Palmer amaranth was first detected in Iowa in 2013 in Harrison County, and until recently the invasive weed had been found in four additional counties. About a month ago, two landowners (both professional agronomists) detected Palmer amaranth in fields planted in spring 2016 with native seed mixes for conservation purposes. In the time since those July detections, Palmer amaranth has been found in an additional five counties (multiple fields in several of the counties). We think it’s safe to say the calm before the storm has ended.
Palmer amaranth in conservation plantings
The finding of Palmer amaranth in new conservation plantings was not a complete surprise since Ohio had reported similar problems one or two years ago. However, what was unexpected is that the Palmer amaranth was found in fields planted with native seed mixes purchased from several seed vendors, some Iowa-based and others from states with widespread Palmer amaranth infestations. Our initial thought was that Palmer amaranth would likely be brought in with seed mixes from states with greater Palmer amaranth problems than Iowa.
Earlier this week I spent a full day inspecting fields of an Iowa-based vendor of native seeds. We did not find any Palmer amaranth, yet their seed mixes have been the source of several infestations. Most providers of native seed mixes are unable to produce all the species desired in these mixes, thus they purchase seed from other producers. The externally purchased seed may be the source of the Palmer amaranth in Iowa produced seed mixes. I was impressed by the commitment of the owner to identify how Palmer amaranth was getting into their seed mixes and eliminate the source of contamination.
Palmer amaranth in conventional crop fields
It is important to recognize that Palmer amaranth can be introduced by other mechanisms. The initial Iowa infestations were started with Palmer amaranth seed introduced via movement of equipment and materials associated with traditional crop production. The most recently discovered Palmer amaranth infestation is at a dairy farm in Dubuque county; the weed is believed to have been introduced via hay imported from the southern United States. We appreciate the diligence of the coop agronomist that identified the infestation. A dairy farm in southeast Iowa was the site of an earlier Palmer amaranth infestation.
Preventing permanent infestation
The good news is that Palmer amaranth is maturing, thus it is easier to spot among the sea of waterhemp due to the presence of seedheads. The bad news is that most plants will already have produced viable seed. It is important to be vigilant and identify infested fields before Palmer amaranth establishes a permanent seed bank. If possible, remove female plants from the field and dispose in a manner that kills the seed (burn, compost, bury). Regardless of whether plants are removed, in future years closely monitor these fields and implement effective control tactics. Even for a prolific seed producer such as Palmer amaranth, it typically takes a few years to establish a permanent population. Spending extra dollars now to prevent a permanent infestation will more than pay for itself in the long haul.
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