Lens-podded whitetop or Lens-podded hoarycress [Cardaria chalepensis (L.) Hand.-Mazz.][CADDC][CalEPPC: B][CDFA list: B] Photographs Map of Distribution
Hoary cress or Heart-podded hoarycress [Cardaria draba (L.) Desv.][CADDR][CalEPPC: A-2][CDFA list: B] Photographs Map of Distribution
Hairy whitetop or Globe-podded hoarycress [Cardaria pubescens (C. Meyer) Jarmol.][CADPU][CDFA list: B] Photographs Map of Distribution
SYNONYMS:
GENERAL DESCRIPTION:Noxious perennials to 0.4(0.5) m tall, with creeping horizontal roots that vigorously produce new plants. Cardaria species are problematic in natural areas and many crops, especially irrigated crops such as alfalfa and sugar beets. Not until 1933 were 3 distinct Cardaria species known to exist in North America. Consequently, taxonomic references to Cardaria species in articles published prior to 1933 are unclear. Although similar, each species differs in chromosome number and herbicide resistance. All are self-incompatible. Lens-podded (2n = 80) and hoary cress (2n = 62, 64) can hybridize, but only first generation hybrids survive under natural conditions. Lens-podded and hairy whitetop were introduced from Central Asia. Hoary cress (2n = 16) was introduced from Eurasia.
SEEDLINGS:Develop tap roots to a depth of 25 cm and lateral roots with shoot buds within 1 month.Hoary cress: Cotyledons oval to elliptic, 7-9 mm long, ~ 2.5 mm wide, unequal, pale gray-green, with peppery taste. First leaves ovate to oblong, dull, scaly, somewhat larger than the cotyledons, often with slightly wavy margins. Subsequent leaves resemble first leaves, but sometimes have short fine hairs along the margins. First and subsequent leaves have bases tapered to petioles equal to or longer than the length of the blades. No descriptions are available for lens-podded and hairy whitetop. However, seedlings of these species likely resemble those of hoary cress.
MATURE PLANT:Stems +/- erect, sparse to densely covered with simple short hairs. Leaves alternate, gray-green, variable, obovate, (ob)lanceolate, oblong to elliptic. Surfaces, especially lower, sparsely to densely covered with simple, short white hairs. Margins irregularly toothed to entire. Basal leaves short-stalked. Upper leaves sessile, with rounded-acute- to acute-lobed bases that clasp the stem.
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ROOTS and UNDERGROUND STRUCTURES:Plants develop extensive systems of persistent, deep, vertical and horizontal roots that vigorously produce new shoots at irregular intervals. Root fragments can generate new plants. Vertical roots can penetrate the soil to depths of 2 m or more. Roots can account for 75% of the total plant biomass and, as a result, store considerable amounts of carbohydrates. Carbohydrate reserves typically accumulate to maximum levels by mid-summer and are minimal in early to mid-spring. Roots survive cold winter climates and periods of drought. Mycorrhizal associations do not develop.
FLOWERS:Infloresecences often +/- flat-topped (compound corymbs). Flowers fragrant, numerous, 4-petaled, white. Insect-pollinated.
FRUITS and SEEDS:Pods (silicles) 2-chambered, variable, inflated, with a persistent style 1-2 mm long at the apex, do not open (or open slowly) to release seeds. Seeds (0)1-2 per chamber, ovoid, slightly flattened, reddish-brown,1.5-2 mm long, 1-1.5 mm wide, with minutely granular surfaces.
POSTSENESCENCE CHARACTERISTICS:Foliage dies back during extended periods of freezing temperatures or drought.
HABITAT:Disturbed open sites, fields, grain and vegetable crops, especially irrigated crops such as alfalfa and sugar beets, orchards, vineyards, roadsides, ditches. Often grows on moderately moist, alkaline to saline soils, but tolerates a wide range of soil types and moisture conditions.
DISTRIBUTION: All species are scattered throughout California and uncommon in the desert regions.
PROPAGATION/PHENOLOGY:Reproduce vegetatively from creeping roots and less importantly by seed. Root fragments generate new plants, but regeneration is poor in dry soils. Under favorable conditions, plants often increase vegetatively by more than a 61 cm (2 ft) radius per year. Light stimulates seed germination but is not required. Seed germinates in the fall after the first rains. Plants typically do not flower the first year. One flowering stem of lens-podded whitetop or hoary cress can produce up to 850 mature pods. Lens-podded and hairy whitetop (and probably hoary cress) compete poorly with shrubs in natural communities.
MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Colonies are difficult to eliminate because of deep, persistent roots. Cultivation can facilitate spread of plants by dispersing root fragments. However, repeated cultivation (bimonthly to monthly) can destroy colonies in 2-4 years. Flooding an area with 15-25 cm of water for 2 months can eliminate troublesome infestations.
