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Cover crops, cultivation, flaming, soil solarization, and mulching are commonly used for weed control in organic production systems. However, several new herbicides, approved by the Organic Materials Review Institute (OMRI), are recommended as contact, non-selective, post-emergence herbicides for annual grasses and broadleaf weeds. Citric acid (Alldown), clove oil (Matran 2), thyme/clove oil (XPRESS) were compared with glyphosate (Roundup Pro), a systemic broad spectrum herbicide, at three sites in southern and north central Florida during September and October, 2003. Treatments varied at each site but included glyphosate (5% a.i. applied to runoff) organic herbicides at recommended rates (undiluted citrus acid at 61 L·ha-1; 10% clove oil at 76 L·ha-1; 10% clove oil/thyme oil at 76 L·ha-1) and at twice recommended concentrations and application rates. Grasses and broadleaf weed species were different at each site but included Alexander grass, bahia grass, Bermudagrass, carpetweed, crabgrass, hairy indigo, lambs quarters, Florida pusley, goatweed, nutsedge, pigweed, shrubby primrose willow, broadleaf signalgrass, southern sandbur, spurge, torpedograss, and citrus rootstock seedlings. Weed control with the organic herbicides at all three sites at recommended and at higher concentrations and rates was inconsistent, ranging from 10% to 40%, compared with 100% control with glyphosate. Labels for the organic herbicides generally specify application to actively growing weeds less than 10 cm tall, emphasizing their use as early season herbicides. Fall applications to larger weeds, some within the specified maturity and size range and others taller and producing seed, could partially explain poor weed control.

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period is marked by an increase in weed populations when the transition is managed by growers who are accustomed to using synthetic herbicides to control weeds ( Martini et al., 2004 ). Few organically acceptable herbicides have been shown to be

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Three studies were conducted to evaluate preemergence herbicides for weed control efficacy and crop tolerance in radishes (Raphanus sativus L.) grown on organic soils. Herbicides evaluated were CDEC, metolachlor, alachlor, pendimethalin, thiobencarb, propachlor, metribuzin, and pronamide. Control of both broadleaf and grass weed species was provided by most herbicides evaluated with little or no apparent visual loss in crop vigor except mebribuzin. Weed control efficacy was reduced during the 2nd study due to excessive rainfall. Weedy and hand-weeded checks consistently produced some of the highest yields, indicating possibly some toxicity to the crop due to herbicide treatment or an apparent lack of weed competition. Chemical names used: 2-chloroallyl diethyl-dithiocarbamate (CDEC); 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-l-methylethyl)acetamide(metolachlor); 2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide (alachlor); N-(l-ethylpropyl)-3,4-dimethyl-2,6-dinitro benzenamine (pendimethalin); S-[(4-chlorophenyl)methyl] diethylcarbamothioate (thiobencarb); 2-chloro-N-isopropylacetanilide (propachlor); 4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one(metribuzin), and 3,5-dichloro(N-l,l-dimethyl-2-propynyl)benzamide (pronamide).

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Community waste management programs that include the composting of sewage sludge and yard wastes have become a necessity. Using these composts provides many benefits; however, increased levels of organic matter may reduce the effectiveness of preemergence herbicides. Determining how herbicide application rates may need to be adjusted when composted waste is incorporated into the soil may permit the use of these amendments without any decrease in weed control. This experiment examined the effect of two types of compost (composted sewage sludge and composted yard waste) on the weed control provided by four preemergence herbicides. The soil was a Hagerstown silt loam amended with 10%, 20%, or 30% compost by volume. Each mix was placed in half-gallon cardboard milk cartons. The cartons were seeded at 1/2 and 1/4 inches with a mixture of broadleaved weeds and grasses. Each soil mix was treated with simazine, oxyfluorfen, oryzalin, and metolachlor at two rates. Control was evaluated both visually by number and by the dry weight of the harvested weeds. Preliminary results indicate composted sewage sludge causes a greater reduction in herbicide efficacy than composted yard waste. Oryzalin and metolachlor were affected less than oxyfluorfen or simazine. The experiment was repeated using lower application rates. In one replication the soil mixes from the previous experiment were used. The second replication used a Hagerstown silty clay loam soil with fresh compost. The results of this experiment will provide preliminary information for future field studies designed to determine if the application rates of preemergence herbicides need to be adjusted when fields are amended with composted organic matter.

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Abstract

N, N-dimethyl-2, 2-diphenylacetamide (diphenamid) and α, α, α,- trifluoro-2, 6-dinitro-N, N-dipropyl-p-toluidine (trifluralin) caused no injury to transplanted petunias (Petunia hybrida Vilm.) or marigolds (Tagetes patula L.) when incorporated in a mulch. Satisfactory broadleaf and grass weed control as denoted by indicator crops was achieved with both of these herbicides. Dichlobenil, 2,6-dichlorobenzonitrile, caused severe injury to both petunia and marigold transplants when applied in the herbicide-mulch combination. Control of broadleaf and grass indicator weeds with dichlobenil was excellent at all rates. Dimethyl tetrachloro terephthalate (DCPA) resulted in no visible injury to the marigold or petunia transplants when used in the herbicide-mulch combination, but control of grass and broadleaf indicator weeds was poor. When diphenamid and dichlobenil at the same rates were incorporated on peat moss, licorice root, pine bark or sugar cane mulches, no differences were observed among the mulches in regard to their ability to control weed growth.

On established nursery stock, 2-chloro-4, 6-bis (ethylamino)-s-triazine (simazine) plus DCPA, simazine plus diphenamid, and 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea (diuron), applied directly to the soil surface performed better than the herbicide-mulch combination at the lower rates. At higher rates, both the direct spray on the soil surface and the herbicide-mulch combination performed satisfactorily.

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nonselective, although crop tolerance varies. Herbicides derived from natural sources have been investigated for weed control in certified organic crop production with many reports focused on the evaluation of performance on warm-season weeds. These studies

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in controlling weeds in containers than hand-weeding, herbicides, or geotextile fabrics ( Chong, 2003 ). Organic mulches can be used to control weeds, conserve moisture in the soil, regulate soil temperature, and decrease weed seed germination, thus

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, and other plant residues have a high sorption capacity for many herbicides ( Brás et al., 1999 ; Huang et al., 2006 ). Herbicide sorption to organic mulch is dependent upon herbicide properties, namely solubility and soil adsorption coefficient [K oc

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Organic production systems rely heavily on cultural and mechanical weed control, with minimal dependence on herbicides that are approved for use in certified organic crops. Herbicides allowed for use in certified organic production systems are

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), geotextile fabric, herbicides, or a combination of these methods to control weeds ( Marble et al., 2015a , 2015b ). In addition to improving plant growth ( Greenly and Rakow, 1995 ), organic mulches have been shown to reduce weed seed germination ( Cochran

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