Plant Records: More Weed Than You Ever Need

Name: Palmer amaranth
Scientific name: Amaranthus palmeri
Known for: Destroyer of crop yields
Record broken: Weed of the Year two years running (2014-2015)

Amaranthus palmerii,
Amaranthus palmerii, pigweed, photographed in Phoenix, Arizona. Photo: Pompilid / Wikipedia.

Palmer amaranth, also known as Palmer’s pigweed, is a fast growing and competitive plant quickly becoming a serious problem for crop farmers across North America. It is edible, and was initially grown by Native Americans to be eaten as a cooked, or dried, green vegetable, as well as for its abundant seed yield. This is the trait that has turned Palmer amaranth into enemy number one for many farmers across the continent. So much so that it was voted Weed of the Year two years in a row, a feat unmatched by any other plant.

Its status as such is well deserved; it is, reportedly capable of producing up to one million seeds per plant per season, which are viable in the soil for up to six years, making eradication very difficult. It is also a very fast growing plant, growing up to two inches in a day. This allows it to out compete crops and reduce germination by 78%. Not only that, but Palmer amaranth has been shown to exhibit resistance to six different herbicide groups. This resistance spreads rapidly through populations because Palmer amaranth is a dioecious plant species, meaning the male and female plants are distinct organisms (Franssen et al., 2001a). Male plants release pollen, which travels on the wind and pollinates the ovules on the female plants. If the male pollen is carrying a resistance gene, this can easily be transferred to female plants in neighbouring fields, and large areas can soon be compromised.

There has also been some evidence  that this herbicide resistance gene can be transferred via hybridisation within the Amaranthus genus (Franssen et al., 2001b). Many other species in this genus, such as Waterhemp, Amaranthus tuberculatus, also carry weed status. Much like antibiotic resistance in bacteria, herbicide resistance is an increasing concern in the agricultural world. It has the capability of causing seriously negative yield impacts, and leaving farmers with no alternative but to mow down their crops. When there are no more herbicides to use, the only method of destroying this pest will be mechanical removal, which is a costly and time consuming method. Strict eradication methods must be followed to prevent this weed of the year from causing permanent and devastating damage to the agricultural industry in North America (Price et al., 2011).

Reference List

Aaron S. Franssen, Daniel Z. Skinner, Kassim Al-Khatib, Michael J. Horak, 2001a, ' Pollen morphological differences in Amaranthus species and interspecific hybrids ', Weed Science, vol. 49, no. 6, pp. 732-737[0732:pmdias];2

Aaron S. Franssen, Daniel Z. Skinner, Kassim Al-Khatib, Michael J. Horak, Peter A. Kulakow, 2001b, ' Interspecific hybridization and gene flow of ALS resistance in Amaranthus species ', Weed Science, vol. 49, no. 5, pp. 598-606[0598:ihagfo];2

A. J. Price, K. S. Balkcom, S. A. Culpepper, J. A. Kelton, R. L. Nichols, H. Schomberg, 2011, 'Glyphosate-resistant Palmer amaranth: A threat to conservation tillage', Journal of Soil and Water Conservation, vol. 66, no. 4, pp. 265-275