Whitetop, also known as hoary cress was first identified in Montana in 1916.  It is native to northern Eurasia and has been introduced onto every continent in the world.  This plant probably arrived in North American via contaminated alfalfa seed.  Whitetop can be found in 54 of 56 Montana counties and infests a total of around 32,000 acres.  This plant is generally found in disturbed habitats that are associated with other weed species.

Concerns

Whitetop is an herbaceous, relatively long-lived, rhizomatous perennial weed.  It tends to form dense monocultures, which displace native plant species and reduce biodiversity, wildlife habitat, and forage production.  Whitetop is of major concern because of the difficulty to control.  This plant has a root system similar to leafy spurge.  The root growth is rapid.  The vertical taproot can reach depths of 12-30 feet by the second or third growing season.  This taproot develops several lateral roots that eventually turn downwards and become taproots themselves often reaching greater depths than the parent taproot.  All of these roots develop buds, which turn into rhizomes and shoots.  This complex root system combined with the ability of a single plant to produce between 1,200 and 4,800 seeds each year are the reason that it is so difficult to control.  It is also a major concern because it can be toxic to cattle.

Identification

Growth begins early in the spring for this plant because it germinates in the fall.  Blue-green lance shaped leaves appear in a basal rosette very early.   The stem leaves are also a blue-green color with the lower leaves on stalks and the upper leaves clasping the stem.  The flowers are white and consist of four petals laid out in a cross.  The individual flowers are small but dense clusters of the flowers create the white, flat-top appearance.  The seed capsules are broad, flat, and heart-shaped.

What can you do?

If you have a small infestation hand pulling can provide control.  For the control to be successful all underground parts of the plant must also be removed and it may take 2-4 years for complete eradication.  Mowing the plant to ground level will reduce biomass and seed production but does not provide long term control.  Whitetop can be controlled using herbicides but it is difficult and requires aggressive reapplication.  Another option is using sheep to graze on this plant.  The best method for the control of Whitetop is an integrated weed management plan utilizing all of the above options with prolonged effort.  If you have any questions or think that you have a patch of Whitetop on your property call your local county weed district.

Visit the MWCA Weed ID pages for additional information and pictures of whitetop.

This series of articles was developed by Ravalli County.  If you would like to use these articles please contact Ravalli County Weed District Weed Coordinator at (406) 777-5842.

Hoary cress (Cardaria sp.) is also known as whitetop or little whitetop. It is a deep-rooted, creeping perennial mustard that spreads by seed and vegetative root growth. There are three species including lens-podded hoary cress (C. chalapensis), heart-podded hoary cress (C. draba); and globe-podded hoary cress (C. pubescens). Hoary cress is well adapted to moist sites including sub-irrigated and irrigated pastures and hay-fields, rangeland, and disturbed sites such as roadsides, railways, and ditch banks. It can also invade cropland including small grain and alfalfa fields. Hoary cress may reduce crop yields, displace native plants, and reduce biodiversity, wildlife habitat, and forage production. The weed contains glucosinolates that can be toxic to cattle (McInnis et al. 1993). Hoary cress begins growth early in spring from a rhizomatous root system and flowers by April or early May. Seeds are produced about a month after flowering.

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