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US weed ID · Pasture & rangeland weed
Hoary Cress (Whitetop)
Lepidium draba · Brassicaceae (mustard family)
- Perennial
- Broadleaf
- Non-native
- Toxic / hazard
If a field edge or pasture turns white in spring with flat-topped sprays of tiny four-petaled flowers over gray-green, stem-clasping leaves, you are probably looking at hoary cress, also called whitetop.
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Considered at least mildly toxic to livestock. It contains glucosinolates, sulfur-bearing compounds that can turn into toxic substances in an animal's digestive tract, and fresh leaves have tested at 0.7 to 2.7 percent sulfur, well above the 0.4 percent that most grazing animals can tolerate. Cattle that eat it are reported to give dairy products an off flavor. Montana State University Extension lists it as toxic to cattle, and Washington's weed board flags it for livestock and people. Do not rely on it as forage; where animals must graze infested range, a Forest Service review passes along advice to supply extra iodine, use mature animals that are not nursing or milking, and limit how much hoary cress they can reach.
What is hoary cress?
Hoary cress is a creeping perennial mustard from Europe, Asia and northern Africa that turns pastures and field edges white in spring. Accurate hoary cress identification starts with flat-topped clusters of tiny white flowers and arrowhead-shaped upper leaves that clasp the stem. Because several related plants share the whitetop name, hoary cress (whitetop) identification also means checking the seed pods. USDA's PLANTS database still lists it under its former scientific name, Cardaria draba.
It grows into dense monocultures that push out native plants and forage and lower the value of crop land, especially wheat ground. It spreads underground, and root pieces left after digging or tilling grow into new plants, so casual removal tends to make patches bigger.
Hoary Cress identification: what to look for
Look in spring while the white flowers are up. Check the upper leaves for two small lobes that wrap around the stem, then look at the seed pods: hoary cress pods are inflated, hairless and rounded to heart-shaped. Plants usually grow in dense patches linked by creeping roots.
- Leaves: Alternate, blue-green to gray-green, lance- to arrowhead-shaped; upper leaves clasp the stem.
- Stems: Upright to somewhat trailing, branched near the top, about 2 feet tall.
- Flowers: Tiny, white, four petals, on short stalks in dense flat-topped clusters.
- Seed pods: Small, inflated, hairless, rounded to heart-shaped, with two reddish-brown seeds inside.
- Hairs: Foliage often carries soft white hairs, though some plants are nearly hairless.
- Roots: Creeping rhizomes that connect plants into colonies and regrow from fragments; roots commonly run 2 1/2 feet deep or more.
Where it grows in the United States
USDA lists hoary cress as an introduced plant in the lower 48 states and Canada. A Forest Service review records it in 38 states, reaching from New England and the mid-Atlantic through the Midwest and Great Plains to the Rockies, the Southwest and the Pacific coast, and notes that it is largely missing only from the southern California deserts (Mojave and Colorado) and the south-central states. It is a well-known problem in the interior Pacific Northwest and northern Rockies: Washington regulates it as a noxious weed, Montana State University Extension includes it in its guide to Montana's noxious weeds, and the Pacific Northwest weed management handbook gives it its own control entry. The same review calls it the most common whitetop on North American rangeland and finds it most invasive in the big sagebrush country of eastern Oregon, where it covers more than 100,000 hectares (roughly 250,000 acres).
It favors moist or irrigated ground and saline soils, so look for it in irrigated fields and pastures, along ditch and river banks and waterways, on roadsides and on other disturbed sites. In wheat country it competes directly with the crop. Disturbance spreads it, especially tillage that drags root pieces to new spots.
Hoary Cress look-alikes
A few plants get mixed up with it:
- Hairy whitetop (Lepidium appelianum): Nearly identical in flower, but its rounder seed pods are covered with fine hairs; hoary cress pods are hairless.
