More info for the terms: competition, cool-season, fire management, forbs, invasive species, natural, phenology, presence, shrubs
Impacts: Cardaria species occur as crop weeds throughout the Middle East, Europe, Australia, and New Zealand. Heart-podded hoary cress has been considered England's most serious weed pest since 1949, and the most serious weed pest for the Wimmera region of Australia ([20] and references therein). Heart-podded hoary cress is the least invasive of the 3 hoary cresses in Canada, and is probably the most invasive of the hoary cresses in the U.S. [66]. Lens-podded hoary cress is more widespread and troublesome than either of the other 2 species in California [9]. Globe-podded hoary cress is generally not as aggressive as lens-podded hoary cress [66].
Hoary cress' reputation for forming dense monocultures and competing aggressively for soil moisture under some conditions on rangeland and pastures has led some authors to conclude that hoary cress, in turn, excludes most or all other herbaceous vegetation. Thus hoary cress is reputed to reduce crop yields, displace native plants, and reduce biodiversity, wildlife habitat, and forage production, causing "a serious threat to the cattle industry." There is insufficient information available to determine the conditions under which these monocultures might occur in wildlands, but there is abundant evidence that they occur in croplands, "waste lands," roadsides, and lands that are otherwise heavily impacted ([20,60,88], and references therein).
Preserve managers of The Nature Conservancy consider hoary cress a "moderate to serious" threat to native plant species in riparian and wetland settings, and a "minor" threat in native grasslands. Hoary cress is most likely to infest disturbed sites such as those caused by grazing, irrigation, drainage, and cultivation [80,81].
Results presented by McInnis and others [47,60] demonstrate the presence of phytotoxic compounds in heart-podded hoary cress root extracts. According to the authors, glucosinolates in the plant may inhibit germination and initial seedling growth of other plants. Hoary cress is considered by many to be at least mildly toxic to livestock [20,61,70,85]. When eaten by cattle it is said to taint dairy products [71,84].
Control: Effective management of hoary cress requires an integrated approach that includes 1) containment of known infestations; 2) prevention to assure new sites are not invaded; and 3) control to reduce or eliminate known infestations [60].
Containment of existing infestations of hoary cress on agricultural lands can help prevent spread to wildlands, and can be achieved by reducing movement of seeds onto new sites and by treating the boundaries of infestations to prevent spread of lateral roots. Because initial establishment of hoary cress is frequently by seed, proactive management to reduce seed dispersal is one of the most effective and least expensive measures to reduce establishment of new populations [60,88].
A single treatment to remove aboveground portions of hoary cress plants is insufficient to control hoary cress. Hoary cress roots have remained alive for a year even when aboveground growth has been controlled [91]. Eradicating hoary cress infestations requires a persistent, long-term, integrated approach, and may not be justifiable unless hoary cress plants are also eradicated from adjoining ditch banks, fencerows, roads, trails, and other disturbed areas. Small infestations of hoary cress may be eradicated from these areas by several methods (e.g. repeated hoeing, burning, hand-pulling, herbicide application) [82].
Studies by McInnis and others [60] in Oregon suggest 8 steps for land managers and others who frequent wildlands and rangelands to control heart-podded hoary cress: 1) learn to identify hoary cress and look for it whenever you are in the field; 2) take measures to prevent dispersing seeds to noninfested areas; 3) treat infestations when they are small and easy to control; 4) apply control techniques known to be effective; 5) apply translocated herbicides to heart-podded hoary cress when it is flowering; 6) always read and follow herbicide labels; 7) monitor treated infestations and follow up with additional treatment if necessary; and 8) manage rangelands for an abundance of vigorous and diverse vegetation [60].
In all cases where invasive species are targeted for control, no matter what method is employed, the potential for other invasive species to fill the void left by their removal must be considered [12].
