United States Department of Agriculture 

NATURAL RESOURCES CONSERVATION SERVICE 

Invasive Species Technical Note No. MT-22 

November 2008 

Ecology and Management of Hoary Alyssum (Berteroa incana (L.) DC.) 

By 
 

Jim Jacobs, Plant Materials Specialist, NRCS, Bozeman, Montana 
 

Jane Mangold, Assistant Professor of Integrated Invasive Plant Management, 
 

Montana State University, Bozeman, Montana 
 

Figure 1. Hoary alyssum growing next to alfalfa. 

Abstract 

Hoary alyssum, a member of the mustard family, is an annual to short-lived perennial forb native 

to east-central Europe and western Asia. It has a slender tap root, star-shaped hairs on the stems, 

leaves, sepals, and seed pods, and four white, notched petals on flowers clustered at the stem 

tips. Dependent on seed production to maintain populations and to spread, flowering begins in 

late spring and lasts through fall enabling a single plant to produce over 2,500 seeds when 

growing on open ground. First reported from Ontario, Canada in 1893 and the northeast coast of 

the U.S. in 1897, it spread to Montana by 1905 and currently is reported from 27 Montana 

counties (a distribution map of hoary alyssum can be found at the University of Montana 

Invaders Database at http://invader.dbs.umt.edu/). It was designated a category 1 noxious weed 

in Montana in 2008. In the 1950s and 1960s, hoary alyssum began to proliferate in forage crops, 

pastures, and rangelands. It is opportunistic and fills voids left in disturbed areas and on overgrazed range and pastureland (see Figure 1). Toxicity to horses has been reported when green or 

dried forage is contaminated by more than 30%. Infestations reduce the species richness of 

pollinator communities because they attract a minimal number of pollinating insects. 

NRCS−Montana−Technical Note−Invasive Species−MT-22 1 
Hoary alyssum can be controlled using 2, 4-D applied at bloom. Re-applications will be required 

to prevent seed production. There are no biological control insects available and grazing 

management has not been developed. Prescribed grazing to maintain the vigor of forage species 

will prevent invasion and retard spread. In forage production fields, nutrient management, 

irrigation management, and crop rotation to maintain vigorous forage plants will prevent hoary 

alyssum invasion and reduce spread. 

Figure 2. The basal rosette of hoary alyssum with long-stalked, oval-lanceolate leaves. 

PLANT BIOLOGY 

Identification 

Hoary alyssum is a member of the mustard family (Brassicaceae). It has a slender tap root 

capable of deep soil penetration. The basal rosette has oval-lanceolate leaves broadest at the tip 

and 1.2 to 2 inches (3-5 cm) long including the long stalk (see Figure 2). Flower stems are erect 

or semi-erect growing seven to 30 inches (20-70 cm) tall and usually branched at the top (see 

Figure 3). Multiple stems are common. Stem leaves are alternately arranged, similar to rosette 

leaves in shape but smaller with less of a stalk, or stalkless. The margins of all leaves are entire 

with no teeth or lobes. 

NRCS−Montana−Technical Note−Invasive Species−MT-22 2 
Figure 3. The leaves, inflorescence, flower, and silicles of hoary alyssum. 

The small flowers (3 mm across) have long pedicels and are clustered at the stem tips in racemes. 

The four ovate sepals are green with white margins. There are four snowy-white petals narrow 

at the base and spreading at the tip with a deep cleft or notch (see Figure 4). There are two short 

outer stamens and four long inner stamens, four nectaries, and one pistil with a long style. The 

seed pods (silicles) are ellipsoid to ovoid, five to eight millimeters long, slightly inflated with the 

persistent style at the tip. They are appressed to the stem on the long pedicel. The seed pod has 

two chambers divided by a translucent septum or membranous partition which remains on the 

pedicel after seeds disperse. Each seed pod contains four to 12 dark reddish-brown, small (1-1.5 

mm) lens-shaped seeds with narrow wings on the margins. The leaves, stems, sepals, and seed 

pods are covered with star-shaped (stellate) hairs that give the plant a grayish appearance from 

which the common name descriptor, hoary, is derived. 

