Whitetop

Common Name(s)

Whitetop

Hoary cress

Scientific Name

Lepidium draba

Family

Mustard family (Brassicaceae)

Reasons for concern

This plant forms dense monocultures, which displace native plant species and reduce biodiversity, wildlife habitat, and forage production. Its very extensive root system makes it very difficult to control once it has become established. All roots can develop buds that can turn into rhizomes and shoots. It is also toxic to cattle.

Classification

Non-native. Included on the Arizona Noxious Weed List as Cardaria draba.

Botanical description

Upright, broadleaf, herbaceous plant, flat appearance with numerous stems, forming dense colonies.

Leaves

Grayish-green, arrowhead-shaped from ¾ inch to 3 inches long. Smaller upper leaves with two lobes that clasp stem. Leaves lower on stem longer and attached with short stalk.

Stem(s)

Stem grows to 24 inches, or smaller in dry sites. Slightly hairy. Multiple stems from base.

Flowers

Numerous small, fragrant white flowers with 4 petals, in dense flat-topped clusters atop plant. Gives plant a “white topped” appearance. Usually blooms in April through August.

Seeds

Seed pods heart-shaped and tipped with a beak. Seeds mature in June to July.

Roots

Extensive system of persistent, deep, vertical and horizontal roots (rhizomes) that easily produce new shoots at irregular intervals. Root fragments generate new plants. Only ¼ of the plant is seen above ground.

Native to

Mediterranean countries and Middle East

Where it grows

Commonly in dry, disturbed, alkaline soils but thrives in a variety of other soils with good moisture conditions. Likes sun. Elevation 3,300 to 7,900 feet.

Lifecycle

Perennial

Reproduction

By seed and roots

Weedy Characteristics

Whitetop can withstand extensive drought. It can flower and drop seed a second time in late summer and the seeds endure for many years. The root system is vigorous and difficult to eradicate. Rhizomes spread up to 12 feet in one year, and send up many shoots (up to 400 annually) that act as new plants. Both seeds and roots can be carried on equipment to new sites.

Look-alike Plants

Virginia pepperweed

Max Licher, swbiodiversity.org/seinet

There are several native pepperweeds (Lepidium spp.) in Northern Arizona. The leaves of common native species do not clasp the stem.

Control Strategies

Dig or pull individual plants for small infestations, taking care to remove all the root. Monitor frequently as remaining roots can produce new plants. Mow to reduce seed production and till to reduce plant density. Several herbicides are effective on actively growing plants but must be used for several years. Contact your local county extension office for more information on chemical control.

Gallery

References

Hoary cress Noxious Weed Control Board, Washington State University
Hoary cress Integrated Pest Management, Utah State University Extension

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