JoAnne Skelly: Perennial pepperweed is another noxious weed

I have been writing lately on noxious weeds because weed season is going strong and timely action makes all the difference. Weeds, especially noxious ones, seem to be growing and blooming earlier than I remember. Yesterday when I was out walking, I saw perennial pepperweed, more commonly called tall whitetop, already in bloom.

Perennial pepperweed (PPW) is a noxious weed introduced from southeastern Europe and Asia. It is a tall dark-green plant with white flowers. The flowers look like baby’s breath, the fluffy white filler in florist bouquets. PPW forms large dense stands that dominate open space, fields and wetlands. It grows on dry or wet sites. Its roots range in size from the diameter of a pencil to the size of a person’s wrist and may extend three to 10 feet. These rapidly spreading roots readily propagate more plants. In addition, a stand of PPW can produce more than six billion seeds per acre.

PPW pieces and seeds can be spread in fill dirt, hay or straw or top soil moved during construction. It can also spread on eroded soil or on equipment moving from an infested area. Most animals will not eat it, so it can destroy a field for forage. It can pull salts up from deep in the soil and deposit them on the surface, ruining the site for native plants or other salt-intolerant species. Loss of native plant habitats means fewer areas for birds and animals. Native trees cannot establish in dense stands of PPW. And, although PPW has invasive roots, they do not hold soil, which results in increased erosion during rain or flood events.

PPW is quite difficult to control, particularly in sensitive wetland or stream areas. In general, hand pulling produces more plants, so it is only a viable management technique when there are just a few plants that are dug out repeatedly week after week. Mowing or tilling simply produces more plants. Generally, PPW has to be controlled with herbicides applied at bud to early bloom stage. This prevents seed set. A second application to any regrowth in the fall appears to give the best control. In order to get good chemical coverage, it is important to remove the dead previous year’s growth. Herbicides must be applied at the right time of year for multiple years for good control. Only certain chemicals work on PPW, so read the herbicide label to see if it includes it. Revegetation with desirable plants is essential to long-term control.

For more information, contact me at 775-887-2252 or skellyj@unce.unr.edu.

JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at skellyj@unce.unr.edu or 887-2252.