Russian knapweed (Acroptilon repens) is a non-native perennial forb (wild flower) that arrived in the United States in the late 1890s. This weed is well adapted, growing in damp to poorly drained soils with high salinity and/or alkalinity. In the year 2000, Russian knapweed infested about 1.2 million acres across the 17 Western states. This included about 425,000 acres in Idaho, 85,000 acres in Oregon, 60,000 acres in Utah, 500,000 acres in Washington and 64,000 acres in Montana. Russian knapweed probably affects well over 10,000 acres in Nevada, but the exact figure is unknown.
Individual infestations in Nevada often are quite large and occur in native grass-hay meadows, riparian areas, rangelands, agronomic crops, field borders and fence lines, canal and ditch banks, roadsides and other linear corridors used to transport commerce and energy, and waste areas. In addition to displacing more desired forage and crop species, even small contamination in grains (0.01%) can reduce flour quality.
Twenty-three percent of agricultural producers in Nevada rated Russian knapweed as a serious problem. At least 14% of all agricultural producers in every county rated the weed as problematic. Those in the following counties rated Russian knapweed even more problematic: Pershing (61% of producers); Humboldt (45%); Lander and Eureka (40%); Churchill (26%); Douglas and Carson City (25%); Nye, Esmeralda and Mineral (24%); and White Pine (20%). Generally, the north-central and central regions of the state consider the weed most problematic. This coincides with counties that have the greatest amount of agronomic cropland, as well as large expanses of grass-hay meadows. Just over 53% of the public land managers in Nevada rated Russian knapweed as a problematic weed on public lands.
Russian knapweed becomes problematic when a few initial plants rapidly expand their population and establish a nearly complete monoculture. Dense stands can persist for over 75 years, and the typical outcome is reduced crop and livestock production on the affected property. The yield of grain and corn crops has declined as much as 75% to 85%. Desired species are excluded because mature Russian knapweed plants are excellent competitors for soil moisture and nutrients and may have allelopathic properties that inhibit the growth of many desired plants.
Russian knapweed is a long-lived, relatively shade-intolerant plant, with a maximum height of about 3 feet. Widespread reproduction from seed seldom occurs, but is possible. Most populations typically increase from vegetative reproduction by rapidly expanding lateral roots. The roots of Russian knapweed may reach depths of 8 feet their first growing season and 23 feet their second growing season. Their radial spread can cover as much as 130 square feet after two growing seasons.
Every one-inch root segment may have a bud capable of producing a new shoot (Figure 1), and root segments as short as 1 inch have produced new shoots when buried as deep as 6 inches. Tillage that cuts the root system into many small segments typically creates hundreds of new plants.
The buds on the root crown and the creeping roots tend to grow during late fall through March. This growth feature tends to make Russian knapweed susceptible to soil-active herbicides from late fall through the dormant season, as long as the soil is not frozen and there is enough soil moisture to move the herbicide into the root zone, where the active ingredient can be absorbed by the buds.
Buds on the root crown and lateral roots start to develop shoots in March and April, when the ground thaws and soil temperature remains above freezing. The new shoot initially develops a basal rosette of leaves, followed in May and June by a bolting floral stem. Most of the carbohydrates produced by the leaves during this period are reinvested in additional stems and leaves, and not moved to the root system. Flowers develop shortly thereafter, and flowering may continue all summer if soil moisture is adequate for continued growth.
The roots of Russian knapweed store a large amount of soluble carbohydrates as energy reserves. Plants use the energy reserves to keep their roots and buds alive during winter dormancy and to initiate new growth the following spring. Stored energy reserves typically peak at the end of the growing season and are smallest when Russian knapweed initially flowers in the spring or early summer. Russian knapweed moves more carbohydrates to the root crown and the creeping roots at full flowering, with continued movement to the roots through the late summer and fall, as the plants slowly senesce.
A single Russian knapweed plant can produce about 1,200 seeds, but typical seed production is 100 to 300 seeds per plant. Most of the seed is viable for only two to three years, but a small percentage may remain alive in the soil for about eight years. Seed germination rates are greatest when the soil remains wet for at least seven days, with peak germination requiring about 25 to 32 days of moist soil. These conditions often occur in seasonally flooded hay meadows and irrigated crop sites, particularly beneath pivot systems with short rotation intervals. A layer of soil or plant litter above the seed facilitates germination. The initial establishment of Russian knapweed occurs most often in disturbed areas where the desired perennial vegetation (or crop) has been thinned or lost due to disturbance or improper vegetation management.
The flowers and seed of Russian knapweed lack adaptations for long-distance dispersal. Flooding, however, can transport the seed or dislodged root segments long distances. Mud that contains viable seed can move that seed long distances, in a short period of time, when it becomes attached to animals, vehicles, farm equipment or even your boots. Construction activities that transport fill dirt to new locations can break large roots into small segments and facilitate establishment long distances away in previously uninhabited areas.