Prevention: The most cost-effective method for managing hoary cress is to prevent its establishment and spread. Seed and root dispersal can be limited by careful management in the following ways [60,84,86,88]:
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Manage rangelands for plant communities in which all niches are occupied by vigorous plants. Grazing management plans consisting of moderate forage utilization and seasonal rotation of livestock can help desirable perennial plants maintain vigor and competitive ability and minimize hoary cress establishment and spread.
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Livestock should not graze weed-infested areas during flowering and seedset. When animals do graze infested areas during and after seed production, they should be transported to a holding area for 10 to 14 days, to allow time to digest and excrete seeds, before moving to uninfested areas.
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Driving vehicles and machinery through infested areas during the seeding period should be avoided. Undercarriages of vehicles and equipment should be checked for seedheads and cleaned when leaving infested areas.
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Remove hoary cress plants from along waterways to prevent movement of seeds and root fragments in running water. Screen irrigation water before applying it to fields.
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Circumvent hoary cress patches during cultivation and harvest to prevent spreading their roots and seeds throughout fields.
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Do not transport hay from fields infested with hoary cress to uninfested areas. Only certified weed-seed-free feed should be used in wildlands. Seeds in unprocessed hay consumed by livestock before entering National Forests may spread more exotic species than seed in feed pellets [19].
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Recreationists should be careful to brush and clean their equipment and animals and destroy seeds in a hot fire before leaving an area.
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Monitor for new or satellite infestations especially along roadways, railways, and waterways, and eradicate early. The eradication plan should include delimiting the infestation boundaries, applying control treatments, setting a control schedule, revegetating, planning follow-up monitoring, and estimating costs.
Integrated management: No single treatment provides effective, long-term control of hoary cress. Managing hoary cress requires an integrated strategy. Integrated management includes early detection, assessment, and containment of infestations before they spread. Factors to be addressed before a management decision is made also include assessment of nontarget vegetation, soil types, climatic conditions and important water resources; and an evaluation of the benefits and limitations of control methods [68]. Hobbs and Humphries [42] advocate an integrated approach to the management of plant invasions that includes "a focus on the invaded system and its management, rather than on the invader" and "identification of the causal factors enhancing ecosystem invasibility" as an effective approach to controlling invasive species. This type of "ecological control" puts an emphasis on removing the ecological stressors that may be underlying the causes of invasion, rather than on direct control of invasive species [42].
Not many studies explore integrated management of hoary cress except in agricultural settings. Once hoary cress establishes, integrating various combinations of competitive plantings, crop rotations, careful grazing management, and herbicides (e.g. [82,91]) can reduce hoary cress to manageable levels. While these approaches may not be appropriate in wildland management, control of hoary cress in agricultural lands can help prevent its spread to wildlands.
A key component of any integrated weed management program is sustained effort, constant evaluation, and the adoption of improved strategies [88].
Physical/mechanical: Various mechanical control methods have been employed to control hoary cress in agricultural settings. Hoary cress has a massive root system (76% of total plant biomass) that provides it with a large pool of stored carbohydrates for regrowth, and numerous belowground buds that can develop into new shoots [60]. Hoary cress roots have remained alive even when aboveground growth has been inhibited for 1 year [91]. Therefore, methods that remove only the aboveground portion of hoary cress have minimal impact unless repeated over several years in order to starve roots.
Several authors provide details on eradication or control of hoary cress by clean cultivation (e.g. [8,20,66,70,82,84,88,91]). Important considerations include timing initiation of and intervals between cultivations, number of cultivations, depth of tillage, considerations for avoiding spread of root fragments, proper implements to be used, soil moisture considerations, and planting of competitive crops. Authors suggest that 2 to 4 years of cultivation are required to control hoary cress.
