Amaranthus palmeri decreased the yield of no. 1 and jumbo grades at all densities greater than 1 plant m−1 row when compared with weed-free sweetpotato yields . Similarly, D. sanguinalis at 1 plant m−1 row decreased the weight of sweetpotato jumbo grade when compared with the weed-free check . Digitaria sanguinalis densities greater than 2 plants m−1 row decreased no. 1 grade sweetpotato yield relative to the weed-free check, with 16 plants m−1 causing the greatest loss of no. 1 and jumbo grades. These findings further demonstrate the negative impact of A. palmeri and D. sanguinalis on sweetpotato yields at low weed densities. Interspecific competition is also reflected in biomass reduction of one or both plant species competing with each other Interactions between year, crop versus no crop, and weed density were not significant ; therefore, means pooled over years were obtained for density and crop versus no crop combinations for each weed species. Biomass per meter of row of A. palmeri and D. sanguinalis increased with increasing weed density . The presence of sweetpotato reduced overall biomass per meter of row for both weed species at densities of 1, 2, and 4 plants m−1 row. Furthermore, sweetpotato reduced the rate of bio-accumulation for D. sanguinalis, as can be seen when comparing the slopes of biomass accumulation of both weeds . We believe that this was an effect of weed height, as A. palmeri quickly establishes and reduces the light reaching the sweetpotato canopy, vertical farming racks whereas D. sanguinalis does not exceed the sweetpotato canopy height as quickly as A. palmeri and is therefore less competitive with sweetpotato for light.
The impact of A. palmeri on light interception with the sweetpotato canopy has been documented by others . Individual weed biomass of A. palmeri and D. sanguinalis was similar across weed densities when grown with sweetpotato . Individual weed biomass for A. palmeri and D. sanguinalis, however, was lower for all weed densities when grown in the presence of sweetpotato compared with weeds grown without sweetpotato. The reduced individual biomass and biomass per meter of row for both weeds, when grown with sweetpotato, indicate that interspecific interference is occurring between sweetpotato and weeds. Crop biomass reductions are generally associated with increased weed competition and yield losses . However, in this study, although weed biomass was lower when grown with sweetpotato, increased weed density did not reduce sweetpotato biomass, despite the reduction in sweetpotato yield at the same densities.Individual dry biomass of each weed species growing without sweetpotato decreased as weed density increased . In the absence of sweetpotato, individual dry biomass ofboth weeds was fit to a linear-plateau model. Individual weed biomass was greatest for both weeds at the lowest density. Amaranthus palmeri and D. sanguinalis individual plant biomass decreased 71% from 1 to 3 plants m−1 of row and 62% from 1 to 4 plants m−1 of row, respectively, and remained unchanged at densities above 4 plants m−1 row for both weeds . This finding was similar to the trend observed in peanut for A. palmeri. We believe that the reduction in individual weed biomass for A. palmeri and D. sanguinalis at lower weed densities when grown without sweetpotato is due to increasing intraspecific competition as weed density increases. At the higher densities of both weeds, the impact of intraspecific competition has limited effect on further decreasing individual weed biomass.
The established threshold is the density at which all weeds achieve maximum accumulated biomass before intraspecific competition begins. Further biomass increases would require densities resulting in weed mortality due to intraspecies competition, and such densities were not evaluated in this study. This study demonstrates that A. palmeri and D. sanguinalis have the ability to reduce yield at densities as low as 1 to 2 plants m−1 row. Sweetpotato competes with A. palmeri or D. sanguinalis, resulting in reduced weed biomass. This observation suggests that sweetpotato with rapid canopy establishment and dense growth habit may provide additional competition with weeds and reduce yield loss, as proposed by Harrison and Jackson . Future studies should establish critical weed-free periods for these weeds in sweetpotato, investigate competitiveness of resistant weed biotypes with sweetpotato, and determine weed interference with sweetpotato under varying management practices .Dittrichia graveolens Grueter, commonly known as stinkwort, is a member of the Asteraceae, or sunflower, family. This plant is native to the Mediterranean region of Europe, occurring as far east as Turkey, Afghanistan and Pakistan . Stinkwort is an erect, fall flowering annual that can grow about 2.5 feet tall. Its foliage has sticky glandular hairs covered in resin. The resin emits a strong aromatic odor that resembles the smell of tarweeds. The flower heads are 0.2 to 0.3 inch in diameter and consist of short yellow ray flowers on the outer edge and yellow to reddish disk flowers in the center. Stinkwort is closely related to fleabanes, horseweed , goldenasters and telegraph weed , but it also closely resembles the tarweeds .
