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Biological and ecological characteristics of weeds

Over a long period, 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 high proportion of extermination of weeds is compensated by a number of features:

  • less demanding to agroclimatic conditions, in particular more drought- and frost-resistant;
  • high seed production;
  • the ability to reproduce vegetatively;
  • the ability of seeds to spread over long distances in a variety of ways; 
  • preservation of seed germination for a long period;
  • weed seed germination over a long period of time;
  • high survival rate in a variety of habitats.

An effective system of weed management is impossible without a full and comprehensive understanding of their biological characteristics. In addition to a scientifically based farming system, knowledge of the peculiarities of weed life allows us to determine the periods of vegetation in which weeds are least resistant to various agronomic techniques or most sensitive to adverse external influences. That helps to design a rational and economically justified system of preventive and extermination measures, ensuring the effective destruction of both vegetating weeds and their regenerating organs in the soil.

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Biological similarity of weeds and cultivated plants determines their prevalence in crops: spring weeds in spring crops, winter and overwintering weeds in winter grain crops, etc.

The main features that distinguish weeds from cultivated plants:

  1. Ability to reproduce vegetatively. Many perennial weeds reproduce quickly vegetatively. Their underground parts, giving a mass of shoots with dormant buds, later develop into independent plants.
  2. Seeds of many weeds are capable of maintaining germination for many years. There have been recorded cases when the seeds of chickweed (Amaranthus), shepherd’s purse (Capsella), asterisk (Stellaria) and some other weeds did not lose their germination for 10-15 years, field mustard (Sinapis arvensis) – 7 years, field broom (Thlaspi arvense) and plantain (Plantago) – 9 years.
  3. Sprouts of weeds appear irregularly. This significantly complicates the fight against them, because germination can take a very long period. For example, one plant of white maria (Chenopodium album) produces two or three kinds of seeds. Some produce sprouts in the year of maturity, others in the following spring, and still others only in the third year after sprouting. The uneven emergence of sprouts of many weed species is due to the varied quality of the seeds, which have unequal viability and varying ability of the seed coat to pass water.

Seed productivity of weeds

In contrast to cultivated plants, the seed productivity of weed plants is orders of magnitude greater (table).

Table. Seed productivity of some weeds[1]https://de.wikipedia.org/wiki/Unkraut - "Wikipedia.org - Weeds (German)" [2]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. - Moscow: Publishing House "Kolos", 2000. - 551 p. [3]Fundamentals of agricultural production technology. Farming and plant growing. Ed. by V.S. Niklyaev. - Moscow: "Bylina", 2000. - 555 p. [4]Éléments de biologie des mauvaises herbes - Les leviers de gestion de la flore adventice, Agri-Réseau (Translation from the French UniversityAgro.ru).

Name of the plant
Productivity of one plant, seeds per year
Birdeye speedwell (Veronica persica)
50-100 (by other reports 1,500-8,000)
Flax, common flax (Linum usitatissimum)
60-100
Soft wheat (Triticum aestivum)
90-100
Wild oat (Avena fatua)
100-450 (by other reports 500-2,000)
Winter rye (Secale cereale)
120-200
Henbit (Lamium)
200-2,000
Cleavers (Galium aparine)
300-450 (by other reports 50-3,000)
Groundsel (Senecio vulgaris)
1,100-1,200
(Bromus secalinus)
1,420
Foxtail (Alopecurus)
1,500-10,000
Bermuda grass (Cynodon dactylon)
2,000-3,000
Bluebur (Myosotis)
2,000-6,000
Barnyard grass (Echinochloa crus-galli)
2,000-40,000
Large crabgrass (Digitaria sanguinalis)
2,000-150,000
(Lolium)
3,000-20,000
Shepherd’s purse (Capsella bursa-pastoris)
3,500-4,000 (by other reports up to 70,000)
Canada thistle (Cirsium arvense)
4,000-5,000 (by other reports up to 35,000)
Dandelion (Taraxacum officinale)
5,000
(Alchemilla)
over 5,000
Bachelor’s button (Centaurea cyanus)
6,820
Purslane (Portulaca oleracea)
10,000
Wild radish (Raphanus raphanistrum)
12,000
Lamb’s-quarters (Chenopodium album)
13,000-500,000
Corn poppy (Papaver rhoeas)
14,000-19,500 (according to other sources 50,000 to 200,000)
Common chickweed (Stellaria media)
15,000-25,000
(Tripleurospermum inodorum)
15,000-19,000 (according to other data, up to 54,000; 30,000 to 100,000.)
Perennial sow thistle (Sonchus arvensis)
19,000
Goosegrass (Eleusine indica)
50,000-135,000
(Chamaenerion angustifolium)
80,000
(Orobanche cumana)
100,000
Flixweed (Descurainia sophia)
730,000
(Amaranthus albus)
500,000-2,000,000

