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CS 414 Section 1



1CS 414 - Weed Science

Section 1

I. What is weed science?

Weed science is the discipline that investigates the biology and ecology of weeds and how best to manage these plant species for the betterment of mankind.

II. What is a weed?
The term “weed” is defined by man. A weed is often defined as “a plant out of place”, or “a plant growing where it is not wanted”. These definitions are overly simplistic. Designating a plant as a weed simply by its physical location does not take into account problems caused by weeds nor certain characteristics of plants that contribute to their potential to be weeds. These simple definitions do not distinguish plants that possess truly weedy characteristics from those that are only an occasional nuisance. Weeds possess certain definable characteristics that set them apart from other plant species.
The Weed Science Society of America defines a weed as “any plant that is objectionable or interferes with the activities or welfare of man.”
Weeds are perhaps best defined as “plants that are competitive, persistent, and pernicious, and are undesirable because they interfere with human activities.”
III. Characteristics of weedy plants:
A. Ability to colonize disturbed environments; rapid population establishment

B. High reproductive capacity (sometimes multiple methods of reproduction)

C. Short time to reproduction (short time to flowering or between flowering and seed maturity)

D. Germination and reproduction over a wide range of environmental conditions

E. Seed dormancy, discontinuous germination, longevity of seed

F. Presence of vegetative reproductive structures

G. Adaptations for propagule dispersion

H. Ability to compete well with crops

1. Growth and resource capture: Success of plants grown in mixture (such as

weeds in a crop) is associated with early and rapid establishment, rapid canopy development, and rapid root growth. In general, a species that grows faster than its neighbors will use a disproportionate share of the available resources, to the detriment of its neighbors. Several studies have shown that growth parameters related to plant size and leaf area are the best predictors of competitiveness in mixtures of plant species. Weeds often have more rapid root elongation and/or deeper, more extensive root systems than crops.

2. Photosynthetic pathways: Basically referring to C3 vs C4 plants. C4 plants are generally considered to be more efficient at “fixing” carbon. C4 plants have a higher temperature optimum for photosynthesis, a higher light optimum for photosynthesis, higher photosynthesis rates per unit leaf area, higher growth rates under optimum conditions for photosynthesis, and greater dry matter production per unit of water used. Most of the world’s flora (> 99%) are C3 plants. However, the C4 pathway is well represented in agricultural weeds; many of the world’s worse weeds are C4 plants. C4 photosynthesis does not confer an intrinsic advantage to the plant in competition with a C3 species; rather the advantage depends heavily upon environmental conditions. In agricultural environments characterized by water and heat stress, many of the worst summer annual and perennial weeds will be C4, while winter annual weeds are most often C3 species.
IV. Types of Losses Weeds Cause
A. Direct Losses
1. Reduced Crop Yields
a. Competition – Defined as a process that occurs when the combined resource demands of plants within a given area exceeds the available supply. Weeds compete with crops for light, water, nutrients, and possibly essential gasses and space. Utilization of these growth inputs by weeds results in less being available to the crop. As the weed population increases, a point is reached where weeds utilize enough of the growth inputs to limit the amount available to the crop. When growth inputs to the crop are limited, crop growth and development are adversely affected and a yield reduction occurs.
Light is typically the growth input for which there is greatest competition between crop plants and weeds. However, competition for moisture and nutrients can also be significant. Competition for essential gasses (O2 and CO2) is likely of little importance.
i. Intraspecific competition: occurs when two or more plants of the same

species coexist in time and space and simultaneously demand a limited resource. Crop plants of the same species compete with one another. Weeds of the same species compete with one another.

ii. Interspecific competition: occurs when two or more species coesist in

time and space and simultaneously demand a limited resource.

