Pigweed is an increasingly aggressive weed in semiarid environments such as Mediterranean areas, and in general the control of all Amaranthus species is becoming more and more difficult. Increasing pigweed aggressiveness could be a result of its ability to keep a high water use efficiency under drought conditions. An experiment was conducted to study the effect of water stress on the photosynthetic capacity, growth, and leaf water potential of pigweed at the field level and assess if this species, as a model for C4 weeds, is CO2-saturated at the current level of atmospheric CO2 in a Mediterranean area. Pigweed was studied within a naturally occurring weed population in a bell pepper field in southern Italy where a rain-fed treatment (V0) was compared to a fully irrigated one (V100) corresponding to the restoration of 100% of the maximum crop water evapotranspiration. Soil water content was measured periodically, and net assimilation rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration were determined on pigweed leaves. Photosynthetic rates of 37.6 µmol m−2 s−1 in V100 and 13.9 µmol m−2 s−1 in V0 were recorded, with higher transpiration rates in V100; consequently stomatal conductance was significantly lower in rain-fed conditions (0.08 mol m−2 s−1)) compared to the irrigated treatment (0.30 mol m−2 s−1). Photosynthesis in pigweed is not completely CO2-saturated at the current atmospheric CO2 level in the Mediterranean area and this could affect competition and increase of aggressiveness toward crops at the actual CO2 atmospheric concentration in agro-ecosystems. This occurs because unlike other C4 crops already saturated for CO2, weeds that are not CO2-saturated will remain CO2-sensitive to higher ambient CO2 levels. Thus, when they are grown in mixed stands where competition occurs, they can still suppress the slower-growing species.

Rice flatsedge and barnyardgrass are widespread and competitive weeds indirect-seeded rice. Developing integrated weed management strategies thatelevate suppression of weeds by rice through crop density, nutrition, andcultivar choice requires better understanding of the extent to which riceinterferes with the growth of these weeds and how these species respond toresource limitation with crop interference. Rice interference reduced theheight of barnyardgrass but did not affect height of rice flatsedge. Theseweed species were able to grow taller than rice and thus avoided beingshaded. Increased specific stem length under crop interference maydemonstrate a strategy of stem elongation to allow the top portion of theweeds to be kept out of shade. Rice interference reduced inflorescence andshoot biomass of both weed species. Barnyardgrass showed the ability toreduce the effects of rice interference by increasing leaf weight ratio. Thepresent study shows that crop interference alone may reduce weed growth butmay not give complete control of these weed species. This highlights theneed for the integration of management practices to achieve control of theseweed species.

Field experiments were conducted from 2004 through 2006 at Pendleton and Clemson, SC, to determine the influence of seasonal emergence of wild radish on phenological development, survival, and seed and biomass production in a noncompetitive environment. The duration of four developmental phases, emergence to bolting, bolting to flowering, flowering to silique production, and silique production to maturity, were recorded following wild radish sowing at monthly intervals from October 2004 through September 2006. Seedling emergence occurred 2 to 4 wk after sowing. Mortality of seedlings that emerged from December through March was greater than that of seedlings that emerged in all other months. Wild radish that emerged from April through August completed its life cycle by summer or early autumn. Wild radish that emerged from September through November was able to survive the winter and complete its life cycle the following spring. The developmental phases most affected by time of emergence were emergence to bolting and bolting to flowering. The duration of emergence to bolting ranged from 249 to 479 growing degree days (GDD), and bolting to flowering from 270 to 373 GDD, depending on the month of emergence. The total life cycle of wild radish varied from a low of 1,267 GDD following June emergence to 1,503 GDD following November emergence. Multiple regression analysis revealed that emergence to bolting and silique production to maturity phases were dependent on accumulated heat units and photoperiod. Seed and biomass production were influenced by month of emergence. An average of 1,470 seeds plant−1 was produced when emergence occurred in July and 10,170 seeds plant−1 when emergence occurred in November. Plants that emerged in autumn exhibited minimal growth during the winter months, but conditions were conducive for growth in mid-March and April, with biomass production of 809 g plant−1 at silique production.