High seed production allows weeds to meet the challenges of population maintenance and mass spread, but also to be mobilized to quickly mutate to changing environmental conditions and new agronomic practices, including herbicides.

Seed bank

Numerous studies have established that in the arable soil layer in some fields there are seeds of 10-25 different species of weeds with their total number from 120 million to 3-4 billion per 1 ha.

Such a potential stock of weed seeds and fruits in the soil requires planned systematic weed control. This is one of the most important and promising tasks in agriculture.

Ways of spreading the seeds and fruits of weeds

Methods of spreading (dissemination) of seeds and fruits of weeds are divided into:

  1. autochore – seed spreading occurs with the help of special devices, driven by mechanical forces:
    • autobarochoric – spreading of seeds under the action of gravity, shattering of seeds (wild mustard (Sinapis arvensis), field cabbage (Brassica campestris), wild radish (Raphanus raphanistrum), wormseed mustard (Erysimum cheiranthoides), chamomile non-scented (Tripleurospermum inodorum)). 
    • Caused by tension in the desiccated covering tissues of fruits (storksbill (Erodium cicutarium), field violet (Viola arvensis), narrow-leaved pea (Vicia angustifolia)).
    • Dispersal of seeds as a result of wind gusts. For example, fruits in the form of bolls open with denticles when the seeds are ripe (purple cockle (Agrostemma githago), white drema (Silene latifolia), corn spurrey (Spergula arvensis), holes (corn poppy (Papaver rhoeas), turnip-shaped bell (Campanula rapunculoides)), lid (black calamus (Black henbane), eye flower (Anagallis)), etc. д.
  2. Allochoric – by means of various agents.
    • Anemochore – spreading with the help of wind (dandelion (Taraxacum officinale), canada thistle (Cirsium arvense), groundsel (Senecio vulgaris), horseweed (Erigeron canadensis) and many others from the Asteraceae family. Thanks to feathery flyers (pappuses), the seeds are carried to the surrounding fields and distant areas, even in weak winds, to a distance of 2-5 km or more.Some weeds form a branched above-ground mass and by the end of the growing season acquire a spherical shape, a “tumbleweed”. Gusts of wind break the dried stem at the base and the plants are able to roll over to other areas (Salsola ruthenica, Amaranthus albus, Descurainia sophia, Lepidium ruderale, Psammophiliella muralis, etc.).
    • Zoochoric – with the help of animals, birds, and insects. For this purpose, weeds have various devices – Velcro: anchors, hooks, serrated spines, bristles, awns, etc. Clinging by them to animal hair, birds’ feathers, human clothes, they are carried to other areas (Fig.). The fruits of field violet (Viola arvensis) and greater celandine (Chelidonium majus) can spread with the help of ants, carrying them to anthills and using as food.A similar example is the seeds of weed hemp (Cannabis ruderalis) and firebug (Pyrrhocoris apterus).
    • Hydrochoric – dissemination by spring and rainwater into lowered parts of the terrain. Seeds of field broom, field brome (Bromus arvensis), bachelor’s button (Centaurea cyanus), field larkspur (Consolida regalis), lamb’s-quarters (Chenopodium album), toad grass (Juncus bufonius) spread this way.
    • Anthropochoric – spreading as a result of daily human agricultural activities. Insufficient cleaning of seeds (grains, tubers and root crops insufficiently cleaned from soil, roots of seedlings and seedlings). This method periodically brings from the southern regions of the country into the Non-Black Earth zone Shirley grass (Amaranthus blitum), galinsoga (Galinsoga parviflora), purslane (Portulaca oleracea), common ragweed (Ambrosia artemisiifolia), Rhaponticum repens, etc.
Fruits of weeds
Fruits of weed plants with flycatchers: 1 - Cirsium arvense; 2 - Taraxacum officinale; 3- Senecio vulgaris; 4 - Tragopogon orientalis
Taraxacum seeds
Pappuses of Taraxacum officinale
Fruits of weeds
Fruits of weeds with glochidium: 1 - Daucus carota; 2 - Lappula squarrosa; 3 - Galium aparine; 4 - Bidens; 5 - Arctium lappa; 6 - Xanthium.