Competition can be used as a weed management tool. Cultural practices that promote uniform stands of healthy, vigorously growing crop plants whose canopy closes quickly gives the crop a competitive advantage over weeds.
b. Allelopathy - Process by which a plant releases into the environment (air or soil) an organic chemical (an allelochemical) that adversely affects the growth and development of surrounding plants.
Allelopathy can result from root exudates or leaf leachates from living plants, volatile compounds released from living plants, or from compounds released from dead plants as they decay in the soil. Johnsongrass rhizomes, for example, release chemical(s) that inhibit growth of soybeans.
The effects of allelopathy have been widely established and are known to play an important role in some ecosystems. Practical application of the process has been limited. However, allelopathy potentially can be useful in weed management. Crop plants (or their residues) may release chemicals that are inhibitory to weeds. For example, chemicals released from wheat straw inhibit germination and growth of certain broadleaf weeds. A heavy straw mulch helps suppress weeds in no-till environments. Some efforts have been made to breed for crop plants with greater allelopathic effects, but success to date has been limited.
c. Interference: The effects of competition and allelopathy are difficult to

separate under field conditions. "Interference" is a term sometimes used to describe the effects of weeds on crops irregardless of the cause (competition and/or allelopathy; parasitism would also be included in some situations).

As shown in the data below, both the species of weed and the species of crop can influence the extent of interference. Additionally, the length of time the weeds are present, and when they are present, can influence the extent of interference. And, environment will have an impact.

Table 1. Effect of crop species on season-long weed interference by fall panicum; NC data.


Fall panicum density

% yield loss


1 per 16 ft of row

1 per 2 ft of row




1 per 16 ft of row

1 per 2 ft of row




1 per 16 ft of row

1 per 2 ft of row



Table 2. Effect of weed species on season-long weed interference in peanuts. NC data.



Weed density

% yield loss




1 per 16 ft of row

1 per 2 ft of row






1 per 16 ft of row

1 per 2 ft of row



d. Time and duration of weed interference: Weeds that emerge before or at

the time of crop emergence generally have a greater negative impact on the crop than weeds that emerge after crop emergence. And, the longer weeds are permitted to compete (or interfere) with the crop, the greater the negative impact on the crop.

i. Critical weed-free requirement: the minimum period following crop

emergence that the crop must be maintained free of weeds in order to prevent crop yield loss.

Hypothetical yield of crop as affected by length of early season weed control (Critical weed-free requirement)

ii. Critical timing of weed removal (or weed competition period): the

maximum length of time (after crop emergence) that weeds can be allowed to compete with the crop and not reduce yield.
Hypothetical yield of crop as affected by length of early

season weed interference (critical time of weed removal)

iii. Critical period of control (or critical period of weed competition): the

time interval between the critical weed-free requirement and the critical time of removal. More precisely, it is the period of time between that period after planting when weed competition does not reduce yield and the time period after which weed presence does not reduce yield.
Hypothetical critical period for weed control

Table 3. Effect of length of weed interference on watermelon yield. Monks and Schultheis, 1998.

Weeks of interference by

large crabgrass

Marketable yield of watermelon












Table 4. Effect of length of weed control on watermelon yield. Monks and Schultheis, 1998.

Weeks after planting when large crabgrass was seeded

Marketable yield of watermelon












Based upon the data in Tables 3 and 4, what is the critical period of large crabgrass control in watermelon?

Critical period of weed control for cotton

(J. W. Wilcut, unpublished data)

2. Reduced Harvesting Efficiency
a. Harvest of the crop may be delayed while waiting for weeds to dry down.
b. Slower harvest speed.
c. Weeds can cause increase harvesting losses.
d. Additional wear on harvesting equipment.
3. Reduced Quality of Harvested Crop
a. Trash and weed seed may lower the price received for crop. Examples: cotton classed as "grassy"; price dockage for foreign matter in soybeans.

b. Weeds may impart off-flavors in products made from crop. Example: wild garlic bulblets in wheat.

c. Seed of some weeds are toxic. Buyer may refuse delivery of crop if certain toxic weed seed are present. Example: crotalaria.
d. Weeds may increase moisture content of harvested crop, resulting in greater drying costs or a price dockage for excess moisture.
e. There may be a loss of crop quality (crop deterioration or weathering) while waiting for weeds to dry down enough to harvest.
Effect of Palmer amaranth density on harvesting efficiency of grain sorghum.

