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Chapter 10: weeds.

KEY TERMS

contact herbicide fumigation herbicide invasive nonselective herbicide noxious weed post-emergence herbicide pre-emergence herbicide selective herbicide solarization systemic herbicide

Definitions of weeds abound, usually with the statement that "a weed is any plant growing where it is not wanted." Perhaps a better working definition would be "uncultivated plants that grow in gardens or among cultivated plants." (Fig. 10-1) Weeds compete with cultivated plants for air, light, water, and nutrients. They can crowd out desirable species and even choke them out, in the case of some vines. They often send roots underground and sprout shoots at a distance from where they started. Weeds often harbor leafhoppers and other crop pests, so control of the pest requires controlling its host weed(s) as well. The weed scientist studies weed populations, diagnoses problems in the field, and may conduct research or be responsible for developing weed management systems. He or she may also enforce laws or develop regulations concerning the use of herbicides and other weed control technology. An herbicide is a substance that is used to kill plants, especially weeds.

Many weeds were introduced to this country intentionally but their spread has gotten out of hand, and they have become naturalized in wide-ranging locations in gardens, along roadsides, or in fields. Some of these so-called weeds were once considered delectable, edible plants and were cherished for their flavor and nutrition, and others were valued for their medicinal value. Some plants were brought in for gardening purposes, but have since escaped into wild areas where they compete with native plants. We tend to think of this as a historical problem created by people who emigrated here from Europe a century or two ago. However, even today, gardeners and landscapers use plants that have been introduced from other countries or non-native locations for gardening and landscaping purposes and that eventually escape into the wild to become potentially problematic and uncontrollable.

Some weeds have become invasive plants and are of increasing concern because of their propensity to spread into natural areas and overtake native species. This invasion upsets the balance of native ecology, affecting not only the plant species but also the native animals and insects and even the soil. The federal government recognizes invasive weeds as being non-native (alien or exotic) to the ecosystem in which they are a problem and likely to cause economic or environmental harm. Humans are the most likely means of introduction of invasive species. Of utmost concern to crop production are federally designated noxious weeds, plants that can directly or indirectly injure or cause damage to crops, including nursery stock. Table 10-1 lists noxious and invasive species.

WEED TRAITS AND LIFE CYCLES

Weeds have evolved several survival strategies that contribute to their success. These include

1. A prolific seed set

2. A fast, short life cycle from seed to seed

3. Persistence from year to year with a perennial root system

4. Faster and larger growth than cultivated plants

5. Seeds that are easily distributed by wind, birds, and other animals

6. Seeds that persist in the soil while remaining viable for many years

[FIGURE 10-1 OMITTED]

Although most weed seeds will survive in the top 3 to 4 inches of soil for 3 to 10 years, some can survive and remain viable for much longer. See Table 10-2 for a list of some of these species.

Weeds may be annuals or perennials. Annual weeds (see Table 10-2) tend to be the most prolific seed setters. Because of this propensity of weeds to be prolific seed setters, weeds should be removed before flowering and seed set. Some plants, such as dandelions, will sometimes go to seed after they are removed, as long as there is a flower present. This is also true if they are treated with an herbicide, such as 2,4-dichlorophenoxyacetic acid (2,4-D). Dandelions should be collected and placed in a closed container, such as a trash bag, to prevent the spread of seeds. Perennial weeds may also set seed well, but they spread by underground rhizomes, stolons, or both or persist from year to year with large storage roots (Fig. 10-2). For this reason, it is not enough to remove or kill the above-ground portion of the plant, the root system must also be destroyed. See Table 10-3 for a list of common perennial weeds.

[FIGURE 10-2 OMITTED]

SEASONAL WEEDS

Weed seeds germinate at different soil temperatures and can be split into two general groups: warm season and cool season. Warm-season weeds germinate in the spring and grow through the summer. Cool-season weeds germinate in fall and grow through winter.

