Guy B. Kyser

Control of Invasive Weeds with Prescribed Burning

Prescribed burning has primarily been used as a tool for the control of invasive late-season annual broadleaf and grass species, particularly yellow starthistle, medusahead, barb goatgrass, and several bromes. However, timely burning of a few invasive biennial broadleaves (e.g., sweetclover and garlic mustard), perennial grasses (e.g., bluegrasses and smooth brome), and woody species (e.g., brooms and Chinese tallow tree) also has been successful. In many cases, the effectiveness of prescribed burning can be enhanced when incorporated into an integrated vegetation management program. Although there are some excellent examples of successful use of prescribed burning for the control of invasive species, a limited number of species have been evaluated. In addition, few studies have measured the impact of prescribed burning on the long-term changes in plant communities, impacts to endangered plant species, effects on wildlife and insect populations, and alterations in soil biology, including nutrition, mycorrhizae, and hydrology. In this review, we evaluate the current state of knowledge on prescribed burning as a tool for invasive weed management. Nomenclature: Barb goatgrass, Aegilops triuncialis L. #3 AEGTR; Canada bluegrass, Poa compressa L. # POACO; Chinese tallow tree, Sapium sebiferum (L.) Roxb. # SAQSE; downy brome, Bromus tectorum L. # BROTE; French broom, Genista monspessulana (L.) L. Johnson # TLNMO; garlic mustard, Alliaria petiolata Andrz. # ALAPE; Kentucky bluegrass, Poa pratensis L. # POAPR; medusahead, Taeniatherum caput-medusae (L.) Nevski; red brome, Bromus madritensis L. ssp. rubens (L.) Husnot # BRORU; ripgut brome, Bromus diandrus Roth # BRODI; Scotch broom, Cytisus scoparius (L.) Link # SAOSC; smooth brome, Bromus inermis Leysser # BROIN; sweetclover, Melilotus spp.; yellow starthistle, Centaurea solstitialis L. # CENSO. Additional index words: Fire, integrated vegetation management, rangelands, wildlands.

Control of Medusahead (Taeniatherum caput-medusae) and other Annual Grasses with Imazapic

Invasive annual grasses, such as medusahead, can reduce forage production capacity and interfere with revegetation projects in California rangelands. Because of the taxonomic similarity to other more desirable grasses, achieving selective control of invasive annual grasses can be difficult. In selectivity trials conducted in Yolo and Siskiyou counties, CA, the herbicide imazapic gave control of many nonnative annual grasses yet provided some level of selectivity to specific perennial grasses used in revegetation projects throughout the western United States. The selectivity difference between newly seeded perennial and annual grasses was greater with PRE applications than with POST treatments. Both perennial and annual grasses within the tribe Hordeae were more tolerant to imazapic than other grass species. In addition, field experiments were conducted at three sites in northern California (Yuba, Yolo, and Lassen counties) and one in southern Oregon (Lake County) to test the response of imazapic to varying management conditions. Imazapic was applied PRE in fall (and also spring in Lake County) at rates from 35 to 210 g/ha on undisturbed rangeland, in comparison with rangeland cleared of standing plant material and thatch by either tillage, mowing and raking, or burning. Imazapic generally showed enhanced weed control when applied following disturbance. Rates as low as 70 g/ha, if combined with thatch removal, provided significant suppression of medusahead. In addition, disturbance alone generally reduced medusahead cover in the following year. Although imazapic showed potential for control of medusahead and other annual grasses, its selectivity window was relatively narrow. Nomenclature: Imazapic, medusahead, Taeniatherum caput-medusae (L.) Nevski, ELYCM