Moore et al., 2004. Weed Technol. 18:23-29.

Top graph: Effect of Palmer amaranth density on grain sorghum

moisture content.

Center graph: Effect of Palmer amaranth density on foreign matter in harvested grain sorghum.

Bottom graph: Effect of Palmer amaranth density on crop seed loss through the combine.

Impact of Palmer amaranth on cotton yield and brush stripper harvesting.

Smith et al., 2000. Weed Technol. 122-126.




To remove

Total harvest
































Trash in cotton

Foreign matter



Due to weeds

in lint





















B. Indirect Losses in Crops

1. Increased Crop Production Costs
a. Cost of control (herbicides, herbicide application, cultivation, additional land preparation, equipment costs, cost of manager’s time).
2. Crop Damage
a. Herbicide damage to crops (damage to treated crop, drift to another crop, or carryover to rotational crops).
b. Certain weed control practices create conditions favorable for other pests;

example is cultivating peanuts, enhancing soil-borne diseases.

c. Mechanical damage from cultivation or other control measures.
d. Moisture loss from cultivation.

3. Weeds may limit rotational choices

a. There may not be effective management options for some weed species in a particular crop. To be able to manage those weeds, the grower may have to plant a less profitable crop in that field. This may be the case with high-value crops, such as tobacco and various vegetable crops, where herbicide options are limited.
b. Some herbicides may persist long enough to damage certain crops planted

the following year. Herbicides with long persistence may be helpful in control of certain weeds in a given crop, but the grower may have to plant a less profitable rotational crop on that field in the following year because other crops may be sensitive to the herbicide residues. For example, Cadre is very effective on a number of problem weeds in peanuts, but Cadre will carry over and damage cotton the following year. If Cadre is necessary to manage weeds in peanuts, the grower will have to plant a potentially less profitable crop than cotton the following year.

4. Weeds may be alternate hosts for insects, diseases, or nematodes that attack crops.
Examples: horsenettle is an alternate host for tobacco mosaic virus; common ragweed is an alternate host for granville wilt; johnsongrass is an alternate host for maize chlorotic dwarf virus; wild mustard is an alternate host for cabbage root maggot; sicklepod and crotolaria are alternate hosts for the soybean cyst nematode.
C. Other Types of Losses
1. Health Hazards to Humans and Livestock
a. Poisons
i. Dermal poisons - cause skin irritations. Poison ivy is example.
ii. Internal poisons - cause sickness or death if eaten. There are numerous poisonous weeds. Examples include jimsonweed, crotalaria, bracken fern, black nightshade, sicklepod.
b. Allergins - an allergin is something that causes an allergy. Example is ragweed pollen
2. Cover and breeding sites for rodents, snakes, etc - This is mainly a problem in non-cropland and residential areas.
3. Safety issues - example would be weeds obscuring view of traffic signs.
4. Problems in Water Management
a. Weeds interfere with water flow in irrigation and drainage canals. This may cause flooding or block flow of water. Weeds also utilize water in irrigation canals, meaning less water is available for crops.
b. Weeds can make rivers unnavigable.
c. Weeds interfere with recreational uses of water (fishing, boating, swimming). Includes fish kills.
5. Problems in Turf and Ornamentals - Weeds detract from the aesthetics of ornamental plantings, lawns, and various other turf areas. Weeds may also detract from the recreational utility of turf areas.
6. Maintenance of Non-Crop Areas - There are major costs associated with weed control on highway and utility rights-of-ways, railroads, storage areas, etc.
7. Reduced land value - reduced rental value or selling price of land, reduced curb appeal of real estate.
V. Noxious weeds
As defined by the Federal Noxious Weed Act of 1974, a noxious weed is “any living stage (including seeds and reproductive parts) of a parasitic or other plant of a kind which is of foreign origin, is new to or not widely prevalent in the U.S., and can directly or indirectly injure crops, other useful plants, livestock, poultry or other interests of agriculture, including irrigations, navigation, fish and wildlife resources, or the public health”.
This act gives the Secretary of Agriculture the authority to designate plants as noxious weeds by regulation. Movement of all such weeds in interstate or foreign commerce is prohibited except under permit. The Act also gives the Secretary the authority to inspect, seize and destroy products, and to quarantine areas to prevent spread of such weeds. The witchweed quarantine in North Carolina is an example.
Federal Noxious Weed list is on next page. See comments on N.C. Noxious Weed Law following the list of federal noxious weeds.


