In the early 1980s, a devastating parasitic weed was found in a tomato field in California.  The infestation of branched broomrape (Orobanche ramosa) was treated aggressively and the field was quarantined to prevent further spread.  When tomatoes were planted in the same spot more than two decades later, though, the branched broomrape quickly returned.  

According to Lee Van Wychen, Ph.D., science policy director of the Weed Science Society of America, the recurrence is not a surprise.  “When weed seeds drop to the soil, some can remain viable for many decades,” Van Wychen says.  “Effective control requires a long-term commitment.”

Several alternatives are available to manage noxious weed seeds that become part of the soil seed bank.  One is to quarantine the area and leave the seeds undisturbed until they are no longer viable.  But as the broomrape example shows, the length of time the area will need to be quarantined is an unknown.  

In some instances, the soil is fumigated in an attempt to destroy noxious weed seeds.  In other instances, the soil is lightly tilled and a nitrogen fertilizer applied to promote germination and encourage the seeds to sprout.  Once they’ve emerged, the weeds are pulled, tilled or treated with an herbicide to keep them from reseeding.

“None of these options is a magic bullet that will work overnight or kill 100 percent of the weed seeds each and every time,” Van Wychen says.  “Persistence is the key.”