Figure 4. Hoary alyssum flowers. 

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Life History 

Seeds can germinate from early spring to late fall, limited mainly by water. Seedlings 

establishing in early July or sooner can flower and produce seed by early fall, thus reproducing 

as annuals. Seedlings establishing in late July or later will remain as rosettes and produce 

flowers and seeds the following year, reproducing as winter annuals or biennials. Short-lived 

perennial reproduction was observed in plants that re-grew after alfalfa cutting. Rosettes, but not 

flowering plants, accumulate non-structural carbohydrates (sugars) in the roots until the first hard 

frost. These sugars are needed for winter hardiness, respiratory maintenance during dormancy, 

and growth following dormancy, and therefore rosettes generally survive winter, whereas 

flowering plants do not. 

Hoary alyssum has low water requirements. In a study in Germany, germination and 

establishment were limited by the availability of open sites and water. However, once 

established, light was more important for growth and reproduction than water. In a Wisconsin 

alfalfa field, hoary alyssum did poorly where alfalfa was vigorous with a shading canopy, but 

thrived when alfalfa growth was poor. Similar to most mustard plants, hoary alyssum does not 

host arbuscular mycorrhizal (AM) fungi and had high mortality when grown experimentally in 

nitrogen-enriched soil and in the presence of adjacent plants inoculated with AM fungi. 

Hoary alyssum reproduces only by seed. Flowers are insect pollinated, predominantly by small 

insects able to access the shallow nectaries in the small flowers, including syrphid and other flies 

(Diptera), wild bees, and wasps (Hymenoptera). Spring flowering plants are able to produce 

large numbers of seeds when grown in the open. In Minnesota, biomass production was 10 times 

greater, and seed production was 20 times greater from plants grown in open areas compared to 

vegetated areas. In North Dakota, seed production averaged 2,530 seeds per plant. Herbicide 

studies suggest hoary alyssum can form a persistent seed bank and seeds can remain dormant and 

viable for several years. 

Habitat 

Hoary alyssum is adapted to the temperate continental climate characterized by cold winters and 

hot dry summers. Fall seedlings and rosettes are resistant to winterkill, and flowering plants are 

resistant to summer drought. The plant grows well on sandy or gravelly soils with poor soil 

fertility, and is most prolific on dry, disturbed open sites. It is commonly found on limestone and 

calcareous substrata and less so on acidic soils. It is common in fields of alfalfa, clover, or 

birdsfoot trefoil. In Montana it has been found along roads, railroads, trails, and gravelly stream 

and lake banks, in lawns, farmyards, vacant lots, overgrazed pastures and rangeland, and in hay 

meadows. 

Spread 

Hoary alyssum is believed to have been originally transported to North America as a 

contaminant of clover and alfalfa seed. Contaminations of forage and lawn seed, as well as 

contaminated hay, are still considered likely means of long distance seed dispersal. Hoary 

alyssum seed may also disperse long distances on mowers, other machinery, and in contaminated 

soil and gravel. Seeds disperse through valves in the seed pod. It is believed most seed falls 

near parent plants. However, the winged margin and light-weight seed may enable wind and 

NRCS−Montana−Technical Note−Invasive Species−MT-22 4 
 

water dispersal, and animals consuming the seed pods may disperse seed in their droppings, 

although there is no information to support this. 

Impacts 

Hoary alyssum began to increase in hay fields and pastures in the 1950s and 1960s, particularly 

under conditions of drought, overgrazing, or poor soil fertility. It decreases forage value because 

the woody stems of mature plants are low in crude protein and digestible carbohydrates. 