The intensification of crop management in the U.K. over the past 60 years has resulted in the decline of the populations of a number of annual plant species adapted to arable habitats. In contrast, other species continue to be common as arable weeds. A community assembly approach was taken to explain these recent changes in the weed flora using databases of plant functional traits, a pot experiment, and weed surveys of the Broadbalk long-term experiment. The hypothesis was tested that species that have been selected against by increased fertilizer inputs and herbicide use share an adverse combination of traits. An analysis comparing the combination of maximum height, seed weight, and time of first flowering of 29 common and 32 rare or threatened U.K. autumn weeds established that rare or threatened species occupied an area of trait space that was distinct from the common species. A rare weed trait syndrome of short stature, large seed, and late flowering was identified. The theory that species with a trait syndrome that is currently unfavorable are better adapted for less fertile environments was supported by the pot experiment. Species with a combination of short stature and large seed had a relatively greater competitive ability in low compared to high fertility treatments. Analysis of survey data from the Broadbalk long-term experiment confirmed that, as N inputs increased, the abundance of the two functional groups that contained only common species remained stable or increased; whereas, the groups dominated by rare or threatened species declined as fertility increased. An understanding of the response traits of arable plants to management filters, including fertilizer inputs and herbicide, is valuable for designing conservation strategies for rare species or predicting future shifts in the functional diversity of weed communities including the potential for invasive species to establish.

Junglerice is one of the most serious grass weeds of rice in the tropics. Greenhouse studies were conducted to evaluate growth and reproduction of junglerice in response to water stress. Plant height, biomass, and seed production of junglerice grown alone were reduced with increasing water stress. However, most stressed plants (irrigated at 12.5% of field capacity) still produced considerable biomass (8.5 g plant−1) and seeds (>1,600 seeds plant−1). When junglerice and rice were grown together under water-stressed condition, junglerice was taller than rice. The junglerice-to-rice biomass ratio also increased from 4.7 at 100% of field capacity to 7.6 at 12.5% of field capacity, indicating the greater junglerice vigor in water-stress conditions. In another study, the influence of the duration of water stress at intervals between 3 and 15 d on growth and seed production of junglerice was evaluated. Plant height, biomass, and seed production decreased with increasing water-stress duration. However, the weed produced an average of 400 seeds plant−1 in the most stressed treatment (i.e., when irrigation was applied at 15-d intervals). Water-stressed treatments did not affect germination of junglerice seeds in the laboratory. Growth and seed production of junglerice at all moisture levels ensures survival of the population in an unpredictable environment and contributes to the weedy nature of this species. The joint effect of enhanced weed competition and drought stress could severely harm crop yield; therefore, it is important to control such weeds in the early stages of crops and save stored moisture for the crops.

A 50-yr study at Fairbanks, AK, was started in 1984 to determine soil seedlongevity of 17 weed species. Seeds were buried in mesh bags 2- and 15-cmdeep and were recovered 0.7, 1.7, 2.7, 3.7, 4.7, 6.7, 9.7, 19.7, and 24.7 yrlater. Viability was determined using germination and tetrazolium tests. By24.7 yr after burial (YAB), no viable seeds were found for commonhempnettle, flixweed, foxtail barley, quackgrass, and wild oat. Bluejointreedgrass, which had no live seed 19.7 YAB, again had viability (1% at 15cm) 24.7 YAB. Seeds of 11 other species were still viable: Americandragonhead (52%), marsh yellowcress (11 and 3.0% at 2 and 15 cmrespectively), common lambsquarters (2.8%), prostrate knotweed (2.8% at 15cm), shepherd's-purse (2.8%), pineapple-weed (2.6%), rough cinquefoil(2.3%), Pennsylvania smartweed (1.1%), common chickweed (0.4%), wildbuckwheat (0.3%), and corn spurry (0.1%). Seed dormancy 24.7 YAB was verylow (< 10%) for all species except American dragonhead (99%),shepherd's-purse (40%), marsh yellowcress (23% at 2 cm), Pennsylvaniasmartweed (18%), and rough cinquefoil (14%). At the beginning of the study,declines in seed longevity were uniform between replicates, but variabilitybetween replicates increased over time for all species except Americandragonhead, suggesting that some soil microsites are more favorable for seedsurvival and may be seedbank “safe sites.” Results of this study demonstratethat nonrandom seed mortality contributes to the spatial heterogeneity ofseed populations in the soil seedbank.

The spread and dominance of the invasive native hay-scented fern in the understory is one of the most significant changes that has affected the forest ecosystems in the northeastern United States in the last century. We studied changes in the distribution and dynamics of hay-scented fern at a large scale over a 10-yr period in Pennsylvania. The study included 56 stands covering 1,009 ha in two ecoregions. Hay-scented fern was more widely distributed and occurred at higher densities in the Allegheny Plateau ecoregion vs. the Ridge and Valley. Hay-scented fern abundance was positively associated with overstory red maple abundance in both ecoregions. After overstory removal, the density and distribution of hay-scented fern tended to increase and remain at elevated levels in stands that were not treated with herbicide. Herbicide treatments resulted in temporary reductions in fern densities and created a “window of opportunity” for the establishment of tree regeneration.

