Early weed removal in canola gives the crop a competitive advantage over weeds and is therefore widely recommended for optimal crop production, but no studies have been undertaken previously to determine the effect of this practice on insect infestations. Four field experiments were conducted at Lacombe and Beaverlodge, AB, Canada, in 1999–2001 for a total of 10 site-years to determine the effect of time of weed removal on root maggot (Delia spp.) egg deposition and larval damage to taproots. The experiments also investigated the effects of other factors such as cultivar, seeding date, herbicide application rate, fertilizer rate and placement, and use of a nitrification inhibitor on root maggot damage and oviposition. Damage to taproots and oviposition declined by approximately 6 and 23%, respectively, with a delay in weed removal from the two- to six-leaf stage of canola development. The effect of time of weed removal on root maggot damage and egg density sometimes varied with site and cultivar, but the main effect of time of weed removal was more prominent (smaller P values) than interactions with site. The most plausible explanation for this effect relates to the behavioral sequence of events that precedes oviposition in mated, gravid female flies. Heterogeneous environments, such as weedy backgrounds in canola plantings, minimize opportunities for females of Delia spp. to complete the behavioral sequence required for oviposition, leading to reduced infestation levels in weedy systems. However, yield improvements achieved with early weed removal exceeded the yield benefit derived by lowered root maggot pressure when weeds were removed later. Nevertheless, current efforts to reduce pesticide use in agriculture may promote broader adoption of cultural control strategies for weed and root maggot management. In some situations, it may then be appropriate to ameliorate root maggot damage by maintaining some weedy background.

Resistance to herbicides and the lack of viable control options have led to an interest in increasing the role of crop competition as a weed management tool in water-seeded rice production. Weed-suppressive rice cultivars have been suggested as a tool that could improve weed control and reduce the reliance of growers on herbicides. Field studies were conducted at Biggs, CA, in 1999 and 2000 with six to eight semidwarf rice cultivars to identify water-seeded rice traits related to the suppression of watergrass growth. Cultivars S-201 and M-302 were the most suppressive in both years. The dry weight (DW) of watergrass grown with the most suppressive cultivar was only 16% in 1999 and 57% in 2000 of the DW of watergrass grown with the least suppressive cultivar. Rice leaf area and root DW in weed-free plots were linearly related to watergrass DW in both years. Weed-suppressive traits were not inversely correlated with rice yields in monoculture; competitive cultivars also had high yields. This study suggests that an indirect selection program, based on traits that can be identified early in the season under weed-free conditions, has great potential for developing more competitive cultivars for water-seeded rice.

Downy brome is an introduced Mediterranean annual grass that now dominates millions of hectares of western U.S. rangelands. The presence of this grass has eliminated many native species and accelerated wildfire cycles. The objective of this study was to identify soil additives that allowed germination but inhibited emergence of downy brome, while not affecting germination or emergence of the native perennial grass Hilaria jamesii. On the basis of data from previous studies, we focused on additives that altered the availability of soil nitrogen (N), phosphorus (P), and potassium (K). Most water-soluble treatments inhibited downy brome germination and emergence. We attribute the inhibitory effects of these treatments to excessive salinity and ion-specific effects of the additives themselves. An exception to this was oxalic acid, which showed no effect. Most water-insoluble treatments had no effect in soils with high P but did have an effect in soils with low P. Zeolite was effective regardless of P level, probably due to the high amounts of Na+ it added to the soil solution. Most treatments at higher concentrations resulted in lower downy brome emergence rates in soils currently dominated by downy brome than in uninvaded (but theoretically invadable) Hilaria soils. This difference is possibly attributable to inherent differences in labile soil P. In Stipa soils, where Stipa spp. grow, but which are generally considered to be uninvadable by downy brome, additions of high amounts of N resulted in lower emergence. This may have been an effect of NH4+ interference with uptake of K or other cations or toxicity of high N. We also saw a positive relationship between downy brome emergence and pH in Stipa soils. Hilaria development parameters were not as susceptible to the treatments, regardless of concentration, as downy brome. Our results suggest that there are additions that may be effective management tools for inhibiting downy brome in calcareous soils, including (1) high salt applications, (2) K-reducing additions (e.g., Mg), and (3) P-reducing additions.