Azolla pinnata (mosquito fern, water velvet)

Caulerpa taxifolia (Mediterranean clone of caulerpa)

Eichhornia azurea (anchored waterhyacinth)

Hydrilla verticillata (hydrilla)

Hygrophila polysperma (Miramar weed)

Ipomoea aquatica (Chinese waterspinach)

Lagarosiphon major (Oxygen weed)

Limnophila sessiliflora (ambulia)

Melaleuca quinquenervia (melaleuca)

Monochoria hastata (monochoria)

Monochoria vaginalis (pickerel weed)

Ottelia alismoides (duck-lettuce)

Sagittaria sagittifolia (arrowhead)

Salvinia auriculata (giant salvinia)

Salvinia biloba (giant salvinia)

Salvinia herzogii (giant salvinia)

Salvinia molesta (giant salvinia)

Solanum tampicense (wetland nightshade)

Sparganium erectum (exotic bur-reed)

Aeginetia spp.

Alectra spp.

Cuscuta spp. other than native or widely distributed species (dodders); total of 53 species on list

Orobanche spp. other than native or widely distributed species (broomrapes); total of 13 species on list

Striga spp. (witchweeds)


Ageratina adenophora (crofton weed)

Alternanthera sessilis (sessile joyweed)

Asphodelus fistulosus (onionweed)

Avena sterilis L. (animated or wild oat)

Spermacoce alata (borreria)

Carthamus oxyacanthus (wild safflower)

Chrysopogon aciculatus (pilipiliula)

Commelina benghalensis (Benghal dayflower)

Crupina vulgaris (common crupina)

Digitaria scalarum (African couch grass)

Digitaria velutina (velvet fingergrass)

Drymaria arenarioides (lightening weed, alfombrilla)

Emex australis (three-cornered jack)

Emex spinosa (devil's thorn)

Galega officinalis (goatsrue)

Heracleum mantegazzianum (giant hogweed)

Homeria spp. (Cape tulip)

Imperata brasiliensis (Brazilian satintail)

Imperata cylindrica (cogongrass)

Ischaemum rugosum (murainograss)

Leptochloa chinensis (Asian sprangletop)

Lycium ferocissimum (African boxthorn)

Melastoma malabathricum

Mikania cordata (mile-a-minute)

Mikania micrantha

Mimosa invisa (giant sensitive plant)

Mimosa pigra (catclaw mimosa)

Nassella trichotoma (serrated tussock)

Opuntia aurantiaca (jointed prickly pear)

Oryza longistaminata (red rice)

Oryza punctata (red rice)

Oryza rufipogon (red rice)

Paspalum scrobiculatum (Kodo-millet)

Pennisetum clandestinum (kikuyugrass)

Pennisetum macrourum (African feathergrass)

Pennisetum pedicullatum (kyasumagrass)

Pennisetum polystachion (mission grass, thin napiergrass)

Prosopis spp. (total of 25 on list)

Rottboellia cochinchinensis

Rubus fruticosus (wild blackberry)

Rubus moluccanus (wild raspberry)