We explored the allelopathic potential of kudzu as a function of its phenolics. Aqueous and methanol extracts of different kudzu organs (leaf, stem, root, and seed) were assayed for allelopathy with the use of lettuce and radish seeds. Both leaf and root extracts significantly inhibited all of the measured germination indices (total germination, speed of germination, and coefficient of the rate of germination) (all P < 0.01). When treated with leaf extract, the total germination of both species was ∼ 20% less than the control. Furthermore, the leaf extract significantly reduced the speed of germination to 38 and 53% that of the lettuce and radish controls, respectively. Lettuce and radish seeds soaked in leaf and root extracts for 24 h imbibed less water (∼ 30% for both species) than those soaked in distilled water (control), suggesting that a reduction of water imbibition might be one of the mechanisms of germination retardation. Stem and seed extracts affected neither the water uptake nor the germination indices of radish and lettuce seeds. Kudzu leaves and roots contain higher amounts of total phenolics (P = 0.001) and soluble phenolics (P = 0.005) than stems and seeds, consistent with the results of the germination bioassays. In agar plate bioassays, both litter and rhizosphere soil had phytotoxic effects on the radicle growth of radish (P = 0.003) and perennial ryegrass (P = 0.001) seedlings. Perennial ryegrass and cobbler's pegs seedlings grown on leaf and root leachate-amended soil gave ∼ 40% shorter roots and shoots and ∼ 50% less dry weight than those grown in leachate-free soil. Kudzu litter was incorporated and incubated in soil for 6 wk, at which point the soluble phenolics in the soil solution were at a level (> 150 ppm) considered to be allelopathic.

Knowledge of the germination requirements of wild radish will help in determining the favorable conditions for germination and emergence and allow better management of this weed. Experiments were conducted during 2005 to 2006 and 2006 to 2007 to evaluate wild radish temperature and light requirements over a 12-mo period beginning in July on seeds placed on the soil surface and at a 10-cm depth. Germination response was influenced by temperature, light, duration of burial, and burial depth. Freshly harvested seeds (July) had no more than 18% germination whereas seeds allowed to after-ripen in the field for 3 to 6 mo (October to January) had up to 40% germination. The germination of wild radish retrieved from the soil surface was 1.2 to 1.5 times greater at alternating temperatures (2.5/17.5, 7.5/22.5, and 12.5/27.5 C) than at constant temperatures (10, 15, and 20 C) at 0, 3, and 6 mo after maturation. The light requirement for germination varied by time of year with no differences in germination between light and dark conditions for freshly harvested seeds. Far-red light inhibited germination of wild radish, indicating that wild radish may become sensitive to light following an after-ripening period.

Horseweed can be a problematic weed in no-till soybean fields and populations can vary in their response to 2,4-D. The objective of this study was to evaluate the growth and seed production of four horseweed populations after exposure to 2,4-D. 2,4-D amine was applied at 0, 140, 280, and 560 g ae ha−1 to 5- to 10-cm-tall horseweed plants. An additional treatment of 280 g ha−1 of 2,4-D + 840 g ae ha−1 of glyphosate was included in the study. At 2 wk after treatment (WAT), injury ranged from 47 to 98%, but by 6 WAT the injury ranged from 89 to 100% for all four populations. Between 6 and 12 WAT some individual horseweed plants started to recover. No differences in dry weights were observed between the four populations in the untreated checks at 0, 2, 6, and 12 WAT. At 280 g ha−1 of 2,4-D amine, seed production was reduced by greater than 95%. However, three of the four horseweed populations had plants that survived and produced seed after exposure to 840 g ha−1 of glyphosate + 280 g ha−1 of 2,4-D. One plant produced seed after exposure to 560 g ha−1 of 2,4-D. These results suggest that horseweed can evolve resistance to 2,4-D and no fitness penalities were observed in populations that had higher levels of tolerance to 2,4-D.