Late watergrass is a serious weed of California rice that has evolved resistance to molinate, thiobencarb, fenoxaprop-ethyl, and bispyribac-sodium. To obtain an insight into the origin and spread of resistant (R) late watergrass in California rice fields, we evaluated similarities in morphological traits and amplified fragment length polymorphism (AFLP) fingerprints among 15 R strains compared with susceptible (S) strains. All strains were derived by inbreeding from accessions collected in rice fields of the Sacramento Valley, CA. In the field, R plants were shorter than S plants; they also had narrower and shorter flag leaves and thinner culms. Spikelets also appeared smaller and more slender in R plants. There was greater morphological similarity among the 15 R strains than among the eight S strains. The mean coefficients of variation for morphological traits were much smaller among R strains, which in a cluster analysis (Ward's method) were grouped morphologically apart at early clustering stages from the more variable S strains. AFLP electropherograms also showed greater similarity between R strains. R strains were grouped separately from the S strains in a cluster analysis based on calculated Nei and Li coefficients used in an unweighted pair group method using arithmetic means. However, small genetic differences also existed because the R strains were grouped into six clusters, suggesting that R strains were not samples from an identical strain. It was concluded that R strains originated from a preexisting and preadapted mutant late watergrass population in the Sacramento Valley. This study establishes that resistance moved by spikelet dispersal, not independent mutation events, most likely defined the geographical distribution of R late watergrass in California. Prevention and control of this dispersal combined with elimination of seed-producing survivors after herbicide treatment should be relevant components of the integrated management of herbicide-resistant late watergrass in California rice.

Sixteen field experiments were conducted across Oklahoma to evaluate the effects of MON 37500 time of application on cheat control, and winter wheat injury and yield. Winter wheat injury from MON 37500 applied preemergence (PRE) was slight and was influenced by cumulative precipitation for 10 d after application. Winter wheat injury was more frequent with early vs. late postemergence (POST) applications and was influenced by wheat growth stage and mean, high, and low air temperatures before and after application. Cheat control averaged 75% (n = 16 treatments with four to six replicates) when applied PRE and 88% (n = 126 treatments with four to six replicates) when applied POST. Cheat control from MON 37500 applied POST declined with increasing cheat growth stage at application and with decreasing mean diurnal low temperatures 0 to 14 d and 0 to 21 d before application. MON 37500 applied PRE increased yields 52 and 66% compared with the untreated control in Year 1 and Year 2, averaged over eight experiments each year. MON 37500 applied POST increased wheat yields 68 to 69% compared with the untreated check in Year 1 and Year 2, when averaged over all applications in eight experiments each year. Wheat yields were greater from fall POST applications than from late-winter applications.

Yellow starthistle is an invasive plant species common in the semiarid climate of central Idaho and other western states. Early detection of yellow starthistle and estimation of its infestation potential in semiarid grasslands have important scientific and managerial implications. Weed detection and delineation of infestations are often carried out by using ground survey techniques. However, such methods can be inefficient and expensive in detecting sparse infestations. The distribution of yellow starthistle over a large region may be affected by various landscape variables such as elevation, slope, and aspect. These exogenous variables may be used to develop prediction models to estimate the potential for yellow starthistle invasion into new areas. A nonlinear prediction model has been developed using a polar coordinate transformation of landscape characteristics to predict the likelihood of yellow starthistle occurrence in north-central Idaho. The study region included the lower Snake River and parts of the Salmon and Clearwater basins encompassing various land-use (range, pasture, and forest) categories. The model provided accurate estimates of yellow starthistle incidence within each specified land-use category and performed well in subsequent statistical validations. This prediction model can assist land managers in focusing their efforts by identifying specific areas for survey.

One approach to site-specific weed control is to map weeds within a field and then divide the field area into smaller grid units. The decision to apply a herbicide to individual grid units, or decision units, is made by using yield loss models to establish an economic threshold level. However, decision units often contain weed populations with aggregated distributions. Many yield loss models have not considered this because experiments dealing with weed–crop competition typically assume uniform weed distributions. Therefore, these models may overestimate yield losses. Field experiments conducted in 1999 and 2000 compared the effects of common ragweed having a uniform distribution vs. an aggregated distribution on soybean seed yield, moisture content, and dockage. Field experiment data were used to calculate and compare economic thresholds for both distributions. Economic thresholds that considered drying costs and dockage also were compared. There was no significant difference in I parameters (yield loss as density approaches zero) between the two ragweed distributions in either year. Seed moisture content and dockage increased with increasing common ragweed densities, but increases were not significant at the break-even yield loss level. Economic threshold values were similar for both distributions with differences between aggregated and uniform of 0.14 and 0.01 plants m−2 in 1999 and 2000, respectively. The economic threshold values were reduced by 0.01 to 0.06 plants m−2 when drying costs and dockage were considered.