From a distance, stinkwort can resemble Russian-thistle , also called tumbleweed. Because it is fairly unattractive and nondescript in appearance, stinkwort initially passed unnoticed by many botanists and weed managers, and it was not included in the 1993 edition of The Jepson Manual of California flora . In its native range and some introduced regions, stinkwort inhabits riparian woodlands, margins of tidal marshes, vernal pools and alluvial floodplains, although it has not yet invaded these wildland areas in California. In California and other introduced areas of the world, stinkwort is most often found in disturbed places, such as overgrazed rangelands, roadsides, pastures, wastelands, vineyard edges, gravel mines, levees, washes and mining sites, although in California it is seldom found in rangelands or pastures . Stinkwort grows best on well drained, sandy or gravelly soils and thrives in areas with hot, dry summers but can also do well along the margins of wetlands. In addition, this plant tolerates a variety of soil types and survives under a range of soil conditions, temperatures and precipitation regimes . When adequate moisture is available, stinkwort can even survive on serpentine or saline soils. In Europe, this plant was shown to tolerate and to possibly hyper accumulate heavy metals, including mercury, zinc and copper .While stinkwort is native to the Mediterranean region, including Egypt and other areas of North Africa, this species has also been introduced to several European countries where it is not native. Within the last two decades, this weed has been spreading rapidly along the highways of Central Europe. In summer 2008, stinkwort was detected for the first time in Slovenia and Austria . Outside of Europe, stinkwort has been reported as an invasive species in Australia and South Africa . Stinkwort is not considered as a palatable species to animals. In fact, it is reported to cause poisoning in livestock . Although livestock mortality is rare, it appears to be due to enteritis caused by the barbed pappus bristles on the seed, which can puncture the small intestine wall . Stinkwort can also cause contact allergic dermatitis in humans . However, impacts to wildlife, natural ecosystems and working landscapes have not been broadly characterized. This is likely due to its very recent introduction and expansion in California and to the lack of published information on the species elsewhere in the world.The first record of Dittrichia graveolens in California is a collection made in 1984 near Milpitas in Santa Clara County . Although the initial mechanism and time of introduction of stinkwort in the state are not documented, many of the earliest collections were made in the south and east San Francisco Bay Area . Stinkwort has since spread to numerous counties in California, and many additional herbarium collections have been made throughout the state . Using the Consortium of California Herbaria records, we determined the rate of stinkwort’s spread since the first discovery in Santa Clara County. Based on collection date and location data from herbarium records, this weed invasion appears to have had only a brief lag period and to have expanded at an exponential rate over the past 18 years .
This has caused increased concern among resource managers across the state. Although it is still uncommon in many places where it is found, stinkwort has been reported in 36 of the 58 California counties . Stinkwort seeds are likely spread by wind, on the fur and feathers of mammals and birds and on motor vehicles and equipment, thus moving along transportation corridors. While the primary expansion has moved radially from the original infestation in Santa Clara County, vertical grow rack unconnected populations have also been discovered in San Diego and Riverside counties . This is likely due to either separate introductions or long-distance movement on vehicles.Stinkwort has very high seed viability, with an average of about 90% of the seeds capable of germination at the time they disperse from the plant. There does not appear to be primary dormancy in the seeds, which is defined as a seed that is dormant at the time it disperses from the plant . These traits, combined with the small seed size, suggest that seed longevity in the soil should be relatively short, perhaps 2 to 3 years. Seeds are capable of germinating at nearly any time of year in the field, but they typically germinate throughout winter and early spring following periods of precipitation. We have shown that germination is limited by soil moisture, rather than soil temperature or low light conditions . When seeds germinate in winter, the plants remain as small rosettes until mid-May. During late spring and summer, they develop into pyramid- or sphere-shaped plants that superficially resemble Russian-thistle. What makes stinkwort’s life cycle rather unusual is that it matures muchlater in the season than most annuals, even other late-season winter annuals . For example, yellow starthistle begins to send up a flowering shoot in April, begins flowering in late June, and — like most late-winter or summer annuals — has completed its life cycle by September or October. In contrast, stinkwort begins to bolt in mid-May , grows most of its branches and leaves between June and September and flowers and produces seeds from September to December. Flowering in stinkwort appears to be controlled by photoperiod , as all plants initiate flowering at the same time regardless of when they germinated . Aside from the tarweeds, there are few other late-season winter annual species with a similar life cycle in the native California flora. Some other weedy species, such as Russian-thistle, horseweed Cronquist and yellow starthistle , have similar life history strategies, with only Russian-thistle and horse weed flowering within the same time frame as stinkwort. In contrast to stinkwort, Russian-thistle is a summer annual that germinates in spring.The environmental and economic impacts of stinkwort in California have not been fully realized and are largely unknown. Our greenhouse studies have shown that stinkwort is dramatically suppressed when grown under shaded conditions, even at 50% light . Thus, like yellow starthistle, stinkwort is not expected to be competitive in understory communities of woodland and forest ecosystems. However, stinkwort can form dense infestations along highways and in open disturbed areas. In addition, while the establishment of this weed in undisturbed wildlands and rangelands is currently very limited in California, invasion of such areas over time is likely based on the pattern of spread in Australia. We are now conducting studies comparing the below ground growth and development of stinkwort with two other common grassland annual species: yellow starthistle and virgate tarweed . Yellow starthistle is an invasive winter annual, and virgate tarweed is a native species that, like stinkwort, is a late-season winter annual. The goal is to determine whether stinkwort shares the characteristics of yellow starthistle and virgate tarweed that allow them to compete with shallow-rooted grasses. These characteristics are a rapid rate of root growth and deep soil root penetration. Initial results indicate that while stinkwort does eventually grow roots as deep as yellow starthistle and virgate tarweed, this occurs several weeks after these other grassland annuals grow their roots. Thus, it may be that stinkwort will not be a significant invasive plant of rangelands, except in years when there is significant late-season rain or when competitive winter annual species are removed by overgrazing. Nevertheless, we have observed stinkwort in open riparian systems, where water is not a limiting factor and a slow-growing shallow root system will not limit its competitive ability.