Reproductive biology of yellow starthistle: maximizing late-season control

Abstract Field studies at three sites and growth chamber experiments were conducted to determine the reproductive potential, flower phenology, seed viability and germination, and overall seedbank longevity of yellow starthistle in the Central Valley of California. At the three study sites, seedheads contained an average of between 65 and 83 achenes. Overall, 85% of the achenes were the interior pappus-bearing type, and the remaining 15% were the outer nonpappus-bearing type. Germinable seed did not initially develop until the late corolla senescence stage 8 d after flower initiation. Seed germination and viability 1 wk after dispersal were similar (86 and 91%, respectively). Comparison in flower phenology in 1996 and 1997 indicated that development from initial anthesis to achene dispersal more closely corresponded to days, rather than thermal units. In the field, germinable seed was produced when more than 2% of the total seedheads had initiated anthesis. To minimize seed production with late-season control methods, such as prescribed burning, mowing, or herbicide treatment, management strategies should be timed before the plant population has advanced beyond the 2% flower initiation stage. Over 84% of the seed germinated under growth chamber conditions 1 wk after seedheads reached the dispersal stage. This indicates that most yellow starthistle seed had little or no after-ripening requirements. In a field experiment, yellow starthistle seed germination corresponded to seasonal rainfall. A total of 44 and 39% of the pappus-bearing and nonpappus-bearing seed, respectively, germinated after one growing season. Of seed recovered from the soil after the first growing season, 88 and 81% of the pappus-bearing and nonpappus-bearing seed, respectively, was either damaged or degraded. From projected values based on recovered and germinated seed, it was estimated that over 97% of the total seed was removed from the soil seedbank after two growing seasons. These findings should assist land managers in developing long-term yellow starthistle management strategies. Nomenclature: Yellow starthistle, Centaurea solstitialis L. CENSO.

Miscanthus × giganteus and Arundo donax shoot and rhizome tolerance of extreme moisture stress.

Crops grown for bioenergy production are a mandated component of the United States energy portfolio. Giant miscanthus (Miscanthus × giganteus) is a leading bioenergy crop similar in habit to the invasive plant giant reed (Arundo donax). To characterize the environmental tolerance of giant miscanthus, we compared the soil moisture stress tolerance of giant miscanthus and giant reed under glasshouse conditions. We subjected both species to soil moisture conditions of severe drought (−4.2 MPa), mild drought (−0.5 MPa), field‐capacity (control), and flooded soils. These conditions were applied to two cohorts: one in which soil moisture conditions were imposed on newly planted rhizome fragments, and one in which conditions were imposed on established plants after 8 weeks of growth in field‐capacity soil. After 16 weeks, we harvested all plants, measured above‐ and belowground biomass, and evaluated the reproductive viability of rhizome fragments. The total biomass of each species under flooded conditions was not different from the field‐capacity control groups regardless of cohort. However, drought did affect the two cohorts differently. In the cohort treated after 8 weeks of growth, mild and severe drought conditions resulted in 56% and 66% reductions in biomass, averaged over both species, compared with the controls. In the cohort treated for the entire 16 weeks, mild and severe drought conditions resulted in 92% and 94% reductions in biomass. Rhizome fragments from both species and both cohorts showed 100% viability following flooded and control treatments; drought treatments reduced rhizome viability in both species, with a greater impact on giant miscanthus. Although giant miscanthus does not appear to have the potential to escape and establish in relatively dry upland ecosystems, it does show tolerance to flooded conditions similar to giant reed.

Effect of light and density on yellow starthistle (Centaurea solstitialis) root growth and soil moisture use

Abstract Yellow starthistle root growth was measured under field conditions using a minirhizotron camera system. Roots grew 1.0 to 1.3 cm d−1 and quickly reached the bottom of the 100-cm tube. When plants were grown under 80 and 92% shading, roots grew 45 and 64% slower, respectively, than when plants were grown unshaded. Using a neutron probe, we evaluated the effect of yellow starthistle density on soil moisture depletion to 180 cm in the soil profile. At the end of the growing season, we also measured aboveground biomass and seedhead production. Total plant dry weight and seedhead number at the lowest density (0.6 plants m−2) were 43 and 97% of the maximum values, respectively, suggesting that yellow starthistle is capable of reaching near-maximum yield at low densities. Moisture depletion was density dependent, and plants in low-density plots used more soil moisture from deep in the profile than from shallow soil (30 cm) early in the season. In contrast, yellow starthistle densities > 90 plants m−2 rapidly depleted moisture from all depths in the soil profile by preflowering growth stages. High yellow starthistle density expands the moisture depletion zone and leads to increased shallow moisture depletion. In high-density plots, soil moisture did not recharge, compared with bare-ground plots, after subnormal winter and spring precipitation. These results illustrate the importance of reducing yellow starthistle densities in grassland restoration efforts, where shallow soil moisture is critical to the establishment of seeded perennial grasses or annual forbs and where moderate to deep soil moisture is essential for the establishment and survival of transplanted shrubs and trees. Nomenclature: Yellow starthistle, Centaurea solstitialis L. CENSO.

