Palmer amaranth and waterhemp are the core targets of most Midwest and Southern herbicide systems. The aggressive growth patterns and herbicide resistance characteristics for both weeds have made them a major economic problem in row crop production.1 For example, soybean yields were reduced by 78% by Palmer and 56% by waterhemp when there was an average of 0.75 plants per square foot when they started competing at crop emergence. Corn yields have been shown to decrease by as much as 91% with an average Palmer density of 0.84 plants per square foot.2
The grouping of Palmer amaranth and waterhemp as being similar species is sometimes done by growers. In fact, in my travels to the Missouri bootheel, growers tend to refer to all amaranth species as "pigweed." It is important to understand the differences between the two weeds when formulating a weed control system. Both Palmer and waterhemp grow at a rapid pace; however, Palmer is more aggressive. In ideal growing conditions, waterhemp can grow as much as 1-1.25 inches per day, whereas Palmer in the same conditions can grow 2-3 inches per day.3 Both species are C4 plants which is common in grass plants but only in 4% of dicot species.4 C4 plants are very efficient producers of energy in high sunlight and high heat environmental situations, much like corn plants.
The movement of waterhemp into northern areas of the U.S. is somewhat more prolific than that of Palmer. In a study conducted in Southern Illinois, emergence dates were measured for waterhemp and Palmer.5 In the spring, waterhemp seeds emerged earlier than Palmer and in cooler soils, but Palmer seeds emerged over a longer period and later into the growing season compared to waterhemp. Further evidence that might explain the resistance of Palmer to northern movement is photosynthetic rates. Palmer photosynthetic rates are most efficient at temperatures around 100° F, but when temperatures dropped to about 75° F, Palmer photosynthetic rate was reduced by 50%.4
Some characteristics of the two amaranth species are very similar. An interesting and troubling fact is that hybridization can occur between Palmer and waterhemp. It is rare, however; there are documented controlled laboratory experiment cases of herbicide resistance transferred in pollen between common waterhemp and Palmer amaranth.6 This would give merit to my counterparts in the South referring to all amaranth species as "pigweeds."
When comparing the two species, one of the things they have in common is widespread resistance to glyphosate and many other herbicide families. The first confirmed case of glyphosate resistance for waterhemp was in 2005 in Missouri and for Palmer in 2004 in Georgia. This is very important to keep in mind as a management plan is implemented.
The management strategies for Palmer and waterhemp are very similar. The aggressive growth and later growing season emergence of Palmer seedlings versus waterhemp pushes the grower to be slightly more diligent when dealing with Palmer.
A well thought out management plan of Palmer and waterhemp can be a challenge but understanding the growth habits and implementing a well-thought management plan can help manage these weeds. For additional information on weed control, please visit RoundupReadyPLUS.com.