Winter Annuals

Winter annuals germinate at cooler soil temperatures. They begin to grow in fall, maintain their size during colder temperatures, and then resume growing with warmer temperatures in spring. The overwintering form of the plant is often a basal rosette. In spring when temperatures warm up, the plant bolts, sending up a flowering shoot in a short period of time. These weeds are present before spring planting and use soil moisture. Common winter annuals are henbit, common chickweed, wild mustard, and annual bluegrass. Fall control of winter annuals is preferred because the younger plants are more susceptible to herbicides. Use a post-emergent herbicide.

Summer Annuals

Summer annuals germinate in the warmer soil temperatures of late spring to summer. They flower in late summer or fall. Summer annuals are more numerous than winter annuals. Some common grasses are large crabgrass, goosegrass, and giant foxtail. Common broadleaves include smooth pigweed, common lambsquarters, purslane, galinsoga, common ragweed, and tall morning glory.

Annual grasses can be controlled best by taking appropriate measures early in the growing season. Otherwise, they will quickly become a severe problem if they are not controlled when small. For example, large crabgrass, will root into the soil where the nodes of the stem contact soil, allowing it to quickly cover open ground (Fig. 10-3). The fibrous root system of grasses makes them very difficult to pull out of the ground.

WOODY WEEDS

When we think of weeds, images of herbaceous plants usually come to mind. Such plants commonly sprout in our flower beds and vegetable gardens. However, woody plants are often a problem in cultivated areas, too. For example, woody plants, such as raspberries and blackberries, are invasive and can easily escape their designated spot in the garden. They accomplish this by sending rhizomes into lawn areas or other parts of the garden. Getting rid of them requires digging out the offending shoots, preferably underground. Nonsystemic sprays can also aid this process. Mowing lawn areas sometimes keeps the new sprouts at bay but is not always as effective as one wishes.

[FIGURE 10-3 OMITTED]

Other woody invaders include a number of species of trees that germinate where they are blown on the wind, planted by squirrels, or have in some other manner been introduced into the cultivated area. Woody plants can be invasive, particularly in areas where clear-cutting has occurred. Some woody weed species include silver maple, Autumn olive, Russian olive, multiflora rose, Amur honeysuckle, Amur maple, Eastern cottonwood, Siberian elm, Chinese elm, and salt cedar. Perhaps the most infamous woody weed is poison ivy (Fig. 10-4). It is commonly encountered as a vine of up to 150 feet long or even a shrub whose trunk can attain a diameter of nearly 6 inches. It is not inhibited in shade or full sun. While the trifoliate leaves have a vibrant red color in fall, they also cause allergic contact dermatitis in humans, with variable levels of reactions in different people. The spread of seeds is probably aided by birds and mammals that freely consume poison ivy leaves and fruits, as allergic dermatitis is generally not a problem for animals. Poison ivy may be controlled by a systemic pesticide used early in the season, either by spraying it on or by wiping the herbicide directly on the leaves. Other woody weeds may be treated this way or removed manually whenever they appear, if their scale is small enough and there are not too many trees to prohibit this method. It is best to control woody weeds as early as possible once they appear. Woody weeds in a prairie or meadow may be controlled by mowing and/or the use of controlled burning. This latter method is best done by experienced persons. Controlled burns are used regularly every 2 to 3 years in a prairie or meadow planting as a means for weed management.

CONDITIONS THAT ENCOURAGE WEEDS

Acid soils, alkaline soils, overfertilization, poor fertility, lack of fertilization in the cropping system, cultivation, compaction, sandy soils, the presence of a crust or hardpan, and land that has been disturbed are all conditions that encourage weed invasion and growth (Fig. 10-5). Ideal gardening conditions also encourage weeds. By their very nature, weeds are competitive and successful in a range of environments and climates. Although there are practices that encourage introduction and growth of weeds and there are others that discourage weeds, they are nevertheless the number one pest of horticulture and agriculture. Plants are destined to cover bare ground wherever it is found. Few exceptions to this rule exist; two of these are very saline soils or soils with toxic substances in them. Therefore, in any area where cultivation occurs, weeds will appear. See Table 10-4 for weeds that thrive in specific conditions.

