1. Locate and map all infestations.

Since witchweed is so prolific, finding each and every specimen of the foot-tall plant is vital. In addition to farmers checking their own land, scouts are sent out to locate infested sites – traveling on foot and on horseback early in the program and later in all-terrain vehicles. There are penalties for failing to report the weed…and bounties paid for sightings by eligible parties. “We typically have great cooperation from local farmers, who know the danger witchweed poses to their crops,” Tasker said.

2. Quarantine.

Though tiny witchweed seeds can be spread by wind or by water, human activity is often the most common culprit. For that reason, the areas where witchweed is found are quickly quarantined. Machinery used at the site and any crops harvested with soil attached must be cleaned thoroughly to remove witchweed seeds. Growing the grass crops favored by witchweed is strictly prohibited until the pest is totally eradicated in the quarantined segment of a field. Instead, farmers must leave the land fallow or convert to crops that are harvested well above the soil line.

3. Control.

Officials remove witchweed plants when they are found and use both foliar-applied and soil-applied herbicides for deterrence. Destroying the tens of thousands of microscopic seeds that may already be in the soil, though, is tougher and more time-consuming. Some seeds sprout right away and are killed by the herbicide. But others can lurk for years.

“Allowing grassy weeds to grow in the area or planting a crop that is susceptible to witchweed can actually trigger dormant sends to germinate, even if they are deep underground,” Tasker says. “We have examples of parasitic weed seeds remaining dormant in a fallow field for 50 years and then sprouting as soon as the preferred host crop is planted.”

One effective technique for controlling dormant seed is “suicide germination.” Ethylene gas – a natural ripening agent produced by fruits, vegetables and flowers – is injected into the soil and causes the seeds to sprout. Without a host plant to attach to, the new seedlings wither and die. Another approach is to fumigate the soil with chemicals to destroy any seeds that remain.

“Regardless of the technique used, we spot-check the fields for years afterwards to make certain the weed is truly eradicated,” Tasker says.

Dr. Tasker and Dr. Jim Westwood of Virginia Tech have organized a special day-long symposium on the witchweed program and other parasitic weeds that will be featured during the 51st annual meeting of the Weed Science Society of America. The event is scheduled for February 7-10, 2011, in Portland, Ore. To register or to find out more, visit http://www.wssa.net.