Biological properties of seeds

Resting seeds

Weed seeds immediately after shattering have a dormancy period, which is subdivided into:

  1. natural (deep or physiological) rest. It is conditioned by:
    • incomplete physiological and biochemical processes (common hogweed (Heracleum sphondylium), cleavers (Galium aparine), shepherd’s purse (Capsella), four-seeded pea (Vicia tetrasperma), etc.);
    • presence of covering tissues impermeable to water and air (white sweet clover (Melilotus albus), Persicaria lapathifolia, Galeopsis speciosa, Wild radish (Raphanus raphanistrum), celandine (Stachys annua), field bindweed (Convolvulus arvensis), blueweed (Echium vulgare), eastern cricket (Bunias orientalis), wild buckwheat (Fallopia convolvulus), etc. );
    • inhibitors in covering tissues (white mustard (Sinapis alba), wild mustard (Sinapis arvensis), field violet (Víola arvensis), wild oat (Avena fatua), black henbane (Hyoscyamus niger), horseweed (Erigeron canadensis), dandelion (Taraxacum officinale), corn poppy (Papaver rhoeas), Verbascum phlomoides, common plantain (Plantago major), black nightshade (Solanum nigrum), etc.). ).
  2. Compelled (secondary or ecological) rest. It is caused by lack of favorable external factors of life (lack of moisture, excessive heat, absence of light, presence of plant inhibitors produced by other species, etc.).

Sprouts of spring weeds appear in spring, plants bear fruit and die at the end of the growing season. Their seeds usually sprout only after overwintering. Seeds of early spring weeds germinate at soil temperatures of 2-4°C; seeds of late spring weeds germinate at 12-14°C or higher.

Weed seeds have an extended germination period, which significantly increases their survival as a species. The same is characteristic of seeds in the soil. This period is sometimes stretched for several years: In the Non-Black Earth Zone, seeds of cleavers (Galium aparine), redroot pigweed (Amaranthus retroflexus), canada thistle (Cirsium arvense), perennial sow thistle (Sonchus arvensis) keep their germination period for two years, Rough mountain thistle (Persicaria lapathifolia) and corn spurrey (Spergula arvensis) – 5-6 years, wild radish (Raphanus raphanistrum), curly mountain thistle (Polygonum ? ), hemp nettle (Galeopsis tetrahit), lamb’s-quarters (Chenopodium album), stinkweed (Thlaspi arvense), Fumaria officinalis – over 10 years.

Longevity

Seeds of most cultivated plants retain their germination under optimal storage conditions for no more than 4-7 years. Seeds and fruits of weeds retain their germination after dozens of years in the soil.