[FIGURE 10-4 OMITTED]

PREVENTING WEEDS

Nonchemical

Mulch is a nonchemical approach to weed control that can be quite successful if done properly. Pine needles have a degree of inhibitory effects on plant growth and thus have some degree of weed suppression ability. Partially composted bark mulch is commonly used in landscaped beds and borders and around trees. This prevents weed seeds that are already present in the soil from germinating. Sometimes a layer of landscape fabric or weed barrier is placed over the area first, and it may be pinned into place with U-shaped or plastic pins that are provided with the fabric. A layer of 2 to 4 inches of mulch should be used. The higher amount is necessary when landscape fabric is not used. Because mulch is organic, it will decompose over time. This decomposition can be beneficial to the soil and the plants, but eventually additional mulch will have to be added to maintain an adequate amount for weed prevention.

Weed seeds may still be introduced by wind or birds or other means, but these are usually minimal. The weeds can easily be removed by hand, especially if landscape fabric was used, because the roots usually grow only in the mulch layer. Corn meal gluten has been shown to be effective for crabgrass prevention. It has germination-inhibiting properties and thus must be applied before weed seed germination. In addition to crabgrass, it can also be used to inhibit germination of clover and dandelion.

[FIGURE 10-5 OMITTED]

SOLARIZATION. Solarization is a technique that can be effective, at least in starting out with a weed-free area. It is a chemical-free technique that requires several months of warm weather to be effective. This is a time-consuming practice that requires some foresight to be successful, but it is friendly to the environment and may require less effort than other methods. The benefits of solarization are that it can kill weed seeds, pests that live in the soil, and soil-borne pathogens to a depth of up to 8 inches.

After selecting the site for your flower bed, remove the turf with a sod remover or turn up weeds by tilling. Water the area because the hot moisture will be more detrimental to seeds and pathogens than dry heat. If possible, lay a soaker hose or drip tubing down before covering with plastic to ensure good moisture levels during the solarization process. You may water in advance to cause weed seeds to germinate, then till lightly to uproot them. Cover the area with clear plastic film and cover the edges of the plastic with soil to hold it in place. Clear polyethylene plastic film is readily available from plastic mulch producers or specialty gardening catalogs. This method is best used in the summer, when temperatures are at their maximum. Ideally, the soil under the plastic should heat up to well over 100[degrees]F. Leave the plastic in place for enough time to kill any seeds or harmful organisms to a depth of several inches. It will take several weeks to a few months for this method to work.

NEWSPAPERS. Another technique is to cover the area to be cultivated with newspapers. Begin the process in fall by placing the newspaper on the area to be planted. Use only recyclable sections of the newspaper; avoid glossy sections and inserts. Most newspapers print with soy ink, which should be safe for plants and the soil, but glossy materials may be printed with inks that contain heavy metals that are not plant-friendly. It may also be possible to obtain ream ends of unprinted paper from newspaper publishers. It is not necessary to till or work the ground first, or even to remove existing plants. Moisten the paper as you go, or even before laying it down, so as to hold it in place. Lay the paper in 5 to 10 layers or more at a time, overlapping the newspaper edges by 3 inches. When you have completely covered the desired area, cover the newspaper with 6 inches of dead leaves and hold them down with a few light prunings from branches of evergreens. You may also use grass clippings with the leaves. Be sure to water the leaves, too. After a few weeks the weeds and sod underneath the paper will be dead. Cultivate in the late spring, turning under the leaves and paper. You may prefer to plant directly into the newspaper and leaves, allowing them to remain in place. Add more mulch as required to keep the area weed-free.

Shredded newspaper can also be used as weed-preventing mulch. It is important to use it only after the soil temperature has reached desired levels, as the newspaper has an insulating effect on the soil. It may be shredded or chopped and watered-in for best effect. As mentioned earlier, glossy newsprint should be avoided.

Chemical

Another way to prevent weeds is to use a pre-emergence herbicide. Pre-emergence herbicides are available under a number of trade names (Fig. 10-6). They control weeds at the germination stage by forming a film layer over the soil. As seeds emerge, they are killed by the herbicide. If this method is chosen it is important to read the chemical label thoroughly and use as directed; otherwise, the effectiveness of the herbicide may be reduced. Do not use pre-emergence herbicides in areas where you expect to plant seeds! You can only use them where you are using transplants or after your desired crop has come up. Pre-emergence herbicides are usually applied in spring for germinating perennial seeds and for warm-season annual weeds. Apply in fall for winter annuals.

