- Author: Lynn M. Sosnoskie
- Author: Ted Webster
- Author: Stanley Culpepper
Despite the fact that I am currently working in orchards and vineyards, I receive lots of requests to discuss Palmer amaranth (Amaranthus palmeri S. Wats.), a weed that I spent almost six years studying. It's the scourge of the Southeastern United States - and a weed that I think everyone should be (or at least become) very conscious of.
Considering that Palmer amaranth is starting to rear an ugly head in California cotton and perennial cropping systems, I thought that I might start a series of blogs revisiting some of the research that I was a partner to in Georgia.
First things first, though...what is Palmer amaranth?
Palmer amaranth, also commonly known as careless-weed (because it can get away from you if you are careless...), is a tall (up to two or three meters) and erect summer annual that is native to the Mexican states of Sonora and Baja California, as well parts of southern Arizona and California. A review of the USDA PLANTS database (http://plants.usda.gov) suggests that the species has been encountered in 30 US states and one Canadian province.
The amaranths are sometimes difficult to distinguish from each other, especially at the seedling stage. The best identifiers of Palmer amaranth, to date, include: 1) petioles that are as long, or longer, than the leaf blades and 2) male and female flowers that are held on separate plants in long, terminal spikes. The female flowers possess pointed bracts that make the flower-heads sharp to the touch; male flower-heads, in comparison, are much softer. The reproductive potential of the species is astounding; under ideal growing conditions, a single female plant can produce >500,000 seeds.
For more detailed information regarding the identification of Palmer amaranth, you can refer to the following weed guides: 'Weeds of the South' and 'Weeds of the Midwestern United States and Central Canada' (Disclaimer: I am a contributor to both guides), as well as 'Weeds of the West.'
Figure 2. Male Palmer amaranth. Photo credit: Lynn Sosnoskie, UC-Davis (CA)
Palmer amaranth plants can grow rapidly; the species can easily out-compete corn, soybean, peanuts and cotton if the seedlings become established early in the cropping season. Palmer's rooting structure may also contribute to its competitiveness; results from published studies suggest that Palmer amaranth may produce more roots than soybean and that these roots are potentially more effective at penetrating compacted soil layers (and, therefore accessing additional moisture and nutrients).
According to the International Survey of Herbicide Resistant Weeds (http://www.weedscience.org/In.asp), Palmer amaranth biotypes in the US have developed resistance to the dinitroanilines, photosystem II-inhibitors, ALS-inhibitors , 4-HPPD-inhibitors and glyphosate. Mississippi, Georgia, Kansas and Tennessee have confirmed cases of multiple resistances in Palmer amaranth (i.e. individual biotypes that are resistant to more than one herbicide mechanism of action). In some cases, the resistant biotype is thought to occupy only a few acres; in other instances, the resistant biotype is spread over a majority of the arable farmland. For example, in Georgia, county extension agents estimate that 90% of all cotton acres are infested to some degree with glyphosate-resistant Palmer amaranth.
Herbicide-resistance, particularly glyphosate-resistance, in Palmer amaranth is a significant economic concern for cotton growers in the Southeastern US. Prior to the development of glyphosate-resistance in Palmer amaranth, growers in Georgia spent, on average, $25 to $35 per acre on herbicides for weed control in cotton. Since the discovery of glyphosate-resistance (in 2004), herbicide costs have nearly doubled. The cost of additional, cultural control methods has also increased. From 2000-2005, approximately 3% to 5% of Georgia's cotton acres were hand-weeded at a cost of $2 to $4 per acre (when averaged over all farm acres); from 2006-2010, 50% to 70% of all cotton acres were hand-weeded at an average cost of $22 to $24 per acre.
Figure 3. Palmer amaranth infesting cotton. Photo credit: Larry Steckel, UT (TN).
- Author: Ben Faber
Occasionally plants show up in our office for identification and no one in the office knows what it is. So it's sent off to to others who might know. This was the case of a perennial amaranth, also called goosefoot for some reason. The is Chenopodium californicum, also know as Blitum californicum.
