What's the pretty white flower?

Solid patches of creamy white flowers are showing up across Northern Nevada. A gardener called, asking whether this white-flowered plant was a weed. She brought it in for identification. It is an invasive weed, hoary cress, Cardaria draba, that is listed as a noxious weed in Nevada Revised Statutes.

This member of the mustard family is a long-lived, highly competitive perennial with invasive roots called rhizomes. We see hoary cress along roadsides, ditch banks and other disturbed areas. It also grows in underirrigated pastures, hayfields and meadows, and even in home landscapes. It reproduces from seed and root fragments. It can produce up to 850 seeds per stem and 4,800 seeds per plant. The seeds spread by wind, animals, vehicles, people and water, and stay viable in the soil for up to three years. Roots can spread up to 12 feet the first season, and two to five feet each subsequent season. These roots can also grow several feet deep. One hoary cress plant can produce 450 new shoots from its roots in one growing season. This ability to reproduce so efficiently explains why hoary cress is so invasive!

Hoary cress can grow to two feet tall, but is generally shorter in dry sites. It has multiple stems rising from the base and bluish-green, waxy leaves. The flowers form rounded clusters at the top of each stalk.

Although tilling, hoeing and other mechanical methods of control are not widely recommended because they spread the plant, relentlessly digging out small patches of the weed each week may reduce infestations over several years. To maximize results, remove as many roots as possible each time you dig. You can limit seed dispersal by cutting the flower heads off before they go to seed, but this will also stimulate more production from the roots. Chemical control is usually required, but is challenging if the hoary cress is mixed in with desirable plants. Products containing 2,4-D or glyphosate as the active ingredient can provide decent control in home landscapes. But, you must carefully apply these products, as both chemicals also kill flowers, trees and shrubs. Glyphosate also kills lawn and other grasses.

Revegetating treated areas with competitive desirable plants is critical to achieving long-term management of hoary cress. In addition, any treated area must be monitored and spot-treated for years.

It is imperative to control this weed early, before it takes over your landscape, yard or fields. Early control will save you money, time and effort, and protect the surrounding environment.

For more information on controlling weeds and gardening, contact me, 887-2252 or skellyj@unce.unr.edu, or your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at www.unce.unr.edu. "Ask a Master Gardener" by e-mailing mastegardeners@unce.unr.edu.

• JoAnne Skelly is the Carson City / Storey County Extension Educator for University of Nevada Cooperative Extension.

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