Where physical conditions permit, hoeing at intervals of 3 to 4 weeks (depending on rate of regrowth) may be as effective as cultivation for eradication of hoary cress. Stands of globe-podded hoary cress were eradicated in 1.5 to 2 seasons by hoeing at intervals of 4 weeks. Soils must remain moist between hoeing so that plants can regrow and deplete their root reserves [82]. Similarly, Sheley and Stivers [88] suggest that digging can be a useful method for controlling hoary cress, especially small patches. Plants must be completely removed within 10 days after emergence throughout the growing season for 2 to 4 years [88].
Mowing alone will not provide effective long-term control of hoary cress [59,60]. Some authors suggest that hoary cress plants can survive repeated removal of top-growth for at least 1 season without noticeable loss in vigor [21,84,91]. Two consecutive years of mowing may have a more noticeable effect; however, hoary cress plants often preserve some of their vitality even after 3 years of mowing ([21] and references therein). Bellue [9] observed that lens-podded hoary cress and globe-podded hoary cress quickly recovered after mowing by producing new lateral growth and flowerheads. Recovery of heart-podded hoary cress was less vigorous [9]. Additionally, mowing may adversely affect desirable plant species. In Saskatchewan, densities of competing forbs remained relatively constant in unmowed plots, but ultimately decreased in mowed plots [86].
Other observations of mowed hoary cress stands suggest mowing reduces biomass and seed production in hoary cress and may result in stands with fewer shoots [59,86]. The date of mowing influences subsequent reproductive effort in heart-podded hoary cress. Plants mowed during flowering produced fewer viable seeds than plants mowed during bolting [59,60]. Phenology of subsequent regrowth of defoliated plants was more uniform at any given date than that of unclipped plants. The authors suggest that while defoliation alone is not expected to be an effective long-term control of heart-podded hoary cress, properly-timed grazing followed by herbicide application may increase mortality [59].
Heart-podded hoary cress survived mulching treatments with straw or tar paper [84].
Flooding: For control of heart-podded hoary cress, flooding to a depth of 6 to 10 inches (15-25 cm) for about 3 months can produce 90% control of the plant. However, short-term submergence lasting a week has no effect on the plant ([20] and references therein).
Rosenfels and Headly [82] discuss flooding as means of eradicating hoary cress from fields where the soil is clayey enough to hold water without excessive seepage, and where there is no great loss of soil fertility following prolonged submersion. They did not conduct experiments, but discussed the method with farmers who had tried it in a region with fine-textured soils around Stillwater, Nevada. Applying water in May or June and maintaining a depth of several inches until early September met with success. Hoary cress plants must be completely submerged. Flooding has limited application, because many areas are not suited to its use [82].
Fire: See the Fire Management Considerations section of this summary.
Biological: Biological control of invasive species has a long history, and there are many important considerations to be made before the implementation of a biological control program. The reader is referred to other sources (e.g. [79,106]) and the Weed Control Methods Handbook [96] for background information on biological control. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.
As of this writing, no insects or fungi for use as biological control agents are available for use on hoary cress in the U.S. Insects have not been developed as biological controls of hoary cress for 2 reasons. First, while hoary cress is listed among the noxious weeds of many states, it is of lower priority than other widely distributed and economically important weeds. Second, hoary cress is a member of the mustard family that contains numerous important agronomic plants such as canola, turnip, radish, and commercial mustards. Identifying insects that feed specifically on hoary cress and not on closely related mustards is difficult [11,60].
Grazing: Hoary cress has some forage value for grazing animals [61] and is sometimes grazed by cattle [20,61] and domestic sheep [20,61,69,88]. However, hoary cress may be at least mildly toxic to livestock ([61], and references therein). McInnis and others [61] suggest managers use caution when allowing animals to graze infested rangelands by providing supplemental iodine, utilizing mature and nonlactating animals, and reducing opportunities for animals to consume hoary cress [61]. Grazing animals can also serve as vectors for hoary cress seed and plant dispersal [20,57].
McInnis and others [59] demonstrated that defoliation of heart-podded hoary cress during its early growth stages could reduce its reproductive effort, and speculated that properly timed grazing combined with subsequent herbicide application could be a practical control measure.