Experiments started by W. Bill in 1879. at Michigan College, USA, showed the ability to germinate the seeds of Stellaria media, Sinapis arvensis, Capsella bursa-pastoris, Amaranthus retroflexus, Melilotus officinalis after 30 years in the soil; seeds of field bindweed (Convolvulus arvensis), curled dock (Rumex crispus), black mustard (Brassica nigra) – 50 years.

Table. Seed longevity of weed plants[5]Éléments de biologie des mauvaises herbes - Les leviers de gestion de la flore adventice, Agri-Réseau (Translation from French UniversityAgro.ru)

Name of the plant
Seed longevity, years
(Bromus sterilis)
1
(Veronica)
10
Wild oat (Avena fatua)
15
(Alopecurus)
15
Annual bluegrass (Poa annua)
over 50
(Bromus secalinus)
20
(Galium)
40
Corn poppy (Papaver rhoeas)
40
(Stellaria)
50 (maximum 500)
Common knotgrass (Polygonum aviculare)
60 (maximum 500)
(Geranium)
over 50
(Chenopodium)
over 50 (maximum 1700)
Wild mustard (Sinapis arvensis)
200

A measure of the ability of weed seeds to retain germination is the ADR (Annual Rate of Decline). Depending on the value of this indicator, weed plants are divided into:

  1. short-lived, ADR close to 100%, that is, in the first few years of being in the soil, the seeds completely lose their ability to germinate (bromegrass (Bromus), mother-jellybeans (Tussilago));
  2. transitional, ADR of 70 to 85% (annual grasses, some dicotyledons such as warthog (Lapsana) and chamomile (Matricaria);
  3. moderate-resistant, ADR about 50%, i.e. within a few years half of the weed seeds lose germination (mainly dicotyledonous weeds: lamb’s-quarters (Chenopodium album), wild mustard (Sinapis arvensis), corn poppy (Papaver rhoeas), flaxseed (Linaria), shepherd’s purse (Capsella), gorgonium (Amaranthus), mountain worm (Polygonum);
  4. resistant, ADR about 10% (Veronica, Stellaria, Anagallis).

ADR value for some weed plants: redroot pigweed (Amaranthus retroflexus) – 33%, wild oat (Avena fatua) – 83%, lamb’s-quarters (Chenopodium album) – 37-48%, corn poppy (Papaver rhoeas) – 35-54%, black nightshade (Solanum nigrum) – 35%, wild mustard (Sinapis arvensis) – 34-43%, mullein multiflorum (Lolium multiflorum) – 75%, common knotgrass (Polygonum aviculare) – 54%, Persicaria maculosa – 32-40%, Setaria verticillata – 75%.

Longevity of weed seeds
Dependence of weed seed viability on time in the soil

Different fruits (heterocarpia)

Heterocarpy is the property of some species of weeds to form seeds and fruits with different morphological and physiological characteristics in the same inflorescence. Heterocarpy increases establishment in new territories and helps to adapt to developing agrocenoses.

Examples.

  1. The lamb’s-quarters (Chenopodium album) forms seeds of three kinds:
    • large, flat, greenish-brown in light tones, germinating in the fall in the year of formation;
    • medium-sized, rounded-convex, thin-coated, greenish-black, germinate in the second year;
    • very small, rounded-oval, densely black, germinating usually in the third year and later.
  2. On the tips of branches in panicles, oatmeal (Avena) forms spikelets with grains of various biological types. Small dark-colored grains, which fall off easily, are formed in the upper part of the spikelet, with a dormant period of 16-22 months. Such seeds produce plants similar in rhythm to late spring cereals, capable of germinating from a depth of 10-12 cm. In the lower part of the spike, larger, lighter-colored grains are formed, falling off later, which leads to the clogging of the seed. They have a resting period of about 2-3 months, sprouting from a depth of 18-25 cm and developing as early maturing plants. Grains intermediate in morphological and biological characters are formed in the middle part of the panicle.
  3. Seeds of weeds of the Asteraceae family, such as spring cress (Senecio vernalis) and greater goatweed (Tragopogon), differ in the length of the dormancy period depending on their location in the inflorescence. Seeds formed in the center of the inflorescence (basket) have a shorter dormancy period than those formed near the edge.