- Perennial pepperweed (Lepidium latifolium): Also a white-flowered perennial mustard, but often taller (up to 6 feet), with larger, waxy, bright green to gray-green leaves that have a whitish midvein and do not clasp the stem.
- Hoary alyssum (Berteroa incana): Another hairy, white-flowered mustard, but its four petals are deeply notched, its hairs are star-shaped, its stem leaves press flat against the stem, and its flattened oval pods hold 6 to 14 seeds.
How to get rid of hoary cress
Pulling, digging and mowing: Digging and pulling are strongly discouraged on established patches, because small root pieces left behind become new plants and the patch often grows. In crop fields, intensive tillage repeated over three to four years can kill the root system; anything less tends to spread fragments. Clean root pieces off tillage equipment before moving to a new field.
Cultural control matters most, because the right conditions keep it from coming back.
- Graze sheep or goats repeatedly to keep plants from setting seed; talk with your extension office about livestock safety first, since toxicity to cattle is reported.
- Keep irrigated pastures and field margins in dense, competitive cover so it has less room to establish.
- In grain fields, a well-timed in-crop treatment helps; spraying before spring plowing, when possible, gets ahead of it.
- Avoid moving soil or equipment from infested fields without cleaning it.
Herbicides: The Pacific Northwest Weed Management Handbook names several active ingredients for hoary cress. 2,4-D works only when applied early, before flower buds form, and is labeled for non-crop sites and for selective use in cereals; spraying before spring plowing helps. Chlorsulfuron and metsulfuron are applied from prebloom to bloom or to fall rosettes and are limited to pasture, rangeland and non-crop sites (chlorsulfuron also on CRP land). Imazapic goes on after full bloom until plants dry down, or on fall rosettes, and carries crop rotation restrictions. A combination of aminocyclopyrachlor and chlorsulfuron is applied to actively growing plants but can injure nearby trees and shrubs. Re-treat in fall if new growth appears. Read the label for your site (crop, pasture, rangeland, roadside or yard), rotation and grazing limits; the label is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.
Keeping it from coming back
Hoary cress moves mostly as root pieces and seed carried on equipment and in soil. Clean tillage implements and mowers before leaving an infested field, and do not haul soil or fill from infested ground.
Scout irrigated fields, ditch banks and roadsides in spring, when the white flowers make it easy to spot, and treat new patches before they knit together underground. Keep competitive vegetation on field margins and avoid leaving disturbed soil bare.
Questions we get about hoary cress
Is hoary cress the same as whitetop?
Yes. Whitetop is a common name for hoary cress (Lepidium draba, formerly Cardaria draba). Related plants such as hairy whitetop also get called whitetop; pod hairiness tells them apart.
Should I pull or till hoary cress?
Not casually. Root fragments left behind regrow, so pulling and occasional tillage often spread it. Only intensive, repeated tillage over several years kills the roots.
Is hoary cress poisonous to livestock?
Mildly, by most accounts. It contains sulfur-rich glucosinolates that can form toxic compounds in the gut, and its leaves can carry up to several times the sulfur that grazing animals tolerate. Montana State University Extension lists it as toxic to cattle. Do not count on it as forage.
When should I spray whitetop?
It depends on the active ingredient: some work only early before bud stage, others from prebloom through bloom, after bloom or on fall rosettes. Follow the label for your product and site.
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Sources: Washington State Noxious Weed Control Board: Hoary Cress · Washington State Noxious Weed Control Board: Noxious Weeds That Harm Washington State, Eastern WA Field Guide · PNW Weed Management Handbook: Control of Problem Weeds · Montana State University Extension: Montana's Noxious Weeds (EB0159) · USDA PLANTS Database: Cardaria draba (CADR) · USDA APHIS: Federal Noxious Weed List · Colorado Department of Agriculture: Noxious Weed Species · USDA Forest Service Fire Effects Information System: Cardaria spp. (hoary cress, whitetop)
Herbicides: always read and follow the product label. Pesticide applicator licensing rules vary by state.