United States Department of Agriculture 

NATURAL RESOURCES CONSERVATION SERVICE 

Invasive Species Technical Note No. MT-12 

May 2007 

Ecology and Management of Whitetop [Cardaria draba (L.) Desv.]

By 

Jim Jacobs, NRCS Invasive Species Specialist, Bozeman, Montana 

Figure 1. A whitetop infestation along a drainage ditch in Madison County, Montana. 

Abstract 

Whitetop, also called hoary cress, is a rhizomatous, long-lived perennial weed invading 

rangeland, pastures, moist meadows, and floodplain meadows in Montana. It is common along 

roadsides, railroad tracks, ditch banks, riverbanks, and often abundant on alkali soils (see Figure 

1). It is in the mustard family (Brassicaceae) and there are three species in Montana, the most 

common being heart-podded whitetop (Cardaria draba). All three species spread by creeping 

roots and rhizomes, and prolific seed production. They can form dense patches that compete 

with crops, forage plants, and riparian plants. Infestations reduce crop yield and increase 

production costs, reduce livestock carrying capacity, and control is costly. 

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Whitetop is native to Russia, other eastern European countries, and the Middle East. First 

reported in Montana from Gallatin county in 1916, whitetop has spread to every county except 

Daniels and Roosevelt as of 2006 (http://invader.dbs.umt.edu). Whitetop is ranked as a Category 

1 noxious weed in Montana, infesting an estimated 83,539 acres as of 2005 (Montana 

Department of Agriculture). In Montana’s noxious weed management plan, the goal for 

management of Category 1 noxious weeds is to reduce established infestations and contain their 

spread by expanding cooperative weed management areas. 

Whitetop can be temporarily suppressed using herbicides that contain metsulfuron, 

chlorsulfuron, or imazapic as active ingredients. Long-term herbicidal control requires repeated 

applications. Repeated cultivations or cultivation combined with herbicidal control are required 

to reduce whitetop populations in cropland. Irrigation favors increases in whitetop populations. 

Cattle, sheep, and goats will graze whitetop, but the effects on population fitness of long-term 

grazing with these animals are not known. Mowing and sheep grazing reduce density and 

standing crop for up to one year. Shrubby species including Rosa and Symphoricarpos are more 

competitive with whitetop than native grasses. 

Biology and Ecology

Taxonomy. There are three species of whitetop in North America all of which are believed to be 

in Montana; lens-pod whitetop (Cardaria chalapensis), heart-podded whitetop (C. draba), and 

globepodded whitetop (C. pubescens). As the common names imply, the three species can be 

distinguished by the morphological characteristics of their fruits (silicles). Silicles of C. draba, 

the most commonly reported species in Montana, are hairless (glabrous), egg-shaped (ovate) and 

cordate-based (like the shape of a heart), and often notched at the tip where the style attaches. 

Cardaria pubescens silicles are covered with fine hairs (pubescent), more finely pointed (acute) 

than cordate at the base, more spherical than ovoid (obovoid), and often without a notch at the 

tip. Silicles of C. chalapensis are glabrous, rounded at the base and generally acute at the tip. 

Cardaria draba is taller in stature than the other two species. Otherwise, the root, leaf and stem 

features of the three species are similar. Resistances to herbicides differ slightly among species. 

Cardaria draba has the greatest economic impact in the United States, whereas C. chalepensis is 

the most common whitetop species in Canada and C. pubescens is most common in California. 

Hybrids of C. draba and C. chalepensis have been found in Canada but there is no evidence of 

their survival beyond the first generation. No natural hybrids between C. pubescens and the 

other two species have been found. Whitetop is also known by the common name hoary cress. 

The genus name, Cardaria is derived from the Greek word kardia which means heart and refers 

to the shape of the silicle of C. draba. 

Roots. Whitetop populations rely on vegetative reproduction from extensive root systems, not 

seed production, to increase plant density and local population expansion. The perennating 

adventitious buds form on all parts of the root system and are capable of producing rhizomes 

(underground stems) and aboveground shoots. Whitetop produces vertical taproots and 

horizontal lateral roots that eventually turn downward and become taproots. Secondary lateral 

roots often form just below the point where lateral roots turn down. Shoots that give rise to 

rosettes tend to form where lateral roots are near the soil surface. Rhizomes form from more 

deeply buried lateral roots. Lateral roots develop from the seedling radicle two to three weeks 

after seedling emergence. The vertical taproots frequently penetrate two to three feet (74 to 81 

cm) into the soil but can penetrate five feet (1.6 m) deep. Taproots of C. pubescens in 
Saskatchewan were tracked to a depth of 18 feet (5.6 m) in the soil. Roots are capable of rapid 

growth and can extend 12 to 30 feet (4 to 9 m) in two or three growing seasons. 