Different time of ripening of fruits and seeds

In the course of evolution, weeds have acquired the property of ending their life cycle earlier than cultivated plants, which allows them to disperse their seeds into the soil before the optimal harvesting date.

For example, plants of wild radish (Raphanus raphanistrum), shepherd’s purse (Capsella bursa-pastoris), stinkweed (Thlaspi arvense), knawel (Scleranthus annuus), red thornberry (Spergularia rubra), which grow in the winter crops, die off one to two weeks before threshing, so most of the seeds get into the soil. Seeds of chamomile non-scented, cornflower blue, larkspur, rye brome, and field broom also manage to drop their seeds a few days before harvesting.

In addition to early seed maturation, some weeds are characterized by their uneven maturation. Data from the South-East Research Institute of Agriculture show the irregularity of falling of oatmeal seeds, half of which, ripening by the threshing period, manages to get into the soil (table).

Table. Falling of oat seeds in spring wheat crops (according to Smirnov)[6]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. - Moscow: Publishing House "Kolos", 2000. - 551 p.

Date of observation
Number of fallen wild oats (Avena fatua) seeds
seeds per m2
% of total
24.07
1066
9
27.07 (beginning of wax ripeness (yellow-ripe stage) of wheat 25.07)
1658
14
30.07 (full wax ripeness of wheat 29.07)
3316
28
5.08 (full ripeness of wheat 04.08)
5092
43
08.08
6987
59
14.08
8409
71
22.08
11369
96

Postharvest (stubbles) weeds are weeds that form seeds and fruits after harvesting.For example, in the steppe regions of Russia, the barnyard grass (Echinochloa crusgalli), blue broom (Setaria ?), buckthorn (Setaria glauca), Russian thistle (Salsola ?), do not form seeds during the vegetation of grain crops. However, after threshing, under favorable agroclimatic conditions, short daylight hours and optimal nutrient and water regimes in unthreshed stubble, they rapidly go through the life cycle and form additional 40-60 mln seeds per 1 ha in 2-3 weeks.

Germination of seeds and fruits of weeds

Seeds of weeds have the property of regulation of germination depending on external conditions. Seeds immediately after falling have a low ability to germinate – no more than 3-8%. After a long stay in the soil, as a consequence of physiological maturation and increased permeability of covering tissues, which is observed after the autumn-winter period, the germination ability of weed seeds increases significantly.

The decisive factors for seed germination are growth factors: moisture and heat. For example, seeds of green foxtail (Setaria viridis), barnyard grass (Echinochloa crusgalli), redroot pigweed (Amaranthus retroflexus), annual celandine (Stachys annua) require 12-15% soil moisture for germination, for broom field seeds – at least 30%.

The requirements of weeds to the minimum soil temperature necessary for seed germination are different. Seeds of common chickweed (Stellaria media), wild radish (Raphanus raphanistrum), corn spurrey (Spergula arvensis), wild oat (Avena fatua), lamb’s-quarters (Chenopodium album), stinkweed (Thlaspi arvense), chamomile (Tripleurospermum inodorum), etc. begin to germinate at soil temperatures above 3-7 °C. Seeds of Setaria glauca, Echinochloa oryzoides, Amaranthus blitoides, Salsola tragus and other late crops require temperatures above 18-20°C for germination.

Light, like other life factors, affects seed germination by accelerating their germination. Seeds of purslane (Portulaca oleracea), Hypericum perforatum, Apera spica-venti, curled dock (Rumex crispus), etc. are most sensitive to light.

Most weed species have an extended period of sprouting, which can last throughout the growing season of cultivated plants, making it difficult to control them in crops, especially during the ripening of crops.