CONTROL OF WEEDS

Growth of weeds cannot always be prevented. They can be introduced into an area by wind, birds, animals, and even human activity. Sometimes a seed mix has some weed seeds in it. But, regardless of the reason, it is sometimes desirable to control weeds after they have come up in a cultivated area. Mechanical and chemical means are most commonly used to control weeds once they become a problem.

Nonchemical

MECHANICAL. Weeds may be removed by hand pulling or hoeing or by cultivating with equipment. If this method is used, it is important to pull early and often. Weeds may be easy to hoe when very small, but remember they tend to grow quickly! Delays of even a few days can result in a much tougher job. If you are not sure what plant is a weed and what is your desired vegetable or flower, it is very important to learn to identify the seedlings. Some seed packets include a drawing of the seedling you are planting. Local extension agents can provide information on how to identify weed seedlings. This crucial step will give you a big advantage in early weed control in your garden.

Cultivation is often used on larger scale plots. The disadvantage of this approach is that it tends to turn up new weed seeds that may have been lying too deep to germinate. Each round of cultivation brings more seeds to the surface, and thus a nasty cycle is perpetuated. It is extremely important to prevent seed set on weeds in this case, although it may still prove impossible to ever eradicate all those that are already there.

[FIGURE 10-6 OMITTED]

Chemical

Chemical controls may be used in horticulture in a variety of ways. Chemicals that are used to control weeds are called herbicides. Pre-emergence herbicides were discussed in the section on preventing weeds. Fumigation is another chemical weed control method used before planting on commercial-scale operations. The chemicals available for fumigation are limited and highly toxic. Post-emergence herbicides are applied after seeds have emerged, just as the name suggests.

Contact herbicides kill the part of the plant they are applied to, but do not affect other parts of the plant. Systemic herbicides kill a plant by entering the vascular system and translocating throughout the plant. Therefore, these herbicides kill not only the portion of the plant contacted by the herbicide killed but also the entire plant, including underground portions or parts that come up in distant places, as long as they are connected to some part of the plant, such as runners, stolons, or rhizomes, or even a vine that grows over a large area.

Grasses and grass-like weeds are monocots, whereas broad-leaved weeds are dicots. Selective herbicides kill only a few or a limited number of weeds. The weeds they do kill are usually related in their physiology, and so the chemical has a similar mode of action. Thus, an herbicide may control grasses, or it may control broad-leaved plants. Nonselective herbicides are able to kill both categories of weeds. Grass weeds include crabgrass, bermudagrass, foxtail, and annual bluegrass, whereas nongrass monocots include wild garlic and nutsedge.

Herbicide Damage

Herbicide damage is a potential hazard of herbicide application. Any herbicide could cause unwanted damage to nontarget plants if it is not used with care and caution. In addition to application on the wrong plants, drift of the mist can spread to nontarget areas. Thus, it is important to apply sprays when there is very little or no wind. Some herbicides, such as the ester form of 2,4-D, are volatile above certain temperatures; thus, they can affect plants grown in an area where herbicides have not been used. This often happens to garden plants when common lawn chemicals are applied. Other methods of introducing unwanted herbicides into the landscape or garden include using grass clippings that have been treated with herbicide and using compost or manure from piles that have been treated with herbicides. Symptoms of herbicide damage include curling and twisting of leaves, flattened or twisted stems, elongated and malformed leaves, and crinkling of leaves and leaf margins. Insect damage is usually easily eliminated as a possible cause, as evidence of insects is lacking. However, herbicide damage could be mistaken for a virus. Roses, grapes, redbuds, and tomatoes are particularly susceptible to 2,4-D damage, and this diversity in plants experiencing damage is a clue to its source. A virus usually only affects a few plants of the same species.