Like other amaranths, it can be upright to 3 feet in height, or if mowed or grazed be more flattened or decumbent. It has a thick, fleshy stem that along with the leaves can be eaten. I guess pigs like it, because it's also called pigweed.
The leaves look sort of lettuce like, which gives it another name - Indian Lettuce.
The stem has also been used for making soap, which gives it another name of soaproot. Which is not to be confused with another soaproot, Chloragulum. Plant names can be confusing.
Chenopodium californicum, Blitum,goosefoot, pigweed, soaproot grows in the chaparral on slopes and in foothill woodlands, mainly along the coast.
Calflora: http://www.calflora.org/cgi-bin/species_query.cgi?where-calrecnum=1968
Plants for a Future: https://www.pfaf.org/USER/Plant.aspx?LatinName=Chenopodium+californicum
![chenopodium californicum chenopodium californicum](https://ucanr.edu/blogs/Topics/blogfiles/51544.jpg)
- Author: Oleg Daugovish
Using organic herbicides in production fields and non-crop areas.
The forecasts call for rainy winter and that means a lot of weeds. During dry times perennial weeds tend to grow better than annual weeds, since perennial structures such as underground rhizomes or tubers can support them and give competitive advantage. Seed of annual weeds in dry soil may have been losing viability, senescing or eaten during this time, but many have remained dormant and look forward to the wet winter us much as the rest of us.
Controlling weeds ‘organically' is always an extra challenge whether you are in a certified field or in an area where synthetic herbicides are not desired. Hand-weeding, already expensive, is even a greater burden with limited labor availability, and frankly not much fun either. Of course sanitation and prevention, mechanical and cultural management are essential in organic systems. That requires time and commitment and can quickly become your not-so-favorite pastime.
Organic herbicides have traditionally been contact materials with no systemic activity. This means that they only affect tissue that they contact and do not translocate through the plant like most synthetic herbicides. Thus, good coverage is critical for these contact materials. Many years ago the first herbicides were sulfuric acid and diesel fuel, current organic materials are often acids or oils too, although a lot more benign.
Recent trials by the University of California weed scientists showed that several organic herbicides provided decent control of easy to control pigweed and nightshade when they were small. When weeds were 12 days old, a mixture of 45% clove and 45% cinnamon oil, 20%-acetic acid and d-limonene gave 61-89% control; however only d-limonene controlled 19-day old weeds and none was effective on one-month old ones. As weeds get bigger they also develop a protective cuticle that minimizes efficacy of these herbicides.
This year we conducted trials with a recently OMRI approved herbicide for row crops, trees and vines that is a mix of caprylic and capric acids. It disrupts cell membranes of plans and causes the contents to leak and plants to desiccate. It worked well at 6 to 9% by volume mixture with water and gave 90% control of little mallow and >95% of annual sowthistle compared to untreated checks. We have also tested it in organic strawberry furrows before planting the crop to prevent potential injury from drift. Furrow cultivation does not get close to the plastic mulch that covers the beds to prevent tears, so the weeds in that zone are good target for the herbicide. This fatty acid herbicide provided excellent control of common lambsquarter, reduced the growth of common purslane but didn't do much for yellow nutsedge - one of our notoriously difficult to control perennial weeds (Figure). The bigger weeds need higher rates (9% is the maximum labeled rate) and better coverage. When you have multiple layers of weed leaf canopy and diverse architecture some plants or their parts may be protected by others that intercept the deposition of the herbicide. When on target, this contact material acts fast – you can see results within 2-3 days, however, it does nothing to weed propagules in soil and has no residual activity against wind-dispersed weed seed that fly in after application. This means the control does not last and you will need additional applications or other control measures. Repeated application is not a problem in a non-crop area and is a great way to deplete your weed seedbank, but crop protection from drift, such as shielded sprayers, is necessary to avoid off target plant injury.
Figure. Weed control in strawberry furrows prior to planting with 9% by volume of fatty acid herbicide (top) and weeds in untreated check (bottom)
![organic herbicide dead organic herbicide dead](https://ucanr.edu/blogs/Topics/blogfiles/32924.jpg)
![organic herbicide live organic herbicide live](https://ucanr.edu/blogs/Topics/blogfiles/32925.jpg)