SIMILAR SPECIES:Unlike Cardaria species, perennial pepperweed [Lepidium latifolium L.] has glabrous foliage, smaller pods (~ 2 mm long) that are +/- flattened and not inflated, with sessile stigmas less than 1 mm long at the apex, and stem leaves that lack clasping lobed bases. In addition, perennial pepperweed is typically taller (> 0.5 m), and its pods open at maturity to release seed.
CONTROL METHODS:
Biology and prevention: These Cardaria species are aggressive perennials native to southwest Asia. They were likely introduced in multiple shipments of contaminated alfalfa seed from Turkestan into North America over a period of 40-50 years. Cardaria draba was first found in Yreka California in 1876, C. chalepensis was first collected in Chino, California in 1918, and C. pubescens was first collected in Alberta Canada and Michigan in 1919. All three species now occur primarily throughout the Western United States, and are found throughout California. They persist under a wide range of environmental conditions and are found in irrigated croplands, roadsides, rangelands, and wildland areas. They are also found in riparian-upland ecotones and are somewhat salt and alkaline tolerant, but generally not shade tolerant. All three species readily establish in disturbed areas in range and wildlands and are favored during years of above average precipitation. Invasion potential is greater under heavily grazed conditions or other disturbances. Infestations rapidly establish dense stands and may exclude native species, reduce biodiversity and decrease rangeland productivity and forage quality. In agricultural areas, they are most aggressive in irrigated fields and in areas where cultivation is infrequent. They can cause considerable yield losses in alfalfa, cereal crops, and orchards. Seed production is important for establishment in new areas. Seed production is greater in C. draba and C. chalepensis than C. pubescens. Seed viability may be less than three years for C. draba, is likely longer for C. chalepensis and is unknown for C. pubescens. Seed viability through animals is unknown. However, seed do not persist for more than a month in manure. Seeds are dispersed by water, vehicles, farm machinery, and contaminated hay and crop seeds. New populations along roadsides and field edges and irrigation ditches should be aggressively controlled to prevent seed spread to new areas. Plant only certified seed to avoid spreading the problem to other fields. Although seed production is somewhat important, the aggressive nature and stubborn persistence of these weeds is due to an extensive system of vertical and lateral roots. New shoots arise from buds on creeping laterals and may form many independent, clonal plants if severed. Vertical roots may penetrate the soil to a depth of 1.5-6 m, and allow the plant to withstand extensive drought. Approximately 75% of the total biomass of C. draba is belowground. C. chalepensis seems to be more robust than the others to both mechanical injury and certain herbicides. Mechanical: Mechanical control is extremely variable with these deeply-rooted species. Rapid replenishment of depleted energy reserves in the roots is likely for all three species. Therefore, any mechanical control must be aggressively maintained for several years. Hoeing or hand pulling will require considerable effort and may reduce other competitive species due to increased disturbance. Infrequent tillage may also increase stem densities by severing rootstocks that form numerous, independent plants. The extremely intensive tillage necessary to eradicate these weeds may require repeated cultivation every 10 days during the growing season for 2-5 years. Research from Australia has indicated repeated cultivation even before new shoots emerge is critical. Mowing alone is generally ineffective for control, as rapid regrowth occurs. However research on a similar species, perennial pepperweed (Lepidium latifolium) has shown that mowing followed by certain herbicides such as glyphosate has dramatically improved control. The most effective timing for mowing is when plants are in the late bud to early flowering stage. This will also reduce seed production, but may also decrease competitive vegetation. Biological: There are no biological control agents for use on these species. Cattle and sheep will graze them; however, dairy animals grazing Cardaria spp. may produce milk with objectionable taste and odor. Fire: It is unlikely that prescribed burning will control Cardaria spp. Numerous observations indicate that Cardaria spp. populations expand following fire. This is likely due to emergence of new shoots from root buds on lateral roots. In Australia, C. draba seedlings have been reported to rapidly appear after grassland fires. Chemical: Chemical control can be effective for all three species, but must be maintained for several years to obtain long term control. Table 1 provides herbicide information for control of Cardaria spp. in California. There are certain differences and problems that may arise with herbicide applications. The most effective timing of application for all three species appears to be in the bud to early flower stage. However, in California, the timing of flowering is later for C. chalepensis than C. draba. Additionally, C. chalepensis is very tolerant of 2,4-D compared to C. draba and C. pubescens. When these species are growing together, herbicide applications may select for the species not at the proper growth stage, and control will appear poor. Table 1. Herbicides labeled for control of Cardaria spp.
Integrated management strategies: One of the prime areas for invasion in irrigated crops is thinning stands of alfalfa. If possible, rotation to a winter annual grain would increase early season competition and allow for intensive cultivation and herbicide applications during the fallow season. 2,4-D and MCPA may also be used in cereals during the growing season, at rates that will provide some Cardaria suppression. In heavily infested range and wildland areas, mowing followed by an herbicide application to regrowth at the bud stage may be the most effective treatment. Established perennial grasses will provide additional suppression of Cardaria spp. However, heavy grazing may reduce suppression. Where possible, shrub establishment may provide the most effective long-term suppression. This is an intensive management process that may be very difficult in heavily infested areas.