A complete grazing management program has not been developed for hoary cress [88]. According to Olson [69], the dominance of rangeland ecosystems by nonnative, invasive plants such as yellow starthistle (Centaurea solstitialis), spotted knapweed, leafy spurge, and hoary cress may represent "steady states" [56], especially if these rangeland systems continue to be grazed only by cattle and horses, which tend to avoid these plants. A potential solution is to introduce or reintroduce small ruminants to these disturbed systems, restoring a balance by using grazers that prefer nonnative forbs [69].
Chemical: Herbicides are effective in gaining initial control of new or severe infestations, but are rarely a complete or long-term solution to invasive species management [18]. Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change conditions that allow infestations to occur (e.g. [109]). See the Weed Control Methods Handbook [96] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.
Most research on chemical control of hoary cress has focused on cropland - usually alfalfa, clover, or wheat fields. Experiments commonly include combinations of herbicides and other nonchemical methods [20]. Sodium chlorate and 2,4-D were the most commonly used herbicides for hoary cress in agricultural settings in the early 1900s (e.g. [21,82]).
Chemicals tested with limited success at controlling hoary cress include 2,4-D [20,24,49,88], metsulfuron [24,49,105], dicamba [20], chlorsulfuron [20,24,27,105], and glyphosate [20]. High application rates of imazethapyr in alfalfa-orchardgrass (Dactylis glomerata) pastures in western Montana provided greater than 90% control during the year of application. However, single applications provided no control the year following application, regardless of application rate. Yields of alfalfa and orchardgrass either decreased or were unaffected [95]. Chemicals that are ineffective at controlling hoary cress include picloram [88], bromoxynil, and fluroxypr [24]. Managers are encouraged to refer to individual herbicide studies, as timing and rate of application, mixtures, and use of adjuvants are important considerations in chemical control and are beyond the scope of this review.
Heart-podded hoary cress is difficult to control chemically due to the short period of maximum carbon allocation to belowground tissue, the large proportion of belowground dormant buds, and the wide variation in phenology among plants at any give time. According to Miller and others [64], chemical control of heart-podded hoary cress is most effective during flowering, when herbicides are translocated with carbon into roots and rhizomes. Herbicides applied prior to flowering may not be as effective because the flow of carbon is mainly to aboveground tissues; therefore, herbicides applied prior to flowering may damage shoots but will not enter the root system in high enough concentrations to kill the plant. Herbicides applied after flowering will not be effective because low soil moisture causes leaves to senesce, thus reducing photosynthesis and translocation within the plant. Effective chemical control is often variable because phenological stage of individual heart-podded hoary cress stems may not be uniform at any given calendar date, with some stems in the flowering stage while others are still in the rosette stage [64].
Because the effectiveness of chemical control for hoary cress is highly variable and unpredictable, chemical treatments must usually be repeated several times or combined with mechanical treatments to be successful [49,64,85,86,87,88,95]. Thus, chemical treatments are often prohibitively expensive on rangelands or natural areas [64].
Cultural: In agricultural settings, hoary cress can be controlled by tilling and planting competitive crops [82,85,86]. Alfalfa is an effective competitor where moisture is abundant, while perennial grasses and shrubs are more effective under dryland conditions and in years of lower than average precipitation [85,86].
The concentration of heart-podded hoary cress roots in the upper 20 cm of the soil profile is similar to the root distribution of many cool-season perennial grasses. The overlap in root distribution may increase the intensity of interspecific competition between heart-podded hoary cress and other herbaceous species [64].
Preliminary results presented by Mealor [62] suggest that native grasses growing among long-lived infestations of nonnative invasive species such as hoary cress and Russian knapweed may be better adapted to compete with invasive populations. More research is needed to understand the "implications of using exotic-selected native genotypes to restore native communities that are more resistant to invasion" [62].