In 1968 the first report of weed resistance to an herbicide was reported, and since then numerous others have followed. The danger of herbicide resistance is caused by repeated use of the same herbicide or herbicides with the same site of action in a plant. Resistance is naturally present but usually at very low levels. However, with the repeated use of the same herbicide, resistant plants are under heavy selection pressure. As long as resistant plants continue to reproduce, they will grow in numbers over a period of time, until they are the dominant genotype present. This can occur in only a few years for some species. Because many weeds have excellent survival skills, including the ability to produce numerous seeds coupled with good seed dispersal ability, an herbicide-resistant weed can be very difficult to eradicate once it has developed.

There are some strategies that the home gardener or commercial horticulturist can use to avoid herbicide resistance in weeds. These include rotating crops and rotating use of herbicides. It is particularly important to rotate herbicides with different modes of action. This rotation can also be accomplished by combining two herbicides with differing modes of action together in one application. Avoid overuse of herbicides by supplementing chemical weed control with prevention methods and mechanical control methods. Cultivation of weeds before seed set can be an effective component of a weed management program. Economic thresholds have been developed for some crops, and these can be used to determine when to apply herbicides. Table 10-5 lists herbicide-resistant weeds.

SUMMARY

Weeds compete with cultivated plants for air, light, water, and nutrients. They also harbor insect pests and disease-causing pathogens. Weeds can be very competitive with cultivated plants, often germinating in many different soil types and growing at a much faster rate then cultivated crops. Some were intentionally introduced as cultivated plants and have become wild by self-seeding and spreading to new areas. Some are invasive under favorable conditions and are even considered noxious. Weed seeds may remain viable in the soil for many years. Weeds may be annual or perennial and are categorized according to when they appear during the year.

Weeds can be prevented, controlled, or managed by several different methods. These include chemical and nonchemical approaches. Herbicide damage is an unwanted side effect of some herbicide applications.

HANDS-ON ACTIVITIES

* Make a weed collection. Include annuals, perennials, monocots, and dicots. Mount and label the weeds after press-drying them. Include information about where the weed was growing, the habitat (sun/shade, wet/dry, etc.) and whether it was a cultivated or noncultivated area, and the date it was collected. If possible, include flowers and fruit.

* Construct a collection of drawings or photographs of the most common weeds and what they look like as seedlings.

* Collect one edible weed to bring into class, and provide a background on the weed's origin and various uses, including whether wildlife feed on parts of the weed. The weed should be collected early in a fall semester, as weeds may not be covered in class until after a frost occurs.

* Study invasive plants in your state and do one of the following: construct a list of the top invasive plant species in your area; write a 1-page report about one of the invasive weeds, including its origin, how it is distributed, and how and why it has become problematic.

REVIEW QUESTIONS

1. Define a weed and discuss the meaning and impact on other plants.

2. Where do weeds come from?

3. Weeds that spread into noncultivated areas and upset the balance of native ecology are classified as--.

4. Weeds that directly or indirectly injure or cause damage to crops, including nursery stock are designated--weeds.

5. Name six strategies for successful survival that weeds have evolved.

6. True or False: Weed seeds can survive and remain viable for 2 to 200 years or longer.

7. Name two nonchemical methods of weed prevention and explain how they work.

8. Chemicals used to kill weeds are called--.

9. The type of herbicide that translocates to all parts of a plant is called a--.

10. What type of herbicide kills both broad-leaved and grass-type weeds?

SELECTED REFERENCES

Dana, M., Katz, D., Kemery, R., Hoelscher, L., & Brown, T. (2004). Picture the damage! Herbicide damage symptoms on ornamentals [CD]. Layfayette, IN: Agricultural Communication Service.

Golden Guides: weeds (2001). New York: St. Martin's Press.

Riotte, L. (1975). Carrots love tomatoes. Cleveland, OH: Garden Way.

U.S. Department of Agriculture. (1971). Common weeds of the United States. Mineola, NY: Dover Publications.

Walters, C. (1999). Weeds: control without poisons. Austin, TX: Acres, USA.