Contamination of 30% or more of forage with hoary alyssum is toxic to horses causing laminitis, 

limb edema, diarrhea, intravascular haemolysis, and hypovolemic shock. Its ability to persist 

under dry conditions and its continuous flowering and fruiting enables it to compete with native 

plants on range and wildlands and reduce biodiversity. In Minnesota it has been implicated in 

the reduction in species richness of pollinator communities because it attracted a minimal 

number of pollinating insects. 

MANAGEMENT ALTERNATIVES 

Herbicide1/ 

Hoary alyssum populations can be temporarily suppressed using 2, 4-D applied at label rates. 

Spring applications when plants are actively growing and prior to bolting will be most effective. 

Because hoary alyssum germinates and establishes throughout the growing season, repeated 

applications will be needed to target plants regenerating from the seed bank. Reports indicate 

metsulfuron applied at 0.5 ounce product (Escort®, Rainbowgreen Metsulfuron®, and others) 

per acre will control hoary alyssum, although hoary alyssum is not listed on the label. Other 

sulfonylurea herbicides including chlorsulfuron, and trisulfuron products as well as dicamba, and 

imazapic products may also be effective. Consult your Extension Agent for herbicide 

recommendations. Glyphosate will control hoary alyssum on seedbed preparation treatments. 

Studies are needed to develop herbicide recommendations for this weed. 

Hand Pulling. Hand pulling that removes the root crown is an effective method to temporarily 

reduce hoary alyssum on small-scale infestations and scattered plants that are either newly 

invading or persisting after herbicide treatments. Pulling or grubbing the root crown is most 

easily accomplished when the soil is moist and a shovel is used to pry-up the tap root. When the 

soil is dry, the plant tends to break off above the root crown enabling regeneration. Pulling 

should be repeated as plants regenerate from the seed bank or from remaining root crowns. 

Plants with seed pods should be burned if conditions permit or sealed in plastic bags and 

disposed of in the trash to prevent seed spread. 

Mowing. Mowing will not control hoary alyssum and may increase infestations by cutting down 

a shading canopy and spreading seed pods. Regular, repeated mowing to a six-inch stubble 

height may reduce seed production when combined with irrigation and nutrient management to 

increase the vigor of desired plants. If the mowing equipment is subsequently used on weed-free 

areas, washing equipment after use will help prevent the spread of this weed. 

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. 

NRCS−Montana−Technical Note−Invasive Species−MT-22 5 
Tilling. Shallow tilling that severs the tap root below the root crown will kill hoary alyssum 

plants. However, this type of disturbance will favor hoary alyssum regeneration from the seed 

bank. Multiple tilling or tilling followed by application of herbicide to target establishing 

seedlings can be used to exhaust the supply of viable seeds in the soil. Tilling is only 

recommended in cropland or in combination with seeding perennial competitive plants. As with 

any mechanical treatment, equipment should be washed after working in an infestation and 

before it is used on weed-free areas. 

Irrigation. Hoary alyssum thrives under dry conditions. Irrigation can be used to increase the 

production of many forage plant species in cropland and pastures and increases their 

competitiveness with hoary alyssum. Good irrigation management combined with nutrient 

management of forage crops can prevent hoary alyssum invasion and help reduce infestations 

through plant competition. 

Fertilization. Hoary alyssum thrives on sites with poor soil fertility. On cultivated pastures and 

hay meadows, nutrient management is important to maintaining the competitiveness of desired 

perennial grasses over hoary alyssum. Nutrient management combined with judicious use of 

herbicides and crop rotation is recommended where hoary alyssum invades non-native pastures 

and hay meadows. 

Prescribed Burning. There is no information on prescribed fire effects on hoary alyssum. Fall 

burns may reduce seed production if seed pods are burned before seed release. Fire may create a 

disturbance favorable to hoary alyssum establishment and sites should be monitored for weed 

occurrence and follow-up weed control should be applied where weeds are found. 