Smellmelon is becoming a problem weed in southern Texas and Louisiana cotton-producing areas. Experiments evaluating the density and duration of smellmelon competition are necessary for the recommendation of appropriate control practices to minimize yield loss. In 1999 and 2000, field experiments were initiated to determine the density, critical period of competition, and potential control measures for smellmelon in cotton. Although the degree of competition differed between the years, similar trends were observed with respect to smellmelon density. As few as 2 or 3 smellmelon plants 10 m−1 row reduced yield at least 17% each year. When densities were increased to 5 or 10 plants 10 m−1 row, yield reductions increased to at least 34%. Results from the duration of smellmelon competition in 1999 and 2000 indicated that cotton should remain smellmelon free for 1 to 7 wk after planting (WAP) and 2.5 to 6 WAP, respectively. Results from the control studies indicate that smellmelon size at application influences control. Early-season control strategies are necessary to reduce yield losses associated with smellmelon competition.

The most common strategy recommended for management of jointed goatgrass infestations is to rotate from winter wheat to a spring crop for several years. A field study was conducted at three locations in 1998 and 1999 to determine the effects of spring seeding date on the ability of jointed goatgrass to flower and produce viable seed in the presence or absence of spring wheat and to determine the effect of jointed goatgrass competition and crop seeding date on spring wheat grain yield. Spring wheat was seeded on four dates at each location in both hand-sown and natural jointed goatgrass infestations. Jointed goatgrass plants from hand-sown spikelets flowered and developed spikelets on all seeding dates except the last; viable seed was produced on the two earliest seeding dates. Jointed goatgrass plant densities from natural infestations were from 1 to 12 plants m−2, and spikelet production ranged from 0 to 480 spikelets m−2. Natural jointed goatgrass infestations produced spikelets containing viable seed on all seeding dates at one location in 1998, the driest location. Spring wheat yield was not affected by jointed goatgrass competition; however, jointed goatgrass spikelet production was reduced by spring wheat competition compared with that of monoculture jointed goatgrass. The last seeding date of spring wheat was associated with 51% less crop yield compared with the recommended seeding date. The decision to manage jointed goatgrass infestations with a spring crop rotation should consider delayed seeding dates to minimize viable spikelet production by spring-germinating jointed goatgrass; however, the cost of this decision may include grain yield reduction.

Velvetleaf has been a major concern of Southern cotton growers, yet information on its competitiveness and seed production in cotton is lacking. Experiments were conducted in 1997 and 1998 at the Central Crops Research Station in Clayton, NC, to evaluate density-dependent effects of velvetleaf interference and seed production dynamics in conventional tillage cotton. Velvetleaf at densities of up to 3.5 plants m−1 of row did not influence cotton height until at least 4 wk after planting. Velvetleaf height increased as plant density increased throughout the season in 1997, but it was not affected until 9 wk after planting in 1998. Because of differences in environmental conditions, velvetleaf and cotton achieved maximum height later in 1998 than in 1997; however, velvetleaf seed production and cotton yields were higher in 1998 than in 1997 regardless of velvetleaf density. Velvetleaf density had no effect on the fresh weight, dry weight, and stem diameter of velvetleaf plants in 1997. But in 1998, all these parameters decreased significantly with increasing velvetleaf density. Velvetleaf seed production in 1998 was nearly twice that in 1997. Averaged over velvetleaf densities, the greatest number of seed were produced between nodes 6 and 20 in 1997 and between nodes 1 and 10 in 1998. In both years, cotton yield loss increased with velvetleaf density. Maximum yield loss was 84% at 3.5 velvetleaf plants m−1 of row. Yield losses of 5 and 10% were caused by 0.2 and 0.4 velvetleaf plants m−1 of row (1,930 and 4,110 plants ha−1), respectively, in 1997 and by 0.03 and 0.08 velvetleaf plants m−1 of row (360 and 850 plants ha−1), respectively, in 1998. To understand better the applicability of these results, we hypothesized that environmental variation caused differences in measured responses between 1997 and 1998. Therefore, kriging methods were used to fit correlations between observed rainfall and growing degree days (GDD) each year at the experiment site. Results based on climate data from 4 yr at 110 sites indicated that inference space was higher for GDD than for rainfall. The conditions observed at the experiment site in 1997 were deemed most appropriate for the recommendations made in the surrounding area.