Dr. Marietta Loehrlein currently teaches horticulture classes at Western Illinois University in Macomb, Illinois. She earned both her bachelor's degree in Agronomy and her master's degree in Plant Genetics at The University of Arizona. Her master's research project was concerned with germination problems associated with triploid seeds, from which seedless watermelons grow. Following that she worked for 5 years in a breeding and research program for Sunworld, International near Bakersfield, California. She worked with peaches, nectarines, plums, apricots, and cherries. Then she returned to school to earn her Ph.D. in Horticultural Genetics at The Pennsylvania State University. Her Ph.D. research focused on flowering processes in regal pelargonium (also called Martha Washington geraniums). While at The Pennsylvania State University, she bred a new cultivar of regal pelargonium, "Camelot." At Western Illinois University, Dr. Loehrlein teaches nine courses: Greenhouse and Nursery Management, Introductory Horticulture, Landscape Design, Landscape Management, Home Horticulture, Plant Propagation, Turf Management, and two courses in Plant Identification.
TABLE 10-1
Select Noxious and Invasive Plants

NAME                      GENUS AND SPECIES          TYPE

Autumn olive              Elaeagnus umbellata        Invasive

Canada thistle            Cirsium arvense            Noxious

Dodder                    Cuscuta spp.               Noxious

Downy brome               Bromus tectorum            Invasive

Field bindweed            Convolvulus arvensis       Noxious

Field sowthistle          Sonchus arvensis           Noxious

Garlic mustard            Alliaria petiolate         Invasive

Hoary cress, whitetop     Cardaria draba             Noxious

Japanese honeysuckle      Lonicera japonica          Invasive

Japanese knotweed         Polygonum cuspidata        Invasive

Johnsongrass              Sorghum halepense          Noxious

Kudzu                     Pueraria montana           Invasive
                            var. lobata

Leafy spurge              Euphorbia esula            Invasive and
                                                       noxious

Mile-a-minute weed        Polygonum perfoliatum      Invasive

Multiflora rose           Rosa multiflora            Invasive

Musk thistle,             Carduus nutans             Invasive and
  nodding thistle                                      noxious

Purple loosestrife        Lythrum salicaria          Noxious

Quackgrass                Agropyron repens           Noxious

Russian knapweed          Acroptilon repens          Invasive

Russian olive             Elaeagnus angustifolia     Invasive

Saltcedar                 Tamarix spp.               Invasive

Scotch broom              Cytisus scoparius          Invasive

Scotch thistle            Onopordum acanthium        Invasive

Spotted knapweed          Centaurea maculosa         Invasive and
                                                       noxious

Tree of heaven            Ailanthus altissima        Invasive

Yellow star-thistle       Centaurea solstitialis     Invasive

Yellow toadflax           Linaria vulgaris           Invasive

TABLE 10-2
Seed Longevity of Some Weed Seeds

                                                    NUMBER OF
NAME                      SPECIES                   YEARS VIABLE

Annual bluegrass          Poa annua                  100

Chickweed                 Cerastium spp.             600

Field violet/European     Viola arvensis             400
  field pansy or
  "wild pansy"

Groundsel                 Senecio vulgaris           100

Lambsquarters             Chenopodium album         1600+

Prostrate knotweed        Polygonum aviculare        400

Shepherd's purse          Capsella bursapastoris      50

TABLE 10-3
Common Annual and Perennial Weeds

WEED                         SPECIES                      TYPE

Annuals

Barnyardgrass                Echinochloa crus-galli       Monocot

Cheeseweed mallow *          Malva parviflora             Dicot

Crabgrass                    Digitaria spp.               Monocot

Deadnettle                   Lamium maculatum             Dicot

Fall panicum                 Panicum dichotomiflorum      Monocot

Foxtail                      Alopecurus spp.              Monocot

Lambsquarters                Chenopodium album            Dicot

Night-flowering catchfly     Silene noctiflora            Dicot

Shepherd's purse             Capsella bursa-pastoris      Dicot

Sticky chickweed             Cerastium glomeratum         Dicot

Stinking chamomile           Anthemis cotula              Dicot

Stork's-bill                 Erodium cicutarium           Dicot

Violet/European field        Viola arvensis               Dicot
  pansy or wild pansy

Wild oats                    Avena fatua                  Monocot

Perennials

Canada thistle               Cirsium arvense              Dicot

Common mouse-ear             Cerastium fontanum           Dicot
  chickweed ([dagger])

Dandelion                    Taraxacum officinale         Dicot

Field bindweed               Convolvulus arvensis         Dicot

Jerusalem artichoke          Helianthus tuberosus         Dicot
  (sunchoke)

Johnsongrass                 Sorghum halepense            Monocot

Yellow Nutsedge              Carex castanea               Monocot

* May also be biennial or perennial. ([dagger]) May also be biennial.