A single treatment that severs Russian knapweed’s roots does not kill the plant and typically increases the number of new plants. However, one study in Russia found that multiple cuttings of the roots to at least 12 inches deep, over a three-year period, destroyed the root system in the top 3 feet of the soil. Root fragments up to 16 inches long showed high mortality when buried at least 12 inches deep. This suggests possible control with repeated deep plowing.
Root carbohydrate reserves are lowest when Russian knapweed begins to flower. A deep plowing treatment that coincides with flower initiation should have a greater chance of success than deep plowing later in the season, when stored energy reserves are larger.
Shallow tillage usually enhances an existing infestation, because new shoots readily emerge from short root segments (1 to 2 inches long) that are buried 6 inches deep, or shallower. Shallow tillage can effectively control young seedlings that have not yet developed buds (i.e., become perennial). This treatment should be successful where seedlings of Russian knapweed have recently emerged in fallow fields or in fields where an annual crop was harvested early in the growing season.
Infrequent or single-mowing events generally stimulate Russian knapweed to produce new shoots from the buds located on the root crown and the creeping roots. Research in a grain field in Russia that was infested with Russian knapweed found that when the crop was harvested for silage for four consecutive years, the control of Russian knapweed reached 99%. Mowing the grain crop at a relatively early growth stage coincided with the flowering stage of the Russian knapweed, which was when plants had their lowest stored energy reserves. Repeated harvest when energy reserves were lowest slowly depleted the plants’ energy reserves, and they essentially starved to death.
Fire is not recommended as a direct control method. Burning eliminates the top growth of Russian knapweed, but does not kill the buds on either the root crown or the roots. Removal of the shoots may stimulate development of a large number of buds and increase Russian knapweed abundance, particularly if the fire also removes any overstory plants that shade the knapweed. Russian knapweed grows very well in high-sunlight environments, and any additional sunlight after an unsuccessful control treatment probably benefits the Russian knapweed.
Flaming or other heat treatments that kill the top growth can control current seed production if the plants are treated at flowering or the early stage of seed formation. After a flaming treatment, regrowth of new shoots from the buds is possible if soil moisture is adequate; therefore, several treatments per season may be necessary. The initial regrowth after flaming, however, comes from stored energy in the roots. The plant uses the stored energy for growth until the bud to early flowering stage. Application of a systemic herbicide at this time may result in better control than an herbicide-only treatment because stored energy reserves in the root are low, which may increase bud mortality. Burning the standing dead material can remove physical barriers that reduce herbicide placement on the leaves or soil surface. This should place more of the active ingredient on the leaf surface or the soil (for soil-active herbicides), which should improve treatment efficacy. Burning can also be a valuable seedbed preparation treatment when it removes plant litter that could adversely affect the seeding of desired species.
Grazing treatments generally do not work well when they are a stand-alone management tool, and at best are only one component of an integrated weed management program. Cattle typically avoid the weed due to its bitter taste, unless it is the only forage available. Cattle forced to consume large quantities of Russian knapweed, particularly if it is a novel forage for them, are likely to be less productive, due to less forage intake. Russian knapweed’s high protein content may permit its use as a protein supplement when cattle are on low-quality forage at the mid-gestation stage. Sheep and goats will graze Russian knapweed more readily than cattle, and may provide control if the plants are heavily grazed three or more times throughout the growing season, for at least three consecutive years. (Longer treatment periods often are necessary.)
Within a single grazing season, the best control of Russian knapweed occurs when livestock can regraze the weed when shoots reach 8 to 10 inches tall. When residual perennial grasses are present, grazing treatments should allow residual grasses to increase in density, biomass and vigor, so the desired vegetation can fully occupy the site and competitively exclude the Russian knapweed. Russian knapweed in large quantities (60% of body weight over two months) is toxic to horses, and horses should not be placed in pastures that have a large population of Russian knapweed.
To achieve a permanent decline in Russian knapweed, treated sites must establish a dense and vigorous stand of desired vegetation. For most pasture and rangeland settings, this means a dense stand of tall and robust perennial grasses. Annual or perennial crops also must be managed to maintain a high density and cover of the crop. Annual crops that are harvested early on sites that will have sufficient soil moisture for continued growth of the Russian knapweed, usually will need a postharvest treatment (herbicide or other tools) to reduce the knapweed. The vegetation/crop management goal is twofold: 1) provide at least partial shade of the Russian knapweed; and 2) for the crop or pasture plants to have a large, robust root system to extract more soil moisture and soil nutrients than are consumed by the knapweed. For many range and pasture systems, the widespread establishment of Russian knapweed coincided with one or more management actions that either thinned the perennial grasses or maintained previously thinned stands in a degraded state (regardless of the initial cause of the degradation). On these sites, vegetation management (e.g., grazing management, harvest management, fertilization, etc.) probably must change to provide perennial grasses an opportunity to increase and eventually outcompete Russian knapweed. Without a change in management, the weed will probably return and expand toward and possibly exceed its previous level.