Four species of biocontrol insects (knapweed root weevil, lesser knapweed flower weevil, spotted knapweed seedhead moth, and bronze knapweed root borer) were released at a diffuse knapweed site located about 10 km east of the Colorado Front Range. Two other biocontrol agents (banded gall fly and knapweed seed head fly) were already present at this site. Densities of rosettes and flowering plants, seedhead production per plant, and seeds per seedhead on mowed and unmowed areas were studied for 5 yr, 1997–2001. Of the six biocontrols, five (Urophora spp., bronze knapweed root borer, knapweed root weevil, and lesser knapweed flower weevil) obtained sizable densities relative to weed abundance. Diffuse knapweed declined from 8.3% in absolute cover in June 2000 to 1.9% by September 2001. Vegetation transects closest to the insect release areas showed the largest declines, with diffuse knapweed disappearing entirely on one transect. In contrast, diffuse knapweed abundance at a nearby prairie increased during the same interval from an absolute cover value of 14.5 to 17%. Seed production of diffuse knapweed on the insect release site declined from nearly 5,000 seeds m−2 in 1997 to less than 100 seeds m−2 in 2001. Lesser knapweed flower weevil larvae appeared responsible for much of the seed reduction, whereas adults of this species were effective in damaging bolting plants. The extent to which grazing removal and individual insect species contributed to this reduction in diffuse knapweed abundance cannot be identified from this study. These results support the contention that a significant reduction in abundance of diffuse knapweed using insects is possible at least in some regions of the western United States.

Corn yield loss associated with common lambsquarters and giant foxtail in mixed-weed species communities was estimated from empirical equations based on early-season weed density, weed relative leaf area, or weed relative shoot volume in 1998 and 1999. The estimated maximum corn yield loss ranged up to 20% in 1998 but was 50% or more in 1999. Competition coefficients estimated from weed density (I values) or weed relative shoot volume (qV values) indicated that the weed species were equally competitive in 1998 but that common lambsquarters was more competitive than giant foxtail in 1999. In contrast, the relative leaf area–based competition coefficients (qL values) indicated that common lambsquarters and giant foxtail were equally competitive in both years. Weed species emerged at the same time as corn in 1998, whereas in 1999, common lambsquarters emerged 3 d earlier than corn and 1 d earlier than did giant foxtail. Earlier emergence of common lambsquarters was associated with greater cumulative intercepted photosynthetically active radiation (IPAR) per plant compared with that of giant foxtail. Competition coefficients estimated from weed relative leaf area were similar between years for common lambsquarters but differed for giant foxtail. Similarly, the relationship between cumulative estimated IPAR and early-season relative leaf area was stable between years for common lambsquarters but not for giant foxtail. Consequently, competition coefficients were more consistent for common lambsquarters than for giant foxtail in mixed communities. The results suggest that the competitive ability of common lambsquarters and giant foxtail may not differ greatly in corn, but variability in corn yield loss between years was not adequately explained by these empirical models.

Pygmyflower is becoming increasingly abundant in conservation-tillage cropping systems of the Canadian Prairies. Increased knowledge of pygmyflower biology would facilitate development of integrated programs for its control. A controlled environment study was conducted to determine the combined effect of various soil temperature and soil water levels on pygmyflower emergence. Pygmyflower emerged at soil temperatures ranging from 10 to 25 C with optimal emergence at 15 C. Emergence was inhibited completely at 5 or 30 C. Pygmyflower emergence markedly declined as soil water content decreased. At progressively lower soil water levels of − 0.03, − 0.28, − 0.53, − 0.78, − 1.03, and − 1.53 MPa, pygmyflower emergence at 15 C was 71, 50, 49, 29, 24, and 15%, respectively. The interaction of warm and dry soils caused the greatest inhibition of pygmyflower emergence. Rate of pygmyflower emergence was reduced greatly by either lowering soil temperature or reducing soil water content. A decrease in soil temperature from 25 to 10 C increased the time to reach 50% emergence (ET50) by 9 to 14 d, whereas a decrease in soil water content from − 0.03 to − 1.53 MPa increased ET50 by 7 to 16 d. Results suggest that pygmyflower may pose the greatest problem in winter crops or early-planted spring crops. Information gained in this study will contribute to the development of an improved pygmyflower management program in annual cropping systems.