TABLE 10-4
Soil Conditions That Encourage Weeds

Acid soils                  Docks
                            Fingerleaf weeds
                            Lady's thumb
                            Sorrels

Slightly acid               Hawkweed
                            Knapweed

Alkaline soils              Barnaby's thistle
                            Canada bluegrass
                            Cornelian cherry
                            Field madder
                            Hare's ear mustard
                            Mountain bluet
                            Penny cress
                            Pepper grass
                            Sagebrush
                            Woody aster
                            Worm seed
                            Yellow chamomile

Compacted/crust/hardpan     Chamomiles
                            Field mustard
                            Horse nettle
                            Morning glory
                            Pennycress
                            Pineapple weed
                            Quack grass

Cultivated land             Buttercup
                            Carpetweed
                            Chickweed
                            Dandelion
                            Lambsquarters
                            Mallows
                            Nettle
                            Plantain
                            Prickly lettuce
                            Prostrate knotweed
                            Rough pigweed

Moist areas                 Annual bluegrass
                            Nutsedge

Nitrogen deficiency         Clovers

Sandy soils                 Arrow-leaved wild lettuce
                            Broom bush
                            Flowered aster
                            Goldenrod
                            Partridge pea
                            Yellow toad flax

TABLE 10-5
Herbicide Resistant Weeds of the United States

                                  STATES
                GENUS AND         RESISTANCE       HERBICIDE(S)
COMMON NAME     SPECIES           OBSERVED         RESISTANT TO

Barnyardgrass   Echinochloa       AK, CA, LA,      Cyhalofop-butyl,
                  crusgalli         MD, MI, TX       fenoxaprop-p-
                                                     ethyl,
                                                     molinate, and
                                                     Thiobencarb,
                                                     Benthiocarb

Common          Xanthium          AL, AR, IA,      Disodium
  cocklebur       strumarium        KS, LA, MD,      methylarso-
                                    MN, MO, MS,      nate (DSMA),
                                    NC, OH, OK,      monosodium
                                    SC, TN           methylarsonate
                                                     (MSMA)

Common          Chenopodium       CT, DE, IA,      Atrazine, Trazine
  lambs-          album             IL, IN, MA,
  quarters                          MD, ME, MI,
                                    MN, NC, NH,
                                    NY, OH, PA,
                                    RI, VA, VT,
                                    WI

Goosegrass      Eleusine          AL, AR, FL,      Trifluralin
                  indica            GA, MS, NC,
                                    SC, TN

Green foxtail   Setaria           ND, WI           Trifluralin
                  viridis

Horseweed       Conyza            AR, CA, DE,      Glyphosate
                  canadensis        IN, KY, MD,      (Round-up)
                                    MI, MS, NC,
                                    NJ, OH, TN

Kochia          Kochia            CO, IA, ID,      Metsulfuron-
                  scoparia          IL, IN, KS,      methyl, tria-
                                    MN, MT, ND,      sulfuron, and
                                    NM, OK, OR,      synthetic
                                    SD, TX, UT,      auxins
                                    WA, WI, WY

Redroot         Amaranthus        AR, CO, CT,      Atrazine,
  pigweed         retroflexus       IN, KS, MD,      cyanizine
                                    ME, MI, MN,
                                    ND, NH, OR,
                                    PA, VA, VT

Rigid           Lolium rigidum    CA               Glycines
  ryegrass

Smooth          Amaranthus        DE, IL, KY,      Atrazine
  pigweed         hybridus          MA, MD, MI,
                                    NC, NJ, PA,
                                    VA, WI

Wild oat        Avena fatua       CA, CO, ID,      Difenzoquat,
                                    MN, MT, ND,      thiocarbamates,
                                    OR               pyrazolines,
                                                     and others
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Author:Loehrlein, Marietta M.
Publication:Home Horticulture: Principles and Practices
Date:Jan 1, 2008
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