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Whitetop is formidable weed

Posted: Friday, Mar 15th, 2013

SLVWMA Coordinator

VALLEY ó Cardaria draba, better known as Hoary Cress, whiteweed or small whitetop, is a very familiar foe here in the San Luis Valley.

Once established in a pasture, it is a highly competitive weed found unpalatable to most livestock and wild grazers, and it has been creeping its way into pastures, fields, croplands and meadows across most of the United States for many years. White top is native to western Asia and Eastern Europe and most likely entered the United State by contaminated alfalfa seeds in the early 1900s.

Identifying white top in the Valley is rather simple because itís everywhere. White top is a small perennial plant, meaning it can grow throughout the year and persist for many years thereafter. The plant emerges in early spring and blooms between May and June, producing many white flowers with four petals with a flat-topped appearance, hence the name. They typically grow between 0.1 to 0.5 m tall with lance shaped leaves.

Each plant can produce 1,200 to 4,500 seeds annually that can spread by wind, vehicles and even irrigation systems, saturating their surrounding areas very quickly. Buried seeds can remain viable in the soil for up to three years even through harsh, freezing winters that are common here in the Valley. White top does not rely on seed dispersal for taking over the landscape. Each plant can establish an extensive lateral root system that can spread out to 30 feet within two to three years, sending off up to 50 new shoots per year from that single root structure.

In general, white top grows better in alkali soils with moderate amounts of moisture. White top has a knack for taking over disturbed sites, including areas with extensive grazing or tilling. It is commonly found in fields, meadows, pastures, open grasslands, waste areas, roadsides, gardens, feedlots, watercourses, along irrigation ditches and riparian habitats.

The most dangerous characteristic of white top is its ability to spread quickly and crowd out native plants. Within two to three years of entering an area, it can become a monoculture, meaning that it is the only plant growing in the area. Even worse, grazing, irrigation and cultivation can promote the spread of white top, all of which are very common practices here in Valley. It is an extremely persistent noxious weed, as many farmers and ranchers have experienced since its introduction into the Valley.

In the last 12 years, white top has invaded nearly 10,000 acres of private and public property in the Valley according to Integrated Land Services GPS data.

Control of white top is very difficult because of the perennial root system, abundant seed production and diverse habitats of the plant. Prevention, early detection, containment and rapid response are key to fighting white top infestations, but no single treatment method will be effective. Instead, an aggressive, integrated weed management approach must be taken.

Prevention is certainly the best way control method for white top. Maintaining healthy native plant communities or planting competitive legumes, such as alfalfa, can reduce the likelihood of an invasion. Also, quarantining grazing animals that have come in contact with white top or grazed in a white top infested area is important, and minimizing soil disturbances will lower seed dispersal and production.

Once an area has been infested, an integrated approach must be taken to ensure proper eradication. This means using a combination of mechanical, chemical and biological control methods. The Colorado Department of Agriculture (CDA) recommends mowing the plant continuously before it flowers in the early spring and then continuing to mow it down throughout the summer. This will drastically limit the plants ability to produce seeds.

Mowing alone, however, will not do the trick and must also be followed up with herbicide treatment. The CDA and Wetlands Dynamic, a local wetland management LLC, recommends using the herbicides Plateau (Imazapic), 2,4-D Amine or Camaign, which usually take about three years of treatment in order to be successful. These herbicides are most effective when applied correctly at the appropriate growth stage. Burning and grazing by goats and sheep can also be used.

A combination of management techniques should be aimed at reducing residual growth and stressing out the plant enough to the point where its growth rate is reduced. Timing is key for all management techniques. For example, Wetlands Dynamic LLC recommends that burning in the fall and then grazing in the spring prior to flowering or burning in the winter and spring, and then applying an herbicide later in the summer during the flowering stage.

Anyone who a problem with white top in the Valley, please contact a local weed district, NRCS or conservation district. Those phone numbers can be found on the San Luis Valley Weed Management Website at www.slvwma.org. Another contact is the SLVWMA coordinator at 589-6432, ex123 or via email at lopitz15@gmail.com. There is also information available through the Colorado Weed Management Associationís website www.cwma.org or the Colorado Department of Agricultureís website www.colorado.ag/gov.

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