Integrating prescribed burning and clopyralid for the management of yellow starthistle (Centaurea solstitialis)

Abstract Prescribed burning and the herbicide clopyralid are very effective tools for the management of yellow starthistle. However, repeated use of either can be impractical or can present other problems. The potential solution is the development of an effective integrated weed management strategy using a combination of the two approaches. In small plot studies (0.2 ha), we tested one of five possible treatments: (1) untreated control, (2) 2 consecutive yr of clopyralid (0.105 kg ha−1), (3) 2 consecutive yr of prescribed summer burning, (4) first-year clopyralid followed by second-year prescribed burning, and (5) first-year prescribed burning followed by second-year clopyralid. Treatments were made in 1999 and 2000 at three study sites in California (San Benito, Yuba, and Siskiyou counties). In 2001, the year following the final treatment, 2 consecutive yr of clopyralid or first-year burning followed by second-year clopyralid consistently reduced yellow starthistle cover in the following year by 92 to 100%. However, at the Yuba site, clopyralid alone increased medusahead and ripgut brome cover. Although 2 consecutive yr of burning was effective in Yuba, very high levels of starthistle infestation in San Benito were not completely burned in the second year because of the lack of available consumable fuel. Clopyralid treatment the first year followed by prescribed burning in the second year stimulated yellow starthistle germination and did not reduce the infestation. In a large-scale study conducted at two sites (13 and 81 ha) in southern Monterey County, we used a first-year burn followed by either 2 yr of clopyralid (0.158 kg ha−1) or a single year of clopyralid (0.210 kg ha−1) and a subsequent burn. Results were in close agreement with those found in the small-scale studies. In the year following the final treatment, control of yellow starthistle was greater than 99% when the burn was followed by 2 yr of clopyralid. In contrast, when a prescribed burn was used in the last year of the program, the level of control was not as good, probably because of the increased germination of the remaining soil seedbank. These results indicate that a first-year prescribed burn followed by a second-year clopyralid treatment can provide consistently good control of yellow starthistle, as well as reduced levels of noxious annual grasses, including medusahead and ripgut brome. Nomenclature: Medusahead, Taeniatherum caput-medusae (L.) Nevski ELYCM; ripgut brome, Bromus diandrus Roth BRODI; yellow starthistle, Centaurea solstitialis L. CENSO.

Instability in a grassland community after the control of yellow starthistle (Centaurea solstitialis) with prescribed burning

Abstract An open grassland at Sugarloaf Ridge State Park, Sonoma County, CA, was burned during three consecutive summers (1993–1995) to control yellow starthistle. By 1996, the yellow starthistle seedbank, seedling density, and mature vegetative cover were reduced by 99, 99, and 91%, respectively, and the plant community had greater diversity and species richness, particularly of native forbs. After the cessation of the prescribed burning after 1995, the community was monitored for 4 yr to determine if the reduced yellow starthistle population represented a stable state or if the population would quickly recover. The yellow starthistle seedbank rose dramatically over 4 yr. Seedling counts and summer vegetative cover also rose, though less rapidly. Total forb cover, particularly native species, total plant cover, and plant diversity decreased significantly after cessation of the burning. Grass cover did not show any strong trends, and year-to-year variation in the grass cover appeared to be more important than the treatment effects. In the absence of some overall changes in management, e.g., periodic prescribed burning, herbicide treatments, or revegetation, it may not be possible to establish and maintain a stable state with a low population of yellow starthistle in annual grasslands in California. Nomenclature: Yellow starthistle, Centaurea solstitialis L. CENSO.

