Whitetop (Hoary Cress) Control in Residential Situations

Whitetop (Hoary Cress) Control in Residential Situations

Taun Beddes, Michael Caron, Jacob Hadfield, and Corey Ransom

Summary

• Whitetop (Lepidium draba, formerly Cardaria draba) is a persistent, perennial weed that grows in open, sunny areas It spreads by seeds and rhizomes by several feet in a single season.

• It is common in wildlands, agricultural, and residential areas.

• It rapidly spreads into unhealthy lawns, landscape beds, and vegetable gardens.

• Non-herbicide control involves consistent, manual removal, shallow cultivation, or covering the affected soil with cardboard followed by a 3-inch layer of mulch. Solarizing affected areas is another option

• Conventional herbicides are effective for suppression and control We could find no organic herbicides labeled to suppress or control whitetop.

Description

Whitetop, also known as hoary cress, is a member of the mustard (Brassicacea) family. It reaches a height of 2 feet and has oblong to lance-shaped leaves. The stems exhibit slight hairiness, while the bluish-green leaves directly attach to the stems without petioles. During spring, densely clustered small white flowers cover the plant (Figure 1). It can sporadically flower on new growth throughout the growing season. The plant produces seeds enclosed in inflated, heart-shaped pods. A mature plant can yield between 1,200 and 4,800 seeds, with seeds remaining viable for 3–4 years.

Whitetop spreads vigorously through an extensive creeping root system, with up to 76% of its biomass located underground. The roots can extend as deep as 12–30 feet. Once established, this invasive species competes with neighboring plants for soil moisture and nutrients. Moreover, it releases allelopathic chemicals that hinder the germination of other species and diminish the vigor of established plants.

Figure 1. Whitetop Blooming in an Unused Field (left) and Growing Through an Ornamental Shrub (right)

Location

Whitetop commonly establishes itself in open, sunny areas, often in areas with recently disturbed soil. It thrives on stored soil moisture from winter and early spring precipitation. It outcompetes other plants and produces seeds rapidly. Capable of growing in salty soil, it is found along roadsides, in new construction areas, areas of the yard without dense plant or mulch cover, and in unhealthy lawns. It grows in most areas from low to high elevations

Lifecycle

Whitetop is perennial and emerges in early spring, with rosettes appearing earlier and flowering in late spring and early summer. If mowed or cut back, it may continue to flower sporadically on new growth In dry areas, it may go dormant until the following spring. The seeds are dispersed by wind, water, and human activities such as the movement of topsoil, mulches, and animal feeds.

Landscape Damage

Whitetop causes significant damage to landscapes by spreading through seed dispersion and rapid underground creeping root growth. It outcompetes many ornamental annuals, perennials, and newly planted shrubs if they are too small. Due to its allelopathic nature, do not add removed plants to compost piles but dispose of them in landfills instead. Whitetop is often not easily eradicated due to its rapid growth and tendency to spread into areas where desirable plants are grown.

Suppression and Control Options

Various control options are available for whitetop. The best strategy is to identify and remove newly germinated plants before they flower.

Smothering

In suitable situations, whitetop can be smothered. Start by cutting plants to ground level in the spring Remove the debris and cover the area with overlapping cardboard sheets. If needed, you can peg the cardboard down with landscape staples. Next, place 3–4 inches of mulch or wood shavings over the top of the cardboard Leave this in place for one year.

Solarizing

Solarization using UV-resistant, clear plastic sheeting is another option. With this method, cut the whitetop down to ground level and cover the area with clear plastic sheeting. The soil underneath the plastic is heated, killing the seeds and roots to 6 inches deep over the season. New sprouts are also prevented by succumbing to the heat under the plastic. Remove the plastic once the season is over. Consider lightly incorporating a few inches of compost into the uncovered soil a few inches deep.

Cultivating

Repeated tilling or cultivation of whitetop can reduce the population when exposed roots are removed. If roots are not removed, the weed may spread. Consistent cutting at ground level during the growing season can deplete the stored carbohydrates over several years, eventually killing the plant.

Herbicide Options

Conventional herbicides provide the fastest, most effective, and most comprehensive control (Table 1) They should be applied in spring before flower buds open, and they are also suitable in early fall.

Of the reduced-risk and organic herbicides we reviewed, none were labeled for controlling whitetop. If you use one of them in an area permitted by the product’s label, only the aboveground portions of the plants will be killed, and they will regenerate from roots. Controlling whitetop in this way requires spraying repeatedly over a few years

Remember, no matter what product you use, you should carefully read and adhere to labels for safety precautions, mixing ratios, and approved application areas.

Product Systemic Conventional Notes

2,4-D X X

Various lawn weed killers

Glyphosate

Metsulfuron

Many formulations are registered for lawn use.

We found no lawn weed killers specifically registered to control whitetop. However, spraying lawn weed killers containing 2,4-D on the lawn or other areas registered on the label to control other weeds will likely suppress or kill whitetop. Most lawn weed killers have use restrictions at temperatures above 85 °F, where they can form a vapor that drifts, damaging nontarget plants.

It is very effective when sprayed according to the label. When sprayed correctly, nearly 100% control can be achieved. Avoid getting spray on desirable plants.

Products containing metsulfuron are the most effective for controlling whitetop. Postemergence herbicides should be applied when plants are in flower to improve effectiveness. Metsulfuron is not readily available to homeowners. Review the label carefully before use.

References

Fire Effects Information System (FEIS). (2023) Cardaria spp. U.S. Forest Service and U.S. Department of Agriculture. https://www.fs.usda.gov/database/feis/plants/forb/carspp3/all.html

Midwest Invasive Species Information Network. (2023) Whitetop (Cardaria draba) Michigan State University. http://www.misin.msu.edu/facts/detail/?project=misin&id=371&cname=Whitetop

Monleon, R. G. (1989). Experiences with ammonium glufosinate in Spain. Industrias Chemicas. Conference Proceedings of the 4th ERWS Mediterranean Symposium on weed problems in the Mediterranean climates. Valencia, Spain. Volume 1.

Qasem, J. R. (2004). Allelopathic Plants 11: Cardaria draba (L.) Desv. Allelopathy Journal, 13(2), 165–172.

Sterling, G. R. (1951) The control of hoary cress in Alberta. Field Crops Branch Report, Alberta Department of Agriculture.

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Table 1. Herbicide Options for Controlling Whitetop

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June 2024

Utah State University Extension

Peer-Reviewed Fact Sheet

The authors provided the photos, and Depositphotos supplied the banner image.

Suggested citation:

Beddes, T., Caron, M., Hadfield, J., & Ransom, C. (2024). Whitetop (hoary cress) control in residential situations [Fact sheet]. Utah State University Extension.