Research was conducted to establish a method to investigate the resistance level of flixweed to tribenuron-methyl and the evolved biochemical resistance mechanism. Four resistant biotypes were collected from wheat fields in Mazhuangcun, Jiacun, Dishangcun, and Bafangcun in the Hebei province of China where tribenuron-methyl had been continuously used for more than 10 yr. Two susceptible biotypes were collected from wheat fields where tribenuron-methyl was never applied. Different biotypes were assessed by petri-dish bioassay, whole-plant bioassay, and acetolactate synthase (ALS) assay. Comparisons of data indicated a similarity between methods and that experiments demonstrated that petri-dish bioassay was a feasible method to identify flixweed resistant to tribenuron-methyl. Data indicated differences among the flixweed biotypes when assessed by the petri-dish bioassay, whole-plant bioassay, or ALS enzyme assay, and a close association was obtained for the three bioassay methods. ALS resistance varied by biotypes with Mazhuangcun > Jiacun > Dishangcun > Bafangcun. Target-site enzyme assay data indicated that the resistant biotype's enhanced ALS activity was the biochemical mechanism that induced flixweed's evolved resistance to tribenuron-methyl. The concentrations of tribenuron-methyl causing 50% inhibition of ALS activity of the four resistant biotypes were 1,359, 513, 184, and 164 nM; in the susceptible biotypes these concentrations were 64 and 65 nM. Resistance indexes were 21, 8, 3, and 3 for Mazhuangcun, Jiacun, Dishangcun, and Bafangcun biotypes, respectively.

Predicting weed emergence dynamics can help farmers to plan more effectiveweed control. The hydrothermal time concept has been used to model emergenceas a function of temperature and water potential. Application of thisconcept is possible if the specific biological thresholds are known. Thisarticle provides a data set of base temperature and water potential of eightmaize weeds (velvetleaf, redroot pigweed, common lambsquarters, largecrabgrass, barnyardgrass, yellow foxtail, green foxtail, and johnsongrass).For five of these species, two ecotypes from two extreme regions of thepredominant maize-growing area in Italy (Veneto and Tuscany), were collectedand compared to check possible differences that may arise from using thesame thresholds for different populations. Seedling emergence of velvetleafand johnsongrass were modeled using three different approaches: (1) thermaltime calculated assuming 5 C as base temperature for both species; (2)thermal time using the specific estimated base temperatures; and (3)hydrothermal time using the specific, estimated base temperatures and waterpotentials. All the species had base temperatures greater than 10 C, withthe exception of velvetleaf (3.9 to 4.4 C) and common lambsquarters (2.0 to2.6 C). All species showed a calculated base-water potential equal or up to−1.00 MPa. The thresholds of the two ecotypes were similar for all thestudied species, with the exception of redroot pigweed, for which the Venetoecotype showed a water potential lower than −0.41 MPa, whereas it was −0.62MPa for the Tuscany ecotype. Similar thresholds have been found to be usefulin hydrothermal time models covering two climatic regions where maize isgrown in Italy. Furthermore, a comparison between the use of specific,estimated, and common thresholds for modeling weed emergence showed that,for a better determination of weed control timing, it is often necessary toestimate the specific thresholds.

This study examined seed ultrastructure in relation to germination of North American dandelion seeds. Based on laboratory rearing observations, it was thought that the design of the pappus acts as a conduit facilitating water entry into the seed. It was hypothesized that seeds without a pappus would yield fewer seedlings and require more time to germinate than seeds with an intact pappus. Seed ultrastructure was investigated using scanning electron microscopy, while relative humidity and fungal association were explored as factors that may confer an advantage to intact seeds. Results indicate that germination for seeds lacking a pappus is 31% lower than control seeds (with an intact pappus) and that the seeds lacking a pappus require more time to germinate. Relative humidity did not differentially affect germination, and while a fungus Cladosporium cladosporioides was recovered internally, its presence neither enhanced germination nor decreased time to germination when tested by antimycotic removal. Electron micrographs revealed that (1) the pappus is hollow and (2) the pericarp of the fruit fuses with and partially encloses the pappus. Fusion of the pappus with the fruit suggests that this structure acts as a device to regulate seed hydration.

Seed deterioration, and therefore seed germination potential, are highlyinfluenced by relative humidity and temperature. However, limitedspecies-specific information is available about the effect of long-termsoaking in water on seed germination potential. Knowing the potential fateof a creeping bentgrass seed that falls in an irrigation canal is importantfor the study of transgene flow in this species at the landscape level. Theobjectives of this study were to evaluate the effect of soaking time andwater temperature on germination of creeping bentgrass seed and to assesshow fast a panicle could be moved in an irrigation canal. Germination wasdetermined for seeds from panicles of three cultivars of creeping bentgrassthat were soaked in water for up to 17 wk at two water temperatures, 4 and20 C. Creeping bentgrass seeds did not lose their ability to germinate after17 wk in water at 20 C and, although reduced, germination was still 46%after 17 wk in water at 4 C. The reduction in germination in seeds frompanicles kept in water at 4 C was due to the induction of secondarydormancy, which was overcome by dry seed storage at room temperature. Wequantified that a panicle that falls in an irrigation canal has thepotential to travel downstream at an average rate of 19 m min−1and move seeds that could potentially establish seedlings elsewhere.Therefore, movement of creeping bentgrass seed by water has to be consideredas a means of gene flow.