Saccharum spontaneum (wild sugarcane)

Salsola vermiculata (wormleaf salsola)

Senecio inaequidens (African ragwort)

Senecio madagascariensis (Madagascar ragwort)

Setaria pallide-fusca (cattail grass)

Solanum torvum (turkey berry)

Solanum viarum (tropical soda apple)

Spermacoce alata

Tridax procumbens (coat buttons)

Urochloa panicoides (liverseed grass)
States, including North Carolina, also have noxious weed laws. North Carolina’s law defines three classes of noxious weeds. Class A is any noxious weed on the Federal Noxious Weed List or any noxious weed that is not native to the state, not currently known to occur in the state, and poses a threat to the state. Class B is nay noxius weed that is not native to the state, is present in fewer than 20 counties statewide, and poses a threat to the state. Class C is any noxious weed not meeting the definition of a Class A or Class B noxious weed for the the Commissioner of Agriculture has determined that eradication is not feasible. The North Carolina law defines a noxious weed as “any palnt in any stage of development, including parasistic palnts whose presence whether direct or indirect, is detrimental to crops or other desirable plants, livestock, land, or other property, or is injurious to the public health.
Class A noxious weeds in NC: All weeds on the federal noxious weed list plus the following:

Lagarosiphon spp. (African elodea)

Salvinia sp. Water fern), all species except S. minima

Polygonum perfoliatum (mile-a-minute)

Crassula helmsii (swamp stonecrop)

Trapa spp. (water-chestnut)
Class B noxious weeds in NC:

Stachys floridana (Florida betony)

Rorippa sylvestris (yellow fieldcress)

Lythrum (any Lythrum species not native to NC)

Tribulus terrestris (puncturevine)

Cirsium arvense (Canada thistle)

Carduus nutans (musk thistle)

Carduus acanthoides (plumeless thistle)

Myriophyllum spicatum (Eurasian watermilfoil)

Ludwigia hexapetala (Uruguay waterprimrose)
Class C noxious weeds in NC:

Celastrus orbiculatus (Oriental bittersweet)
VI. Invasive weeds

Many species of plants (also animals and microorganisms) can be introduced into an area

by man. Most introduced species do not pose an economic or ecological problem, but a

few become invasive and cause significant harm to the environment, the economy, or in

some cases human health. Introductions can be accidental (such as coming in with crop

seed or in some other manner unintentionally introduced) or intentional (deliberately

introduced because of some perceived agricultural or ornamental value).
An invasive species (weed, plant, microorganism) is one which is introduced, by human activities, to a location where it did not previously occur naturally, becomes capable of establishing a breeding population in the new location without further human intervention, spreads rapidly throughout the new area.
Invasive weeds are introduced into an area where they did not naturally evolve. They dominate the habitat, creating problems such as the following:
1. Environmental impacts

a. destroy wildlife habitat

b. displace threatened and endangered species

c. reduce plant and animal diversity because of the monoculture they create

d. disrupt waterfowl and migratory bird flight patterns and nesting habitats

e. interfere with agricultural production

f. increase rate and severity of natural disasters (such as fires)
2. Economic impacts

a. reduce crop, pasture, and forestry production

b. reduce property values

c. reduce recreational values of land

d. interfere with irrigation

e. cost billions in treatment and loss of productivity on privately owned lands

Examples of invasive terrestrial weeds:

Beach vitex Purple loosestrife

Canada thistle Tree-of-heaven

Japanese knotweed

Japanese stiltgrass



Leafy spurge

Multiflora rose

Examples of invasive aquatic weeds:

Eurasian water milfoil

Giant salvinia


Water Hyacinth

Examples of invasive animals:

European starling

Giant African snail

Africanized honeybee

Red imported fire ant

Zebra mussell

Northern snakehead
Examples of invasive disease organisms:

Asian soybean rust

Avian influenza

Sudden oak death

West Nile virus

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