The level of weed infestation is determined not so much by the viability of weed seeds as by the number of emerged seedlings. Optimum depth of germination of seeds and fruits of most species of weeds is not more than 5 cm. Seeds of tartary buckwheat (Fagopyrum tataricum), tufted vetch (Vicia cracca), rye brome (Bromus secalinus), spiny cocklebur (Xanthium spinosum), weedy sunflower (Helianthus ?) germinate from a depth of 10 cm. Wild oat (Avena fatua) seeds may germinate and form complete shoots at the depth of 20-25 cm. Increasing the depth of seeds in the soil reduces their germination, and the formed seedlings do not reach the surface and die.

Studies on the effect of seed storage depth on seed viability conducted by the Association de coordination technique agricole (ACTA, France) showed different results after 2 and 4 years in the soil at 2, 10 and 40 cm depth (graphs).

Depending on the ability of seeds to germinate from different depths, two limits were determined:

  • emergence limit – the maximum depth of weed seeds from which they are able to germinate and give seedlings;
  • germination limit – the maximum depth of seed embedment at which the weed plants are capable of germinating.

Emergence limit is the depth from which a plant is capable of sprouting, while germination limit is the depth at which a plant is capable of sprouting, but if it is greater than the emergence limit, no sprouts will appear at the surface. Below the emergence limit, the seeds go into a dormant phase – canned. Therefore, in the practice of weed control it is important to consider this property.

There is an opinion that the germination limit has a direct correlation with the size of the seeds.

To use the properties of weed seeds, the following should be taken into account: periodic loosening of the arable layer, promotes germination, including in dry conditions of the Volga region. Subsequent tillage destroys the sprouts that appeared, which contributes to the clearing of the field. On the contrary, embedding seeds into deep dense layers creates conditions of hypoxia (lack of air access), which leads to preservation of seeds for a longer time.

Thus, even once fallen seeds weed, is able to ensure the emergence of seedlings for several years to come.

Influence of seed depth on germination
Influence of seed depth during 2 years of spring-summer weeds on their germination
Influence of seed depth on germination
Influence of the depth of seeds for 4 years on their germination

Vegetative propagation of perennial weeds

A feature of many weed species is the ability to vegetatively propagate from roots (rhizomes, bulbs, nodules), called propagation roots. The high ecological plasticity in this case is due to the quantitative abundance of roots or their parts and the high regenerative ability to reproduce (Table). The depth of occurrence of a considerable part of such roots is not limited to arable layer and can reach a depth of 1-2 m. That provides protection of weeds from extermination by tillage implements.

Table. Characteristics of the reproduction roots of perennial weeds in the arable soil layer (according to Smirnov, Kott, Tulikov, etc.)[7]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. - M.: Publishing house "Kolos", 2000. - 551 p.

Plant
Weight, g
Length, m
Number of adventitious buds
Canada thistle (Cirsium arvense)
144.2
80.5
410
(Rhaponticum repens)
592.0
65.8
250
(Lactuca tatarica)
310.5
32.5
130
Perennial sow thistle (Sonchus arvensis)
102.3
24.6
831
Coltsfoot (Tussilago farfara)
1524.0
170.0
2596
(Elytrigia repens)
1520.0
126.5
5550
Field horsetail (Equisetum arvense)
162.5
2625.0
45
Marsh hedgenettle (Stachys palustris)
1079.0
523.0
7009

The ability to vegetative reproduction is caused by the accumulation of plastic substances in the roots, mainly in the form of carbohydrates which content varies from 5-12% to 35-54% depending on the type of plant and time of vegetation.A sufficient number of adventitious buds are located on the roots, which are awakened when the roots are damaged, and a new plant is formed due to the stock of plastic substances.

Root crushing as a result of the action of tillage implements leads to the formation of a large number of scraps. In favorable conditions they take root and form independent plants.Weeds with high adaptability: perennial sow thistle (Sonchus arvensis), couch grass (Elytrigia repens), Tatar lettuce (Lactuca tatarica), field horsetail (Equisetum arvense). Low survival rate is characteristic of creeping weed (Acroptilon repens), Canada thistle (Cirsium arvense), and field bindweed (Convolvulus arvensis).