Biennial wormwood and lanceleaf sage have become serious weeds of several crops within the northern Great Plains of the United States and Canada. Both species are prolific seed producers but little is known about their potential for developing persistent seedbanks. Field studies were conducted to determine the influence of duration (7, 8, 11, 19, 20, and 23 mo) and depth of burial (0, 2.5, and 10 cm) on biennial wormwood and lanceleaf sage seed viability and decay. Biennial wormwood and lanceleaf sage seeds were buried in September 2003 (burial 1) and September 2004 (burial 2). In burial 1, biennial wormwood and lanceleaf sage seed viability was 65 and 66%, respectively, after 23 mo of burial. In burial 2, biennial wormwood and lanceleaf sage seed viability was 8 and 3%, respectively, after 23 mo of burial. The difference was likely because of higher soil moisture during burial 2, which promoted seed decay. Controlled-environment studies sought to determine the influence of stratification environments (freezing, chilling, and freeze–thaw) followed by exposure to diurnally fluctuating temperatures on germination of biennial wormwood and lanceleaf sage seeds. Stratified biennial wormwood seed germination was 95% or greater when incubated in fluctuating day/night temperatures of 37/20 or 37/25 C. Stratified lanceleaf sage seeds from freezing and chilling environments did not differ in germination following incubation in fluctuating temperatures and averaged 56 and 55%, respectively. Germination of stratified lanceleaf sage seeds from the freezing and thawing environment was higher than 50% during the thawing cycle, suggesting the possibility of early season emergence of this species. Our study indicates that biennial wormwood and lanceleaf sage have the potential to develop a seedbank that can persist for more than 2 yr. High moisture levels in the soil seedbank can lead to reduced seed survival.

The spread of buffalobur in China poses a serious threat to existing ecosystems, and control and eradication of this species have become increasingly important. Studies were carried out to ascertain the seed production, morphological characterization, dormancy behavior, and methods for breaking dormancy of buffalobur. The results showed that a single buffalobur plant could produce 1,600 to 43,800 seeds with an average weight of 3.0 mg. Average seed length, width, and thickness were 2.5, 2.0, and 1.0 mm, respectively. Newly ripened buffalobur seeds were innately dormant and exhibited combinational dormancy, which involves a hard seed coat (physical dormancy, PY), a partial dormant embryo (physiological dormancy, PD), and a dark requirement to germinate. PY of buffalobur seeds could be broken by dehusking or acid scarification by 14 M H2SO4 for 15 min, with germination rates of 55 or 50%, respectively. PD was effectively broken by KNO3 or gibberellic acid (GA3). The optimum concentration for KNO3 was between 20 and 40 mM, which resulted in over 70% seed germination. When presoaked with GA3 at 30 C in dark for 24 h, maximum germination (> 98%) was obtained at 2.4 mM, the corresponding germination speed (85%) and germination index (16) were also highest at this concentration. Synergistic effects were observed in seed germination when H2SO4 and GA3 were combined. The most rapid and effective combination in breaking dormancy was when the seeds were immersed in H2SO4 (14 M) for 20 min and presoaked with 2.4 mM GA3 for 24 h. Germination index for this combination was over 35, and 95% of the seeds germinated within 7 d. Knowledge gained in this study will be useful in increasing germination of buffalobur and facilitating further laboratory studies.

