White Top (Hoary Cress)
Scientific Name: Cardaria draba

History/Origin
          There are three different white tops that have been introduced in to the United States. The white top that has a heart-shaped pod, also known as hoary cress, is native to western Mediterranean countries and the Middle East (Turkey, Israel, Syria, Iraq and Iran). Hoary cress probably entered North America in contaminated alfalfa seed. It was first found at Long Island, New York, in 1862. This weed is now introduced on every continent.

Biology/Ecology
          Hoary cress is a member of the mustard family. New plants can grow from both seed and root fragments. Leaves grow very rapidly after seedling emergence, and lateral roots develop within three weeks. Seedlings over-winter as rosettes, and usually bloom in May. After producing seed the plant continues to grow until heavy frost.

          Hoary cress is highly competitive once it is established, and can quickly dominate an area. Each flowering stem can produce 850 seeds annually. With the possibility of producing seed twice a year, the surrounding area can become saturated with seeds. Seeds are spread by wind, irrigation/waterways, and vehicles. Buried seeds remain viable for up to three years.

          Hoary cress doesn't rely on seedling establishment alone. A single plant can send out rhizomes that will spread out over 12 feet in the first year. This spread can continue to grow at a rate of 2-5 feet per year. These rhizomes send up shoots that act as a new plant. An average of 50 new shoots are produced every year. In addition to these creeping rhizomes, an extensive root system can grow up to 30 feet in 2-3 growing seasons. Lateral roots branch off a main tap root and spread though the surrounding area. Each root has adventitious buds that can develop into additional rhizomes and new shoots.

Known Impacts
Detrimental:
         Hoary cress can form a dense monoculture, similar to leafy spurge (Euphorbia esula), that displaces native plants, degrades wildlife habitat, and decreases species diversity. The loss of available forages is a serious threat to the cattle industry. In addition to this, hoary cress contains a toxin (glucosinolates) which can affect some cattle. This weed can also invade cultivated fields and reduce harvest yields.

Beneficial:
          The flowers provide nectar for honeybees, and the seeds can be used as a substitute for pepper.

Potential Locations
          This plant grows in open, unshaded areas, and is usually found with other exotics such as smooth bromegrass (Bromus inermus) and Russian knapweed (Centaurea repens). Hoary cress requires moderately wet sites (12-16 inches). Invasion of dry rangeland is unlikely. It prefers alkaline soils that are wet during late spring, but will grow in other soils. Lands most likely to be invaded are sub-irrigated pastures/croplands, rangelands, ditch banks, roadsides, and waste areas.

Control/Management
          This is a very difficult weed to control. Eradication is only an option with very small patches. Control requires an integrated plan with constant monitoring and work.
• Containment is the best option when dealing with this weed. Create a perimeter and attack any plants that get out.
• Digging can be successful on small new sites. New shoots must be dug up within 10 days after emergence. Sites must be rechecked throughout the growing season, for four years.
• Herbicides are effective, but are best used on small sites or around a perimeter (example herbicides: 2,4-D (4 and 6EC), Telar 75 (DF), Ally (60 DF), and Escort [look at herbicide label: free search]).
• No biocontrol available.
• Tilling or other mechanical means of control are not recommended (mowing with herbicide application can be effective).