Due to a decrease in the amount of plastic substances, the regenerative ability of roots decreases with a decrease in the length of the root segment. The shortest length at which regeneration is possible is observed in such weeds as perennial sow thistle (Sonchus arvensis), couch grass (Elytrigia repens) and some others that can reproduce from 1-5 cm long segments. In addition, a decrease in the length of segments leads to awakening of more adventive buds, which results in a 1.5-2-fold increase in the regeneration capacity of roots (table). Therefore, infrequent tillage in clean fallow or improper cultivation of row-spacing rows of row crops can lead to significant clogging.

Table. Regeneration capacity of propagation roots during their crushing[8]Farming. Textbook for universities/G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. - M.: Publishing house "Kolos", 2000. - 551 p.

Root segment length, cm
Number of shoots formed per 1 m of propagation roots
(Lactuca tatarica)
Perennial sow thistle (Sonchus arvensis)
(Elytrigia repens)
20
100
100
100
15
140
-
104
10
200
173
138
5
400
200
251
3
400
209
-
2
-
-
-
1
-
262
-

In order to exterminate weeds with high ability to vegetative reproduction the roots are chopped into small pieces and buried to a depth of at least 20-25 cm. Due to the limitation of plastic substances and great depth, the cuttings are completely deprived of the ability to regenerate. This method of mechanical weed control is called the strangulation method.

Increase of soil density more than 1.1 g/cm3, lower humidity up to 15-20% and temperature below 5-10 °С) sharply reduces root cuttings engraftment. Thus, rhizomes of bermuda grass (Cynodon dactylon) and johnsongrass (Sorghum halepense) completely die during the winter period in the fields after autumn plowing.

Perennial weeds have a high ecological plasticity and in relation to adverse conditions. Under severe mechanical damage of roots, excessive soil compaction, and prolonged drought, the root system of the weed Acroptilon repens, Canada thistle (Cirsium arvense), perennial sow thistle (Sonchus arvensis), Lactuca tatarica, and field horsetail (Equisetum arvense) fall into a dormant state for 2-3 years. Their surviving root system resumes regeneration with the onset of favorable conditions. This property explains in some cases the unexpectedly abundant appearance on ploughed fields of perennial weeds not previously observed.

Weeds as habitat indicators

The confinement of many species of weeds to certain field communities indicates not only their phytocenotic compatibility with the crop, but also their preferences for the soil conditions of the habitat. In the latter case, the generalized response of weeds should be considered in two aspects: as an indication of natural edaphic conditions and as their response to soil properties, to some extent changed by agronomic measures in the process of human economic activity.

When studying the reaction of weed plants to edaphic conditions we should first of all dwell on their responsiveness to soil moisture supply, reaction of soil environment and provision with elements of mineral food.

In relation to the level of soil moisture the following groups of weeds are distinguished:

  • Hygrophytes are found almost exclusively in damp, poorly aerated soils (Gnaphalium uliginosum, Juncus bufonius, Apera spica-venti, field horsetail (Equisetum arvense), Mentha arvensis, marsh hedgenettle (Stachys palustris), creeping buttercup (Ranunculus repens));
  • Hygromesophytes – prefer sufficiently moist and well-aerated soils (lamb’s-quarters (Chenopodium album), Chenopodium polyspermumFumaria officinalis, cleavers (Galium aparine), Tripleurospermum inodorum, stinkweed (Thlaspi arvense), perennial sow thistle (Sonchus arvensis));
  • Xerophytes prefer well aerated, warm and sometimes strongly drying soils (redroot pigweed (Amaranthus retroflexus), prostrate pigweed (Amaranthus blitoides), Stachys annua, storksbill (Erodium cicutarium), Silene vulgaris, green foxtail (Setaria viridis), barnyard grass (Echinochloa crusgalli), and common ragweed (Ambrosia artemisiifolia)).