Giant ragweed seeds have high nutritional value, consisting of 47% crude protein and 38% crude fat, and may be an important food source for rodent and invertebrate populations in agricultural and early successional ecosystems. We investigated temporal patterns of postdispersal giant ragweed seed predation on the soil surface of a no-tillage cornfield as affected by involucre (seed dispersal unit) size and presence or absence of crop residue. Cage exclusion experiments indicated that rodents and invertebrates were the principal predators of giant ragweed seed, and total predation of involucres over a 12-mo period beginning in November was 88%. Rodents were the greatest predators of giant ragweed involucres during fall and winter, and cumulative predation by February 1 in treatments with rodent access ranged from 39 to 43%. In contrast, giant ragweed involucre predation by invertebrates occurred mainly from May 1 to November 1. When rodent access to involucres was prevented, total involucre predation by invertebrates over a 12-mo period ranged from 57 to 78%. Rodents showed an initial preference for large involucres (> 4.8-mm diameter), and invertebrates preferred small involucres (< 4.8-mm diameter). Involucres covered with corn plant residue underwent less predation by rodents from November to February than uncovered involucres, but residue cover had no effect on seed predation by invertebrates. In a laboratory feeding trial, the carabid Harpalus pensylvanicus preferred seed of smooth pigweed and yellow foxtail to giant ragweed seed, suggesting that giant ragweed seed is an incidental rather than a preferred food source for some carabids. Because giant ragweed exhibits relatively low fecundity and short seed bank persistence, results of this study suggest that postdispersal predation may directly reduce giant ragweed recruitment the next year by reducing new seed bank inputs. However, seed losses from predation alone may be insufficient to maintain giant ragweed populations below economic threshold levels in no-tillage cornfields.

Wild oat, a troublesome weed in cereals, infests about 11 million hectares of cropland in the United States. Diversifying cereal production with alternative crops, such as yellow mustard and canola, can provide flexibility in cropping systems, decrease production risks, and allow for effective weed suppression. The objective of the study was to quantify the competitive ability of yellow mustard and canola relative to wild oat in addition series field experiments, which were conducted in 1999 and 2000 near Genesee, ID. Biomass and seed production of wild oat were reduced 67 and 80%, respectively, in mixtures with yellow mustard, which was three to four times greater than the reduction in corresponding mixtures with canola. In addition, yellow mustard reduced the biomass and seed production of wild oat equally regardless of wild oat density. In contrast, the competitive effect of canola on wild oat biomass decreased 5 to 10 times when wild oat density increased from 100 to 200 plants m−2. Yellow mustard at all densities and at both biomass harvests suppressed wild oat biomass and seed production similarly. But suppression of wild oat by canola increased as canola density increased, and canola plants were more competitive at the flowering stage than at the rosette stage. Wild oat had little or no effect on yellow mustard seed yield but reduced canola seed yield 37%, when averaged over canola densities. Additionally, the oil content of canola seed was reduced 0.4% for every 1% of wild oat seed in the harvested seed. Models developed in this study accurately predicted plant populations of yellow mustard and canola that provided optimal weed suppression and crop yield for different wild oat populations.

The effects of temperature and light on the dormancy of velvetleaf, common waterhemp, and giant foxtail seeds were studied under controlled growth chamber conditions. Seeds were either kept chilled at 4 C for 12 wk under wet conditions or nonchilled at 4 C in dry storage. Then, seeds were germinated under increasing and decreasing temperatures and under continuous red light (R) and far-red light (FR). In addition, chilled and nonchilled seeds were germinated in the dark after being exposed to alternating R and FR flashes. Velvetleaf germination was increased by exposure to high temperatures (36 C) immediately after exposure to low temperatures (4 C), but light had no effect. Chilling increased common waterhemp seed germination and sensitivity to light and temperature. R promoted common waterhemp seed germination, whereas FR inhibited germination and maintained dormancy. In addition, the effect of light was reversible. Therefore, common waterhemp dormancy was phytochrome regulated. However, high temperatures (36 C) promoted the germination of chilled seeds, even when exposed to FR. The germination of chilled giant foxtail seeds was reduced by FR. Giant foxtail seed dormancy was partially phytochrome regulated, but dormancy regulation was more dependent on mean temperature.

