Weed of the Week: Pigweeds

Posted on by km767

Aaron Gabriel
Cornell Cooperative Extension
Capital Area Agriculture and Horticulture Program 

Redroot and smooth pigweed are two common pigweeds in New York that most of us are familiar with, but there are several other species as well.  Notably, waterhemp and Palmer amaranth have become serious weeds because they are difficult to control with herbicides alone.  The New York Flora Atlas (https://newyork.plantatlas.usf.edu/ ) lists 17 pigweeds.  

Table 1. Eight weedy pigweeds in New York, roughly in their order of prevalence- redroot being the most prevalent and spiny being the least. 

Redroot pigweed  Amaranthus retroflexus 
Smooth pigweed  Amaranthus hybridus 
Powell amaranth  Amaranthus powelli 
Tumble pigweed  Amaranthus albus 
Waterhemp (tall, common, roughfruit)  Amaranthus tuberculatus =A. rudis 
Prostrate pigweed  Amaranthus blitoides 
Palmer amaranth  Amaranthus palmeri 
Spiny amaranth  Amaranthus spinosus 

It takes some practice to identify the various pigweed species.  There are differences in the shape of the cotyledons (the seed leaves); hairiness of the stem and leaves; leaf shape; petiole length; flower size, feel, and structures.  There are good resources at Cornell to identify our five common pigweeds (redroot, smooth, Powell, Palmer, and waterhemp): 

You can find identification help for prostrate and tumble pigweeds at: 

   

 

All the pigweeds are summer annuals that germinate in mid to late spring when the soils warm.  However, Palmer amaranth and waterhemp continue to germinate through the summer, making control more difficult.  They all produce tremendous amount of seed, but Palmer amaranth and waterhemp produce the most.  Seed produced earlier in the year are less dormant than seeds produced late in the summer.  Most of the seed remains viable for only three or so years.  Seeds are spread with manure and in soil on machinery.  Germination is enhanced when seeds are near the surface and soils are very warm.  Nitrates promote germination.  Light and soil disturbance can trigger germination.  Seedlings are small and somewhat fragile.  Pigweeds are somewhat drought tolerant (they have the C4 photosynthetic system like corn and sorghum and unlike alfalfa and our cool-season grasses) and respond well to nitrogen and phosphorus fertility.  Shade will reduce pigweed growth, but also cause them to grow taller. 

There are several ways to manage pigweeds.  Since the seeds germinate near the surface, the stale seedbed technique can be effective (shallow tillage when the soil warms, then more shallow tillage to kill the seedlings before and after planting).  The fragile seedlings are easily killed with cultivation.  Small grains produce a lot of shade when pigweed seedlings are trying to establish, so a dense small grain crop can be part of the crop rotation.  It is especially helpful to tine weed small grain crops between the 3-leaf and stem elongation stages.  Surviving plants need to be killed after grain harvest.  Amaranth seeds mostly remain in the seedheads at grain harvest, so combines that collect and destroy weed seeds are helpful.  Dense mulch is also effective since the seedlings cannot push through a dense mulch.  Since pigweeds respond to fertilizer, it is best to direct fertilizer to an established crop to avoid fertilizing the weeds.  Rotations with a few years of sod allows the seeds to die and reduce the seedbank.  Because pigweeds produce so many seeds, any escaped plants should be rogued out.  Broken seedhead fragments left on the ground will continue to mature seed, so plants should be removed from the field. 

The challenge of waterhemp and Palmer amaranth is that they are tolerant of several herbicide families.  The variety of effective herbicides is greater for corn than for soybeans.  Herbicides in Chemical Family Groups 4, 5, 14, and 15 are most effective.  Because pigweeds germinate late into the summer, it is important to include an herbicide with soil activity at planting and again later with a post-emergent application once the crop is established.  Residual soil herbicide activity is need late into the season. 

Pigweeds are edible for both people and livestock.  Quinoa is a grain from a Amaranthus species.  The plant itself can also be eaten.  The one caution is that plants accumulate nitrates, so in high nitrogen environments, they may have too much nitrate to be safe. 

For more information about pigweeds, go to the Cornell Weed Profiles, https://cals.cornell.edu/weed-science/weed-profiles.  

 

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