V.V. Weedy plants of the flora of the USSR. – L.: Nauka, 1983.

There are 650 species of weed plants in the agricultural lands of Europe, France – 220, Czech Republic – 198, Russia – more than 1100 (1330[21]Farming. Textbook for universities/G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 p. [22]Fundamentals of agronomy: textbook / Yu.V. Evtefeev, G.M. Kazantsev. – MOSCOW: FORUM, 2013. – 368 p.: ill.) species. The harmfulness for cultivated plants of each of these species is ambiguous and varies depending on natural zones and the level of intensification of agriculture.

The floristic composition of one agricultural zone is limited to 80-100 species and is determined by the historical, natural conditions of the zone and the established technology of cultivation. The number of dangerous weeds in one crop area usually does not exceed 10-15 species, which composition, besides the above mentioned, also depends on ecological conditions, the type of crop and its agricultural technology.[23]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 p..

In addition to independently developing weeds, there are more than 120 species of parasitic weeds (36 species of Dodderweed (Cuscuta) and more than 30 species of Orobanche) and 220 species of semi-parasitic weeds.

Weed crops

Weed crops – crops plants found in the crops of another cultivated crop or another variety and which are undesirable.

For example, plants of winter rye found in the crops of winter wheat reduce the quality of the resulting grain. The presence of spring wheat plants of this crop of another variety makes it impossible to use the resulting grain for seed production purposes.Spring wheat crops may be heavily littered with sunflower shoots formed from sunflower seeds that fell off during harvesting and overwintered in the soil.

Useful weeds

Useful weeds can perform a number of functions in the garden or yard, including fertilizing the soil, increasing moisture content, acting as shelter or living mulch, deterring pests, attracting beneficial insects, ornamental plants, use as food or other needs.

Useful weeds can include:

  • mimosa pudica (Mimosa pudica) – used as a natural ground cover in tomato and pepper crops, attracts predatory insects;
  • chinchilla thistle (Euphorbia lathyris) – repels moles;
  • vine onion (Allium vineale) – used in crops of fruit trees, nightshades (tomatoes, peppers, potatoes, etc.), cruciferous (cabbage, broccoli, kohlrabi, etc.), carrots to repel slugs, aphids, carrot flies;
  • rosehip (Rosa) – used as an ornamental shrub and medicinal agent (fruit).
Mimosa pudica (Mimosa pudica)
Mimosa pudica (Mimosa pudica)

Accounting and mapping of weeds

The purpose of accounting and mapping of weeds is to obtain systematic accurate data on the floristic and quantitative composition of weeds and their geographical distribution in a particular area (region, locality, farmland, farm) for the purpose of making a sound and effective program of mass distribution prevention and control of weeds.

Weed control measures

Thresholds of harmfulness of weeds

In view of the fact that in practice it is not possible to destroy weeds even for several seasons on agricultural land, the feasibility of weed control is determined based on the degree of weed infestation (threshold of harmfulness) and the economic costs of control.

The following thresholds of harmfulness are distinguished:

  1. Phytocenotic threshold of harmfulness (PTH) – the degree of weed infestation of cultivated crops, in which weeds do not cause harm.
  2. Critical (statistical) threshold of harmfulness (CTH) – the degree of weed infestation of cultivated crops, in which the harm from weeds does not exceed statistically unreliable yield losses, that is 3-6% of the actual yield.
  3. Economic threshold of harmfulness (ETH) is a degree of weed infestation of cultivated crops in which complete elimination of weeds provides an increase in yield that pays for the cost of extermination measures and harvesting of additional products.

Classification of methods of weed control

  1. Preventive measures – identification, localization and elimination of sources, foci and pathways of weeds – are divided into the following control methods:
    • quarantine; 
    • organizational.
  2. Extermination measures – destruction of vegetative weeds on agricultural lands, as well as organs of their generative and vegetative reproduction, being in the soil to reduce the viability of weeds. Subdivided into the following control methods:
    • Agrotechnical (provocation, mechanical and physical destruction, depletion, suffocation, desiccation, freezing, etc.);
    • biological (phytocenotic, ecological, allelopathy, crop rotation, etc.);
    • chemical;
    • special;
    • combined.

Sources

Fundamentals of agricultural production technology. Farming and crop production. Edited by V.S. Niklyaev. – Moscow: Bylina, 2000. – 555 с.

Bazdyrev G.I. Weed plants and measures to control them in modern agriculture: Textbook for universities. M.: Publishing house of the Moscow Agricultural Academy, 1993. – с. 242.

Fundamentals of agronomy: textbook/Y.V. Evtefeev, G.M. Kazantsev. – MOSCOW: FORUM, 2013. – 368 p.: ill.

Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – Moscow: Publishing House “Kolos”, 2000. – 551 с.