The use of this information is usually limited to taking into account the actual situation, while the possibility of its regulation is practically excluded.

According to the reaction to the pH value of the soil solution (actual acidity), it is reasonable to distinguish the following groups of weeds:

  • oxylophytes – found mainly on soils with a pH value < 5.0 (Rumex acetosella, corn spurrey (Spergula arvensis), knawel (Scleranthus annuus), Spergularia arvensis, Tripleurospermum inodorum, wild radish (Raphanus raphanistrum), Apera spica-venti, Juncus bufonius);
  • oxymesophytes – grow on soils with reaction of soil solution from slightly acidic to slightly neutral (wild oat (Avena fatua), Atriplex patula, wormseed mustard (Erysimum cheiranthoides), stinkweed (Thlaspi arvense), black henbane (Hyosyamus niger), marsh hedgenettle (Stachys palustris), Potentilla anserina, perennial sow thistle (Sonchus arvensis));
  • indifferent to the reaction of the soil solution (lamb’s-quarters (Chenopodium album), shepherd’s purse (Capsella bursa-pastoris), purple cockle (Agrostemma githago), horseweed (Erigeron canadensis), Galeopsis speciosa, Achllea).

The presence of populations of several species from a particular group gives grounds for assessing the advisability of subsequent liming of soils. This method often reduces the infestation of crops due to the change in the floristic composition of weeds and improvement of crop development.

According to the level of responsiveness to the provision of soil with elements of mineral nutrition one usually distinguishes “element-positive” and “element-negative” groups of weeds. However, in farming practice it is important to know the weeds that respond positively to a high content of certain elements of mineral nutrition in the soil.

  • The group of nitrogen-positive weeds includes lamb’s-quarters (Chenopodium album), Chenopodium polyspermum, Atriplex patula, wild radish (Raphanus raphanistrum), wild mustard (Sinapis arvensis), Persicaria lapathifolia, Galeopsis speciosa, Galeopsis bifida, barnyard grass (Echinochloa crusgalli), annual bluegrass (Poa annua), Rumex acetosella.
  • The group of phosphorus-positive weeds includes: groundsel (Senecio vulgaris), field violet (Viola arvensis), corn spurrey (Spergula arvensis), Spergularia rubra, Fumaria officinalis, and henbit (Lamium amplexicaule).
  • The group of potassium-positive weeds includes: cleavers (Galium aparine), Atriplex patula, stinkweed (Thlaspi arvense), perennial sow thistle (Sonchus arvensis).

The above groups of weeds in relation to the various elements of mineral nutrition is not strict, since with a change in mineral nutrition usually changes and other living conditions. As a consequence, the response of weeds to individual elements can change significantly. Nevertheless, this information, supplemented with other information of ecological and phytocenological content, allows the most rational use of lime materials and mineral fertilizers.In doing so, it is possible to successfully avoid increasing the weed infestation of crops or to develop measures for a more complete elimination of weeds in advance.

The considered biological properties and ecological preferences of weeds indicate the diversity of forms and ways to adapt to both phytocenotic and ecological conditions of arable lands. Therefore, only a detailed knowledge of these features and their deep scientific interpretation can contribute to the development of a system of effective measures to reduce the abundance and harmfulness of weeds.

Mimicry

Numerous researchers have found that weed populations of the same species in different natural conditions often behave as plants of different biological groups. For example, wild mustard (Sinapis arvensis) and purple cockle (Agrostemma githago) in northwestern Russia are typical spring early weeds, while in southern areas they behave as wintering plants. Some weeds, such as bachelor’s button (Centaurea cyanus), Tripleurospermum inodorum, and stinkweed (Thlaspi arvense), have both spring and wintering forms. The typical roots sucker Perennial sow thistle (Sonchus arvensis) and field bindweed (Convolvulus arvensis) usually behave as taproot weeds in over-compacted soils.

Sources

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

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

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