Grazing Management 

Grazing animals have not been used to control hoary alyssum. Domestic and wild mammals 

consume hoary alyssum where it occurs in sufficient quantities, and poisoning has only been 

reported in horses. Lambs rejected hoary alyssum in a Minnesota forage field trial. Goat 

utilization of weed-free hay was greater than utilization of hay contaminated with hoary alyssum 

in a Wisconsin study. Studies specific to using these animals to manage hoary alyssum are 

needed. Cattle will utilize hoary alyssum in their forage areas but they generally select more 

digestible forages. Prescribed grazing to maintain the competitiveness of forage plants is 

important to prevent the spread of hoary alyssum in pastures and on rangeland, because the 

disturbance of overgrazing favors hoary alyssum establishment and reproduction. 

Biological Control 

There are no biological control insects available for management of hoary alyssum. 

Revegetation 

Sustainable suppression of hoary alyssum populations is more likely with desirable perennial 

plants that will compete for light, water, and nutrients. On disturbed sites and sites where the 

competitive vegetation has been lost, re-vegetation may be necessary to establish a competitive 

plant community. In forage crop fields, crop rotation is important for weed control and keeping 

the crop vigorous and competitive. For example, hoary alyssum increased in Wisconsin when 

five-year varieties of alfalfa replaced three-year varieties. 

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Species selected for revegetating disturbed sites and hoary alyssum infestations should be 

appropriate for management objectives, adapted to site conditions, and competitive with the 

weed. Management objectives will determine if non-native forage species or native species are 

seeded and species mixture components. The environmental conditions of the site, including 

precipitation, soil texture and depth, slope and aspect will affect species establishment. On 

native rangeland, a diversity of perennial grass and forb species that occupy many niches over 

time and space will most fully utilize available resources and compete effectively with hoary 

alyssum. 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 seeding rate guidance and revegetation species selection. State, area, and 

field resource specialists can help determine the most appropriate, site-specific species mix and 

timing of seeding for local conditions. 

Integrated Pest Management (IPM) 

Hoary alyssum thrives on disturbed ground with poor fertility and dry conditions. On hay 

ground, IPM practices include crop rotation, nutrient management, and irrigation management 

(where available) to maintain competitive shading forage plants. New infestations of hoary 

alyssum should be aggressively controlled using hand pulling and broadleaf herbicide. Based on 

data from Wisconsin alfalfa field trials, herbicide application was recommended as soon as 

possible after the last forage harvest in early fall. On pastures and rangeland, herbicide 

application should be combined with prescribed grazing. In farm yards, herbicide application 

and hand pulling should be used particularly where hay, tillage, and harvest equipment are 

stored. Equipment should be clean before taken to the field. Roadways, trails, and irrigation 

ditches should be maintained free of weeds. 

References 

Geor, R.J., R.L. Becker, E.W. Kanara, L. R. Hovda, W.H. Sweeney, T.F. Winter, J.K. Rorick, 

G.R. Ruth, E. Hope, and M.J. Mutphy. 1992. Toxicosis in horses after ingestion of hoary 

alyssum. J. Am. Vet. Med. Ass. 201: 63-67. 

Hastings, R.E. and C.A. Kust. 1970. Reserve carbohydrate storage and utilization by yellow 

rocket, white cockle, and hoary alyssum. Weed Science. 18: 140-148. 

Leroux, G.D., R. G. Harvey, N. A. Jorgensen, and M. Collins. 1985. Influence of hoary alyssum 

(Berteroa incana) on quality of alfalfa (Medicago sative) forage and its utilization by goats. 

Weed Science. 33: 280-284. 

Reichman, O.J. 1988. Comparison of the effects of crowding and pocket gopher disturbance on 

mortality, growth and seed production of Berteroa incana. The American Midland Naturalist. 

120: 58-69. 

Warwick, S.I. and A. Francis. 2006. The biology of invasive alien plants in Canada. 6. Berteroa 

incana (L.) DC. Can. J. Plant Sci. 86: 1297-1309. 

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