Tall Whitetop 

( Lepidium 

latifolium)

By James A. Young, Rangeland 

Scientist; Robert R. Blank, Soil 

Scientist; and Charlie D. Clements, 

Rangeland Scientist; USDA 

Agricultural Research Service

Background

The Weed Science Society of 

America has designated perennial 

pepperweed as the official common 

name for Lepidium latifolium L. In 

the San Luis Valley of Colorado, 

nearly everyone refers to the plant 

as ironweed. Throughout much of 

the western United States the plant 

is known as tall whitetop. Unfortunately, another weed commonly 

called “whitetop” is widely distributed on rangelands. The scientific 

name of this whitetop is Cardaria 

draba [L.] Desv. and the accepted 

common name is hoary cress. This 

may seem confusing, but it results 

in a communication rather than 

an identification problem. If correspondence arrives reporting a 

“whitetop” problem at a field location, it is necessary to determine 

the exact species that is being 

referred to as the problem. In 

the field, it is generally easy to 

distinguish between Lepidium 

latifolium and Cardaria draba. 

Both weeds known as whitetop are 

members of the Brassicaceae, or 

mustard, family. They both have 

the typical, four-parted mustard 

flower with white petals. Both 

species have vigorous creeping 

roots. These roots have buds 

that can produce either shoots 

or roots, and both species form 

colonies. With all these shared 

characteristics it would seem 

that tall whitetop and whitetop 

are difficult to distinguish. 

The adjective “tall” is the key. 

Whitetop (hoary cress) can color 

entire meadows with a dazzling 

display of white flowers, but a 

month later the leaves dry on the 

fragile 1- to 1.5-foot-tall stems and 

the plants virtually disappear. Tall 

whitetop (perennial pepperweed) 

plants can easily reach heights 

of 6 feet or more and the stems 

are semiwoody at the base. Tall 

whitetop forms thickets that are 

visible all seasons of the year and 

the dead plant material persists for 

several years (Figure).

Monoculture can be an overused 

term in describing weed infestations, but tall whitetop infestations 

come very close to actual monocultures. The mass of creeping 

roots send up numerous shoots 

that form a closed canopy. Tall 

whitetop out-competes other vegetation for light, nutrients, and 

moisture. Tall whitetop infestations 

are closed to the recruitment of 

seedlings of other perennial species 

without substantial weed control 

treatments. Tall whitetop is native 

to eastern Europe and adjacent 

Asia. It is a fairly recent accidental 

introduction to North America 

and is presently found in all western States. It is a major weed in 

tidal marshes of San Francisco Bay 

and at an elevation of 7,500 feet in 

southern Colorado. Tall whitetop 

is highly adapted to saline–alkaline soils, but is not restricted to 

such soils. It is widely distributed 

in meadows of the Lake Tahoe 

Basin of California and Nevada, 

growing on neutral to acid soils. 

Tall whitetop is highly adapted to 

riparian and meadow ecosystems. 

It becomes distributed along irrigation and drainage ditches from 

which it invades agronomic fields. 

It is also a serious pest in alfalfa 

fields. Tall whitetop is a noxious 

weed in most western States. 

Despite its affinity for riparian and 

wetland environments, the weed 

has established roadside colonies 

in the Great Basin in salt desert 

environments in the 4- to 6-inch 

annual precipitation zone.

Discussion

Mechanical control of tall whitetop through tillage is not a valid 

weed control option because 

virtually every cut section of root 

will produce new plants. Grazing 

NO. 76 DATE 01/19/05

Range

Range

Resource Note 76

Page 1

Figure. Tall whitetop (Lepidium latifolium L.) infestation is present along the Trukee River in 

northwestern Nevada.
Range

Resource Note 76

Page 2

management can be used to suppress the dominance of tall whitetop, but only before the weed has 

gained complete dominance of the 

site. Grazing animals do not prefer 

the herbage of tall whitetop—

especially after the flower stalks 

have begun to elongate. A stocking rate that is sufficiently high to 

produce utilization of tall whitetop means that virtually every 

desirable plant in the community 

is excessively utilized. Goats can 

be forced to completely consume 

tall whitetop plants before flowering stalk elongation occurs, but 

this extreme level of utilization has 

no lasting weed control influence. 

These extreme grazing treatments 

do not enhance the efficacy of 

subsequent herbicide applications.

Presently, the only option for 

the control of tall whitetop is 

the repeated use of herbicides in 

conjunction with seeding perennial grasses. Annually, repeated 

applications of an amine form of 

2,4-dichlorophenoxyacetic acid 

(2,4-D) at the rate of 2 pounds of 

the active ingredient per acre will 

sufficiently suppress tall whitetop to allow the establishment of 

seedlings of perennial grasses. In 

the seedling year of the perennial grasses, 0.5 pounds per acre 

of 2,4-D can be applied over the 

grass seedlings after they reach the 

first true leaf stage of growth. On 

highly saline–alkaline soils in the 

Intermountain Area, tall wheatgrass (Elytrigia elongata) is the only 

perennial that will establish on 

the sites. After the tall whitetop is 

suppressed by the combination of 

2,4-D applications and the perennial grass establishment, tillage can 

be used to reduce the grass and 

establish adapted native shrubs, 

grasses, and broadleaf species.

Herbicides in the chemical family 

sulfonylurea are extremely effective 

for the control of tall whitetop, but 

they leave residues in alkaline soils 

that can inhibit the establishment 

of perennial grasses for as long as 

several years, depending on soil 

type and precipitation site. Sold 

under various trade names, these 

herbicides will kill tall whitetop but 

will also leave herbicide residues.

Conclusion

Tall whitetop is an exotic, highly 

invasive perennial weed. It causes 

great ecological damage to riparian and wetland habitats. This 

weed can be suppressed by repeated 

applications of the herbicide 2,4-D 

in conjunction with establishing 

good stands of perennial grasses.

Contact

James A. Young, U.S. Department 

of Agriculture

Exotic and Invasive Weed 

Management Research Unit

920 Valley Road

Reno, NV 89512 

Telephone: 775-784-6051 Ext. 229

Fax: 775-784-1712

Email: jayoung@scs.unr.edu

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