Experiments were conducted in 2006 to 2008 to study growth, phenology, and competitive ability of glyphosate-resistant (GR) and -susceptible (GS) biotypes of horseweeds from San Joaquin Valley (SJV), CA. When grown alone, in pots, the GR horseweeds consistently developed more rapidly than the GS weeds, as evidenced by their earlier bolting, flowering, and seed set; the GR horseweeds set seeds nearly 25 d (approximately 190 fewer growing degree days) sooner than the GS horseweed. At seed set, the relatively slow-developing GS horseweeds had amassed 40% more shoot dry matter than the GR weeds at the same phenological stage, but neither biotype was consistently more fecund than the other. Although the GR biotype had lower shoot dry mass than the GS biotype when grown alone, in mixed populations under increasing levels of competition (in a replacement series design) and limited resources (mainly moisture), the GR weeds were not only taller, but also accumulated more dry matter than the GS weeds. Thus, the GR biotype was more competitive than the GS biotype, particularly when grown at high densities and under moisture-deficit stress. Therefore, under California conditions there is no apparent fitness penalty for this particular GR horseweed biotype, and it is likely to persist in the environment and outcompete the GS biotypes regardless of further glyphosate selection pressure. If so, this biotype of GR horseweed is likely to become increasingly common in the SJV until effective management strategies are developed and adopted.

Much research has been conducted on mesotrione activity on crops and weeds, but information is lacking in regards to the relative contribution of soil and foliar absorption of mesotrione. Three experiments conducted at Virginia Tech's Glade Road Research Facility in Blacksburg, VA, evaluated the effects of 50 and 90% relative humidity (RH) on the activity of mesotrione applied to foliage, soil, and soil plus foliage. Tall fescue injury ranged from 0 to 21% and was significant in 6 of 20 comparisons. Three of these injury events were caused by soil plus foliar applications, which were always more injurious than foliar only applications, which were more injurious than soil-only applications. Both application placement and RH significantly influenced smooth crabgrass responses to mesotrione. Smooth crabgrass phytotoxicity was lowest when mesotrione was applied only to foliage and highest when mesotrione was applied to soil and foliage. Increasing RH from 50 to 90% caused a 4- to 18-fold increase in plant phytotoxicity when mesotrione was applied only to foliage. By dissecting the plant canopy, it was noted at 14 d after treatment, when averaged over RH, that white leaves comprise 16% of leaves when only foliage was treated and 55 and 62% when applied to soil plus foliage and soil only, respectively. Furthermore, white tissue was found predominately in the two youngest leaves when mesotrione was applied to soil or both soil and foliage, but in older leaves when applied only to foliage. Data indicate mesotrione entering plants through soil travels quickly to growing points and has an equal or greater effect on plant phytotoxicity than foliar-absorbed mesotrione. In addition, foliar-absorbed mesotrione appears to increase in plants significantly with increasing RH, but does not move rapidly to growing points.

Experiments were conducted on Palmer amaranth seeds collected in 2004 and 2006 from a natural population near Pendleton, SC, to determine the temperature and light requirements for germination of seeds retrieved from soil surface or from 10-cm depth in the field. A cyclic change in seed germination of Palmer amaranth in response to temperature and light occurred during a 12-mo after-ripening period. Freshly matured seeds collected in November required mean temperatures ≥ 25 C, and natural or red (R) light for increased germination. Following after-ripening in winter, seeds experienced a reduction in dormancy and germinated higher at 25 to 35 C mean compared with 10 to 15 C mean. With after-ripening for an additional 3 mo in May, seeds experienced a broadening of thermal range (10 to 40 C mean), and germination in natural light or R light was more than twice the germination in the absence of light. Fluctuating temperatures (7.5 C amplitude) improved germination over constant temperatures, except in summer and fall (9 and 12 mo after seed maturation). Exposure of seeds to high temperatures during summer caused secondary dormancy induction. Averaged over thermal amplitudes, seeds retrieved in fall required mean temperatures > 25 C for increased germination. Burial in spring for 3 to 6 mo induced seed dormancy, and the relative germination in fall (12 mo after seed maturation) was at least 50% higher for seeds retrieved from soil surface compared to seeds exhumed from 10-cm soil depth. Seeds retrieved in late summer and fall required natural light or R light for promoting germination, whereas far-red (FR) light or darkness inhibited germination. Furthermore, the effect of R and FR light was reversible, indicating a partially phytochrome-mediated germination response of Palmer amaranth seeds following 9 to 12 mo of after-ripening in the field.

Seeding date and the duration of weed emergence influenced the duration ofthe critical weed-free period in carrot. The critical weed-free periodextended up to 930 growing degree days (GDD), when carrot was seeded in lateApril. In contrast, the critical weed-free period was short and lasted 414to 444 GDD, when seeded in mid to late May and weed biomass was less than650 g m−2. It is important for growers to scout fields for weedsuntil 930 GDD to protect the yield potential of the carrot crop in earlierplanted crops; however, for carrot planted in mid to late May, weedsemerging after 444 GDD did not reduce yield. A useful strategy to reducereliance on herbicide application would be to delay planting until late inMay.