Whitetop roots store abundant reserves. Accumulations of carbohydrates peak in August and the 

lowest levels have been found in April. Root fragments are capable of developing into new 

plants; however fragmented roots show poor re-generation in dry soil. 

Whitetop roots produce glucosinolates. When released into the soil, glucosinolates, or the 

byproducts of their breakdown, may act as allelochemicals to reduce root growth and seed 

germination of other plant species. Extracted chemicals from whitetop roots were found to 

inhibit the germination of alfalfa, bluebunch wheatgrass, crested wheatgrass, and whitetop, and 

reduce the germination of winter wheat. Root growth of these species was also reduced by the 

extract. 

Figure 2. Whitetop leaf and silicle features. 

Rosettes and Stems. Whitetop plants produce rosettes and flowering stems. Rosettes emerge 

early in the spring. Rosette leaves are longer than wide and are widest near the tip (elliptic, see 

Figure 2). They taper to a short petiole. They are simple, the margins are entire or irregularly 

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dentate, and they have scattered to dense pubescence with short simple hairs. Stem leaves are 

also longer than wide, 3.5 by 1.5 inches (9 by 4 cm) but vary in shape from slightly egg-shaped 

(obovate) to wider at the tip or consistently wide the entire length (elliptic-oblong), to wider at 

the base and tapering to a point (lanceolate). The margins are entire or irregularly toothed. They 

have no petiole and clasp the stem with heart-shaped or arrowhead-shaped (cordate-sagitate) 

lobed bases (see Figures 2 and 3). Stem leaves are alternate along the stem and grayish-green. 

Leaves are usually shed at the time of seed maturity (July). 

Stems of C. draba are erect to slightly trailing along the ground (procumbent) and grow from 

one to two feet (30 to 60 cm) tall. Stems of C. chalepensis and C. pubescens are shorter than C. 

draba, averaging 8 to 16 inches (20 to 40 cm) and 4 to 16 inches (10 to 40 cm) tall, respectively. 

The upper parts of the stems are hairless (glabrous) and the lower parts are slightly to densely 

pubescent. When rosettes and shoots develop from buds on roots they usually form where lateral 

roots bend down to become taproots. Individual plants can produce from 50 to 450 stems 

depending on environmental conditions and the degree of competition from other plants. 

Figure 3. Whitetop stem leaves showing the leaf shape, clasping leaves, and irregularly toothed 

margins 

Whitetop rosettes, stems, flowers and fruits have high levels of crude protein, favorable levels of 

digestible energy, and adequate levels of minerals to support livestock. Whitetop consumed 

before flowering satisfies the protein and energy requirements of a 130 pound (60 kg) ewe. 

After flowering, crude protein and digestible fiber decrease. Leaves, stems and silicles produce 

glucosinolates that are potentially toxic to livestock. Sulfur content associated with 

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glucosinolates may exceed the maximum tolerable level of 0.4% in whole plants and in silicles. 

Glucosinolates leached from plant material deposited on the soil surface may be an 

allelochemical that prevents seed germination. 

Figure 4. Whitetop plants blooming in Jefferson County, Montana in mid-May. 

Sexual Reproduction. Whitetop is capable of producing large amounts of seeds. The flowers 

are white, small, and have four glabrous white-margined sepals 1.5 to 2 mm long, four white 

petals 3 to 4 mm long, and six stamens. The flowers are attached to the inflorescence by a 

slender pedicel 1/2-inch (1.5 cm) long. The sepals of C. pubescens are pubescent. The flowers 

of all three species are numerous on many-branched, corymbose racemes (see Figure 4). The 

number of flowering stems, and the number and arrangement of flowers on the racemes give an 

infestation a solid white appearance from which the common name “whitetop” is derived. 

Whitetop is self-incompatible and outcrosses by insect pollination. 

 

The flowers develop into fruiting pods (silicles) that vary in shape within and among species as 

described above (see Figure 2). The silicles are inflated at maturity and have two chambers, each 

capable of producing one seed. Seeds are oval to slightly compressed, 1.5 by 2.0 mm, and 

reddish-brown (see Figure 5). One whitetop plant can produce 1,200 to 4,800 seeds. The 

percentage of viable seeds is high. Seeds remain viable in the seed bank for three to four years. 

Seeds are dispersed through ruptures in the silicle side wall or as the silicles fall intact from the 

plant. Seeds exude mucilage when moistened that is believed to increase germination under low 

osmotic conditions. Seeds germinate in the fall or spring. 

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Figure 5. Whitetop seeds. 

Whitetop plants grown from seed may not produce flowers until two years after establishment. 

Flowering and seed production from plants with established root systems is rapid. Rosettes 

growing from the root system in early spring initiate flower stem development about two weeks 

after emergence. Flower buds develop about one month after rosettes form (see Figure 6). 