Control of Medusahead (Taeniatherum caput-medusae) Using Timely Sheep Grazing

Abstract Medusahead is among the most invasive grasses in the western United States. Selective control of this noxious winter annual grass is difficult in California grasslands, as many other desirable annual grasses and both native and nonnative broadleaf forbs are also important components of the rangeland system. Intensive grazing management using sheep is one control option. This study was designed to determine the optimal timing for sheep grazing on heavily infested medusahead sites, and to evaluate the changes in species composition with different grazing regimes. Midspring (April/May) grazing reduced medusahead cover by 86 to 100% relative to ungrazed plots, regardless of whether it was used in combination with early spring or fall grazing. Early spring (March) or fall (October to November) grazing, alone or in combination, was ineffective for control of medusahead. In addition, midspring grazing increased forb cover, native forb species richness, and overall plant diversity. At the midspring grazing timing, medusahead was in the “boot” stage, just prior to exposure of the inflorescences. The success of this timely grazing system required high animal densities for short periods. Although this approach may be effective in some areas, the timing window is fairly narrow and the animal stocking rates are high. Thus, sheep grazing is unlikely to be a practical solution for management of large medusahead infestations. Nomenclature:Medusahead, Taeniatherum caput-medusae (L.) Nevski ELYCA.

Herbicide-Assisted Restoration of Great Basin Sagebrush Steppe Infested With Medusahead and Downy Brome

Abstract Downy brome or cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae) are the most problematic invasive annual grasses in rangelands of the western United States, including sagebrush communities that provide habitat to sage grouse. Rehabilitation of infested sites requires effective weed control strategies combined with seeding of native plants or desirable competitive species. In this study, we evaluated the effect of three fall-applied pre-emergence herbicides (imazapic, rimsulfuron, and chlorsulfuron + sulfometuron), and one spring-applied postemergence herbicide (glyphosate) on the control of downy brome and medusahead and the response of seeded perennial species and resident vegetation in two sagebrush communities in northeastern California. All pre-emergence treatments gave > 93% control of both invasive species at both sites in the first year. Glyphosate was less consistent, giving > 94% control at one site and only 61% control of both species at the other site. Imazapic was the only herbicide to maintain good control (78–88%) of both species 2 yr after treatment. No herbicide caused detectible long-term damage to either perennial grasses or annual forbs, and imazapic treatment resulted in an increase in resident native forb cover 3 yr after treatment. Broadcast seeding with or without soil incorporation did not result in successful establishment of perennial species, probably due to below-average precipitation in the year of seeding. These results indicate that several chemical options can give short-term control of downy brome and medusahead. Over the course of the study, imazapic provided the best management of both invasive annual grasses while increasing native forb cover.

Site Characteristics Determine the Success of Prescribed Burning for Medusahead (Taeniatherum caput-medusae) Control

Abstract Medusahead is one of the most problematic rangeland weeds in the western United States. In previous studies, prescribed burning has been used successfully to control medusahead in some situations, but burning has failed in other circumstances. In this study, trials were conducted using the same protocol at four locations in central to northern California to evaluate plant community response to two consecutive years of summer burning and to determine the conditions resulting in successful medusahead control. During 2002 through 2003 large-scale experiments were established at two low-elevation, warm-winter sites (Fresno and Yolo counties) and two higher elevation, cool-winter sites (Siskiyou and Modoc counties). Plant species cover was estimated using point-intercept transects, and biomass samples were taken in each plot. After 2 yr of burning, medusahead cover was reduced by 99, 96, and 93% for Fresno, Yolo, and Siskiyou counties, respectively, compared to unburned control plots. Other annual grasses were also reduced, but less severely, and broadleaf species increased at all three sites. In contrast, 2 yr of burning resulted in a 55% increase in medusahead at the coolest winter site in Modoc County. In the second season after the final burn, medusahead cover remained low in burned plots at Fresno and Yolo counties (1 and 12% of cover in unburned controls, respectively), but at the Siskiyou site medusahead recovered to 45% relative to untreated controls. The success of prescribed burning was correlated with biomass of annual grasses, excluding medusahead, preceding a burn treatment. It is hypothesized that greater production of combustible forage resulted in increased fire intensity and greater seed mortality in exposed inflorescences. These results demonstrate that burning can be an effective control strategy for medusahead in low elevation, warm-winter areas characterized by high annual grass biomass production, but may not be successful in semiarid cool winter areas. Nomenclature: Medusahead, Taeniatherum caput-medusae (L.) Nevski