I tell students in my ID class that weed identification is easy since very few species have traits that allow them to persist in disturbed habitats, thus you run into the same weeds over and over. After a few years in the business, you seldom encounter a weed you haven’t dealt with before. That premise was blown out of the water when I was involved in identifying a weed whose only known occurrence in the western hemisphere was New York City. Acalypha australis
Agronomists in Black Hawk County found an unknown weed infesting a large seed corn field. They sent a sample to Ames, which I declared ‘insufficient to identify'. Next, they emailed some images, and I was still stumped. Finally, they dug up several plants, put them in a five-gallon bucket and delivered them to my office. Now I had no excuse not to identify the plant (other than I'm a weed scientist, not a plant taxonomist).
I used my Britton & Brown Illustrated Flora of the Northeast United States to narrow it down to something in the spurge family (Euphorbiaceae), but none of the species matched (as a non-taxonomist, I match pictures rather than use plant keys). So, I took it to the curator of the Ada Hayden Herbarium, a true taxonomist, and she also struck out. She sent images to one of the spurge experts in the United States. He identified the plant as Acalypha australis, Asian copperleaf. He informed us that the only known occurrence of this plant in North America was in metropolitan New York City.
The discovery of this weed in a Black Hawk County field raises several questions: 1) how and when was it introduced?, 2) is it in other fields in the area?, and 3) does it pose a threat of becoming a permanent weed in Iowa crop fields? I don’t have good answers for any of these questions, but my thoughts are below.
1) My initial thought on how it got here would be via something involved with seed corn production since these operations frequently move equipment long distances. However, the lack of A. australis in other corn producing areas in the country reduces the likelihood of this. The seed company has been very cooperative and are not aware of this weed being a problem in other areas where they work. A cereal rye cover crop and wheat were grown in the field relatively recently, but I wouldn't expect A. australis seed to be present in those crop seeds. Due to the severity of the infestation, it is apparent that the weed has been present in the field for several years. The last time the field was in seed corn was four years ago. That might be sufficient time for it to build up to the current levels, but new weeds often take longer than that to adapt to new conditions.
2) An agronomist with the seed corn company found a few plants of A. australis in another seed corn field across the road, but the infestation was not severe like in the field where it was identified. I quickly scouted two other seed corn fields in the area and found no A. australis in those fields.
3) This is the most important question. The severity of the infestation in this field raises concern. The plant is not included on the list of weeds considered a threat to the U.S. according to the Federal Noxious Weed Act. However, a 2012 USDA APHIS Report states the plant has potential to be harmful to row crops, and that the plant is adapted to survival in about 75% of the United States.
Virginia copperleaf (Acalypha virginica) is a native plant in the same genus as Asian copperleaf, and this is what I initially thought the weed was. However, the bracts (modified leaves associated with flowers) on the Asian and Virginia copperleaf are completely different. Virginia copperleaf is an agronomic weed in the southeastern United States, but not considered to be one of the more difficult weeds to manage. The APHIS report stated that resistance to glyphosate and group 14 herbicides has been documented in the species.
It would be nice to determine how the plant was introduced to Iowa, but at this time I don’t have the resources to dig further into it (and the pragmatic side of me wonders how that knowledge would help us manage the weed). I collected seed capsules from several plants; hopefully the seed was mature enough not to be affected by the herbicides sprayed on the field following harvest. If the seed is viable, I will do some simple screening to see if this population is resistant to several common herbicide groups.
In the 30+ years I’ve worked in extension I have encountered many ‘uncommon’ weeds, but this is the first time I’ve been involved in identifying a plant previously not known to be present in the Midwest. Hopefully the weed turns out to be an interesting diversion from dealing with the current Palmer amaranth situation, rather than becoming another permanent weed in Iowa’s weed community.