About one and one half months after rosettes form, flowers have opened and developed into 

silicles. Seeds are fully developed within two months after the formation of rosettes from an 

established root system. Generally, plants die back to the root crown after the development of 

seeds and remain dormant during the summer drought period re-initiating growth in the fall if 

moisture conditions are favorable. Otherwise growth resumes in the spring. 

 Figure 6. Whitetop flower buds forming on plants May 10, 2007. 

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Spread. Whitetop spreads by seed, creeping roots, and root fragments. Population expansion is 

mainly through the extensive spreading root system and can be as much as 12 feet (3.7 m) in 

diameter during the first year when growing without competition. In Saskatchewan, patch radial 

expansion ranged from 2.0 to 2.5 feet (61 to 76 cm) per year. Root fragments can be dispersed 

within and between fields by cultivation equipment. Fruits with seeds can be transported long 

distances in wind, river currents, irrigation ditches, contaminated hay, and by animals and birds. 

The source of early introductions of whitetop in North America is believed to be from alfalfa 

seed imported from Turkestan. 

Habitat. Whitetop occurs on a variety of soils on mesic sites. In an area with a 15-inch (38 cm) 

average annual rainfall, whitetop increased when precipitation was 20% above average and 

decreased when precipitation was 15% below average. In Oregon, increases in stem density 

occurred during years with cool, dry Aprils and warm, moist Mays. Whitetop is often abundant 

on alkali soils. Whitetop is commonly found in irrigated pastures and hay meadows, floodplains, 

along irrigation ditches, and along roadways where runoff water accumulates. It grows well in 

full sun. It is commonly found in brome, alfalfa, and growing with Russian knapweed. In 

sagebrush eco-systems, whitetop populations are restricted to moist microsites, toe slope 

positions, and areas of soil disturbance. 

Economic impacts. Because of its clonal reproduction, whitetop often forms near monotypic 

stands that exclude desirable plants. Infestations reduce livestock carrying capacity by 

displacing nutritive forage plants. For every 90 pounds per acre increase in whitetop biomass 

there is an estimated 5% reduction of desirable forage. Hay contaminated by whitetop has lower 

nutrient content than weed-free hay. Wildlife habitat is diminished where whitetop displaces 

riparian plant species that provide browse and bird nesting sites. It reduces the yield of cropland 

and increases management costs. 

Management Alternatives 

Herbicide.1/ Metsulfuron (1.0 oz product/acre) or chlorsulfuron (1.0 oz product/acre) applied 

from bud to early bloom are the most effective herbicide treatments and can suppress whitetop 

for one to two years on rangeland sites. For control of whitetop in annual cereal crops, follow 

label instructions when using metsulfuron. A non-ionic surfactant (0.5% by volume) needs to be 

added to the spray solution when using chlorsulfuron or metsulfuron. Imazapic (8-12 oz. 

product/acre) mixed with methylated seed oil (1 qt./acre MSO) applied after full bloom, when 

the plants go dormant, or in the fall to re-growth will provide similar control. Chlorsulfuron and 

metsulfuron have long residual persistence on alkaline soils. 

In alfalfa, imazethapyr applied at 6 oz./acre product (Pursuit®) with a non-ionic surfactant 

(0.25%) will control whitetop for the growing season but not the year following application. 

Alfalfa yield will not be affected by this application but grass yield may be reduced. Using a 

non-ionic surfactant instead of methylated seed oil may reduce injury to grasses. Combining a 

nitrogen-based fertilizer in the spray solution may increase the risk of grass injury. 

1/

Any mention of products in this publication does not constitute a recommendation by the NRCS. It is a violation 

of Federal law to use herbicides in a manner inconsistent with their labeling. 
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Shoots of whitetop can be killed using 2, 4-D (2 qt. product/acre), but root crowns will re-sprout 

new foliage. Glyphosate (2 qt. product/acre) provides similar control as 2, 4-D, however 

glyphosate is non-selective and re-vegetation may be needed to restore competitive plants. 

Imazapyr has an aquatic label (Habitat®) and can be used on whitetop where it grows in riparian 

areas. Picloram does not control whitetop. Repeated herbicide applications are needed in most 

cases to sustain population reductions regardless of the product used. Because of its clonal 

nature and ability to produce allelochemicals, whitetop can suppress other plants and competitive 

plants may need to be restored after herbicidal control of whitetop. Where herbicides with soil 

residual activity are used (i.e., metsulfuron, chlorsulfuron, and imazapic), consult the herbicide 

labels for replant intervals to avoid herbicide carryover injury to desirable seeded species. 