There is no effective biocontrol of Russian knapweed at this time. A number of insects have been approved for release, but their establishment generally has been poor and there are few, if any, documented cases of successful treatment. The saprophytic fungus, Boeremia exigua isolate FDWSRU 02-059, may cause substantial damage (not necessarily mortality) to Russian knapweed plants, but further study is needed. At best, these biological agents stress the Russian knapweed plants but do not effectively reduce existing populations when they are the only treatment applied. There has been little, if any, research on the influence of biological control agents when their application occurs sequentially or in conjunction with other treatments.
There are about a dozen active ingredients labeled for application on Russian knapweed (Table 1). Most are labeled for range, pasture and noncrop sites; the environmental setting in Nevada where large infestations typically occur. The majority of labeled herbicides are selective, causing little or no damage to desired perennial grasses when applications occur according to instructions on the product label. None of the active ingredients, however, are safe to apply to broadleaf crops when they are actively growing. Many should not be applied when the crops are dormant because the active ingredients have a long period of soil activity.
Soil-active herbicides with a long period of soil activity are important for the control and management of Russian knapweed. These chemicals can effectively kill buds that form in the fall on Russian knapweed’s roots and root crown. They are effective after the plants become completely senescent, as long as the soil remains unfrozen and moist.
Research completed several years ago in Paradise Valley, Nevada, found that completely senescent plants treated the last week of November (Figure 2) resulted in 90% control with aminocyclopyrachlor (specific products were not named at the time of this work) and almost 80% control with 7 ounces of Milestone (aminopyralid), eleven months after treatment. Many soil-active herbicides are still active until spring and continue to kill viable buds and roots. Although numerous herbicides can be treatments completed after fall senescence begins, through complete dormancy, generally are more effective.
The movement of a foliar-applied herbicide, to and then through the large root system, largely follows the movement of carbohydrates from the leaves to the rest of the plant. For Russian knapweed, the plant typically moves more carbohydrates to the root crown and roots in the late summer to early fall, than in the spring through the flowering period. For a foliar herbicide treatment to be effective, however, the leaves must be actively photosynthesizing, which requires adequate soil moisture. The mere presence of green leaves in late summer or early fall does not guarantee the plant is photosynthesizing and moving carbohydrates to the roots. Herbicide applications to green plants under dry soil conditions typically are much less successful than when soil is moist.
It is important to have good to excellent growing conditions at the time of herbicide application and for a couple of weeks thereafter. The ideal conditions include soil that is moist (not saturated) and warm air temperatures, where moisture flows easily from the soil through the plant. Only about 10% of picloram (Tordon®) applied to leaf surface is taken up by the leaves, and most uptake occurs within 30 minutes of application. High photosynthetic rate at time of application is important for maximum uptake and treatment success. Furthermore, only about 10% of absorbed Picloram is translocated out of leaf within four days of application, with about half moving toward the roots, and half toward the shoots. As growing conditions decline shortly after herbicide application, control is likely to decline. All herbicide applications should use a surfactant to improve chemical uptake. Consult the label of the product you use to identify the specific type of surfactant best suited for that herbicide.
There is no single active ingredient listed in Table 1 that is the best herbicide for all Russian knapweed infestations. Some factors to consider are: 1) do you need an herbicide that is selective and not going to adversely affect the residual desired species that occupy the site?; 2) are your short-and mid-term management objectives compatible with a chemical that leaves a residual amount of the active ingredient in the soil?; 3) what will Russian knapweed’s growth stage(s) be when you have the time to fit an herbicide treatment into your overall farming or ranching operation?; and 4) can you make the commitment to any follow-up treatment that is needed? Glyphosate-based herbicides and 2,4-D typically result in less long-term control than the other active ingredients listed in Table 1. Treatment of Russian knapweed with these two chemicals is more likely to need one or more follow-up applications.
An important question of any herbicide treatment is, was I successful? The level of success cannot be determined until the middle of the first growing season after application of the treatment, and perhaps even later. Figures 3a and 3b show a field that was nearly a complete monoculture of Russian knapweed when it was treated with Milestone (aminopyralid) the previous November, after the plants were dormant. The following year, in early July there were no Russian knapweed plants in the treated area, and knapweed growth in the untreated area was 12 to 18 inches tall (Figure 3a). By mid-August, numerous Russian knapweed plants had emerged on the treated area (Figure 3b). Without a follow-up treatment, these relatively few plants will expand and the site eventually will become re-infested with Russian knapweed, or some other weed. The knapweed plants emerged in the treated area about four months after plants in the untreated area, because the treatment killed most of the buds on the shallow roots, but not all the buds on the deeper roots. It took about four months for the surviving plants to move stored energy to the surviving buds and to grow a shoot through the soil until it emerged aboveground. The effectiveness of an herbicide treatment on any weed that has a deep root system with buds that can grow into new plants should not be judged too soon after treatment. The full effect, or lack thereof, of an herbicide treatment may take a year or more to appear.
For the complete document, use the link below to download the PDF version.
B. Schultz, S. Foster, 2021, Nevada’s Priority Agricultural Weeds: Russian Knapweed, Extension, University of Nevada, Reno, SP-21-02
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