Yellow starthistle is a problematic invasive plant in the western United States. Attempts to control it often include the use of herbicides although herbicides can have detrimental effects on desired native species. We studied the effect of clopyralid on a native bunchgrass and vernal pool community in the Central Valley of California. Areas invaded by yellow starthistle were treated with clopyralid, and the presence and cover of all species was monitored for 2 yr. There were significant differences in species cover between treated and untreated plots 2 yr after treatment (P = 0.0001). These included a decline in cover of yellow starthistle and other native and exotic members of the Asteraceae and Apiaceae families in both years and a decline of exotic members of the Fabaceae family in the second year. There also were decreases in the frequency of the Asteraceae (P < 0.0001), Apiaceae (P < 0.05), Fabaceae (P < 0.05), and Polygonaceae (P < 0.05) families 1 yr after treatment, and the Asteraceae (P < 0.05) and Fabaceae (P < 0.05) families 2 yr after treatment. We also observed a significant negative effect of clopyralid on fecundity measures of a native Viola species during the first year of application (P = 0.0074). This effect did not persist into the second year, when violets were more common and produced more flowers in treated plots (0.02 < P < 0.05).

Experiments were conducted to determine the efficacy, absorption, and translocation of glyphosate, glufosinate, and imazethapyr with selected weed species. In the greenhouse glyphosate, glufosinate, and imazethapyr were applied at 0.25, 0.5, and 1 times their label rates of 1,121, 410, and 70 g ha−1, respectively, on 10- to 15-cm black nightshade, common waterhemp, eastern black nightshade, field bindweed, giant ragweed, ivyleaf morningglory, prairie cupgrass, velvetleaf, and yellow nutsedge. Glyphosate applied at the 1-time rate caused injury greater than or similar to injury from the 1-time rate of glufosinate or imazethapyr on black nightshade, common waterhemp, eastern black nightshade, field bindweed, giant ragweed, prairie cupgrass, and velvetleaf. The 1-time rate of glufosinate injured ivyleaf morningglory and yellow nutsedge more than did the 1-time rate of glyphosate or imazethapyr. Under field conditions glyphosate caused the greatest injury to common waterhemp, prairie cupgrass, and velvetleaf across plant growth stages. Giant ragweed and ivyleaf morningglory injury was more dependent on growth stage, with the 15- and 30-cm growth stages more susceptible to glyphosate than to glufosinate or imazethapyr. Differential response of these weed species may be caused by differences in herbicide translocation. Glyphosate was translocated more in both giant ragweed and ivyleaf morningglory, and these species were injured more by glyphosate than by glufosinate or imazethapyr at the larger growth stages.

Greenhouse hydroponic and field experiments were conducted to determine the effect of nitrogen (N) fertilizer on eastern black nightshade growth and N content. After 4 wk in hydroponics, the greatest shoot dry mass occurred in the 5 or 10 mM nitrate treatments. An exponential model was strongly correlated with eastern black nightshade root and shoot growth in the 5 or 10 mM nitrate treatments. Eastern black nightshade reached the exponential stage of growth approximately 21 d after transfer to the nitrate treatments. Root dry mass was only 10% of shoot biomass. There was a substantial increase in height and growth rate when plants were fertilized with 168 kg N ha−1. Eastern black nightshade was at its maximum growth 12 wk after planting, and fresh mass increased with N up to 336 kg N ha−1, the greatest N concentration in the study. The greatest N content was in the upper leaves and the least in the lower stem. These results indicate that eastern black nightshade growth is favored by high rates of N.

Kudzu is an aggressive, nonnative vine that currently dominates an estimated 810,000 ha of mesic forest communities in the eastern United States. To test an integrated method of weed control, abundances of kudzu and other plant species were compared during 4 yr after six herbicide treatments (clopyralid, triclopyr, metsulfuron, picloram + 2,4-D, tebuthiuron, and a nonsprayed check), in which loblolly pines were planted at three densities (0, 1, and 4 seedlings m−2) to induce competition and potentially delay kudzu recovery. This split-plot design was replicated on each of the four kudzu-dominated sites near Aiken, SC. Relative light intensity (RLI) and soil water content (SWC) were measured periodically to identify mechanisms of interference among plant species. Two years after treatment (1999), crown coverage of kudzu averaged < 2% in herbicide plots compared with 93% in the nonsprayed check, and these differences were maintained through 2001, except in clopyralid plots where kudzu cover increased to 15%. In 2001, pine interference was associated with 33, 56, and 67% reductions in biomass of kudzu, blackberry, and herbaceous vegetation, respectively. RLI in kudzu-dominated plots (4 to 15% of full sun) generally was less than half that of herbicide-treated plots. SWC was greatest in tebuthiuron plots, where total vegetation cover averaged 26% compared with 77 to 111% in other plots. None of the treatments eradicated kudzu, but combinations of herbicides and induced pine competition delayed its recovery.