Hand Pulling. Repeated hand pulling, digging, and rouging can reduce populations of whitetop 

where soil conditions allow removal of large portions of the root system. Though labor 

intensive, hand pulling may be practical for small infestations, in riparian areas, or for spot 

elimination of re-growth after herbicide application. 

Grazing and Mowing. Cattle, sheep, and goats will utilize whitetop. Studies on the nutrient 

composition indicate levels of protein, digestible energy, and mineral nutrients are adequate to 

meet the nutritional requirements of most classes of livestock. The concern for livestock 

managers is the presence of toxic levels of glucosinolates which can cause anti-thyroid 

symptoms. The chances of animal poisoning can be lessened by providing supplemental iodine, 

alternative forage plants, and using mature non-lactating animals. Glucosinolate levels are 

lowest when whitetop is in the rosette to bloom stage. This is also when nutrition is greatest and 

is the recommended time for grazing whitetop. Cattle apparently prefer whitetop seed pods in 

the fall when other forages are less available. Glucsinolate levels are high in whitetop seeds and 

it is likely that the small, hard-coated seeds will pass through the digestive tract intact. It is 

therefore not recommended to graze whitetop infestations in the fall where seed pods are present 

to reduce the risk of poisoning and spread of the weed by seed. Grazing fall re-growth where 

seed pods are not present may reduce the fitness of the population. 

Mowing by itself will not reduce whitetop populations. But mowing during flowering may 

reduce seed production. Mowing to reduce seed production followed by herbicide application to 

re-growth may improve the effectiveness of herbicides in controlling whitetop. 

Biological Control. To date there are no biological control agents introduced into the United 

States to manage whitetop. Development of biological control agents is difficult because of the 

risk of damage to valuable crops that are closely related taxonomically, including canola, 

mustard, and cabbage. Currently, one mite and five insect species are being studied as potential 

biological control agents for whitetop. 

Tillage. Whitetop, like other perennial rhizomatous weeds, may be controlled by persistent 

repeated tillage. Cultivation six inches deep must be repeated within ten days of re-growth. 

However, it takes three years of continuous tillage to kill the root system of whitetop. Tillage 

also spreads root fragments and should be followed by herbicidal control (glyphosate at 2 quarts 

per acre) and cropping or re-vegetation. Tillage equipment should be thoroughly cleaned before 

moving to a non-infested field. 
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Re-vegetation. Where whitetop forms dense near-monocultures, re-vegetation may be needed to 

establish competitive plants after control procedures. Establishing competitive perennial grasses 

on disturbed land, followed by prescribed grazing management to maintain grass vigor will 

suppress whitetop and prevent re-establishment and spread by rhizomes and seed. Refer to 

Montana Plant Materials Technical Note 46, ‘Seeding Rates and Recommended Cultivars,’ and 

Extension Bulletin EB19, ‘Dryland Pasture Species for Montana and Wyoming’ 

for recommended re-vegetation species and their appropriate seeding rates. State and area 

resource specialists can help determine the most appropriate, site-specific species mix, timing of 

seeding, and seeding methods. Where herbicides have been applied, chemical carryover should 

be assessed prior to planting permanent vegetation. 

Integrated Pest Management (IPM). Integrated pest management is the application of two or 

more management alternatives so they are complimentary in weed suppression, increase the 

longevity of control procedures, and improve crop production, or conservation of resources. 

Mowing to a height of six inches (15 cm) prior to herbicide application may improve herbicidal 

control of whitetop. To reduce the frequency of herbicide re-applications, prescribed grazing 

with sheep, goats, or cows can be applied to prevent re-establishment from roots that survive 

herbicide treatment (follow herbicide label guidelines for grazing after herbicide treatment). On 

cropland, tillage followed by herbicide treatment will be more effective than either treatment 

applied alone. On pastures and rangeland where competitive plants have been suppressed, revegetation following control of whitetop will improve the longevity of the control application. 

References 

Kiemnec, G.L. and M.L. McInnis. 2002. Hoary cress (Cardaria draba) root extract reduces 

germination and root growth of five plant species. Weed Technology. 16:231-234. 

Larson, L., G. Kiemnec, and T. Smergut. 2000. Hoary cress reproduction in a sagebrush ecosystem. Journal of Range Management. 53:556-559. 

McInnis, M.L., L.L. Larson, and R.F. Miller. 1993. Nutrient composition of whitetop. Journal 

of Range Management. 46:227-231. 

Miller, R.F., T.J. Svejcar, J.A. Rose, and M.L. McInnis. 1994. Plant development, water 

relations, and carbon allocation of heart-podded hoary cress. Agronomy Journal. 86:487-491. 

Mulligan, G.A. and J.N. Findlay. 1974. The biology of Canadian weeds. 3. Cardaria draba, C. 

chalepensis, and C. pubescens. Canadian Journal of Plant Science. 54:149-160.