Noxious Weed of the Month

Hoary cress (Lepidium draba)

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Hoary cress is a perennial noxious weed in the mustard family that displaces native vegetation, reduces property value, and is toxic to humans and livestock. Photo by Tracie Oxford, Spokane County NWCB.

Now is the time to watch for emerging hoary cress, especially in eastern Washington and parts of northwest Washington. This toxic escaped ornamental is native to Southeastern Europe and the Middle East, though it has spread extensively throughout the rest of Europe. In the US, hoary cress was first found in 1862 and in Washington state in 1916, where it probably arrived with contaminated alfalfa seed. Hoary cress is a significant hazard to crop production under moist conditions and on irrigated land and is toxic to humans and livestock. 

Hoary cress easily displaces native vegetation in disturbed sites, pastures, roadsides, saline soils, and along riverbanks and waterways. It can reduce the value of high-price wheat lands. Hoary cress is a rhizomatous perennial that blooms early in April through July. Mature plants can produce nearly 5,000 seeds, making it a noxious weed to keep watch for and remove quickly. Hoary cress plants form extensive root systems, consisting of a vertical taproot that develops several lateral roots, which eventually become additional vertical roots that can burrow deeper than the original taproot. Whitetop can take root right down to the water table, where it can get plenty of moisture. A single plant can spread across an area 12 feet in diameter in its first year and 12-30 feet in its second and third growing seasons.

Hoary cress was listed as a Class C noxious weed in Washington State in 1988. The Class C status means that control is not required by the state. However, counties where hoary cress is a local priority may select the species for mandatory control. Check with your County Noxious Weed Control Board to see if you are required to control hoary cress on your property. Even if control is not required by your county, your Noxious Weed Control Board can provide educational and technical assistance for landowners interested in voluntary control. Many methods, including cultural, mechanical, and chemical, can be effective in controlling hoary cress infestations when used as a part of an Integrated Pest Management plan. Your County Noxious Weed Control Board can assist you in choosing appropriate methods. 

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In this picture, you can see the flat-topped flower clusters, lance shaped leaves common to mustard family plants, and upright yellow-green stems of hoary cress.

How to Identify

Hoary cress is a herbaceous, perennial weed in the mustard family with deep, extensive rhizomes. It generally grows to a height of about 2 feet, though it can sometimes reach 3 feet. The yellow-green stems of hoary cress are generally upright or somewhat trailing on the ground and branch off near their tops. On the lower parts of the stem, leaves have stalks, while upper leaves do not have stalks, instead, they have two lobes clasping the stem. Shiny blue-green leaves are alternately arranged, entire, and lance shaped with smooth or irregular toothing and a covering of soft white hairs. White flowers top the stems in flat-topped clusters of 4-petaled blooms. Hoary cress produces dark brown, inflated, round or heart shaped, hairless seed pods.

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Early in the hoary cress growth cycle, rosettes appear similar to other mustard species.

(Photo: Tracie Oxford, Spokane County NWCB)

The lance shaped leaves of hoary cress tend to be covered in soft white hairs. 
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In larger infestations, hoary cress flowers can look similar to a snow-dusted field. (Photo: Dennis Brown, Spokane County NWCB) Flat-topped clusters of 4-petaled white flowers top the upright stems of hoary cress.

Ways to Help

  • Control hoary cress plants on your property.
  • Contact your local County Noxious Weed Control Board for information about local volunteer opportunities.
  • Encourage friends and family to be on the lookout for hoary cress and other noxious weeds. 
  • Share your knowledge!

Resources for Further Reading:

View our profile on hoary cress

Read our written findings on hoary cress

Lincoln County Guide: Options for Controlling Hoary Cress

Spokane County NWCB Fact Sheet: Hoary Cress

San Juan County NWCB: Hoary Cress Poster

Hoary Cress Weed Report from the book Weed Control in Natural Areas in the Western United States