Pigweed - with and without burrs

[Stephen Tashi]

TL;DR Summary

Are pigweeds with burrs a distinct variety or gender from pigweeds with no burrs?

A photo shows two plants that are known locally (southern NM, USA) as "pigweed". The shorter plant has burrs that develop along the main stem. The taller plant has no burrs. What is the scientific name for these plants? Are these plants distinct varieties? - or different genders of the same variety?

The burrs, known locally as "goatheads", are insidious. When they dry up, they loose their green color and harden. The hard burrs can embed themselves in shoe soles and puncture bicycle tires. The weeds in the photo are about 6 inches high. The weeds can grow several feet high. They don't necessarily develop burrs when they get that large. My impression is that the non-burred plants tend to have reddish stems.

One photo is a close up of the stem with burrs. Another photo shows a close-up of the stem of the plant that has no burrs.

 

[Bystander]

"Pigweed" is a totally different plant from puncture-vine, the source of "goatheads." https://www.google.com/search?q=pun...ome..69i57.12501j0j7&sourceid=chrome&ie=UTF-8

 

[Stephen Tashi]

"Pigweed" is a totally different plant from puncture-vine, the source of "goatheads." https://www.google.com/search?q=pun...ome..69i57.12501j0j7&sourceid=chrome&ie=UTF-8

Ok, but I'm still curious about the burrs and plants in the photo. The burrs are a nusiance.

 

[Bystander]

https://www.google.com/search?q=beg...ome..69i57.13703j0j7&sourceid=chrome&ie=UTF-8 Are you certain you're talking about "pigweed/redroot" and not some other nuisance plants/weeds? https://www.google.com/search?q=coc...me.0.69i59.11599j0j7&sourceid=chrome&ie=UTF-8

 

[jim mcnamara]

Okay. Pigweed is a common name for about 20+ different plant species in the genus Amaranthus.
The noxious weed species of pigweed:
redroot pigweed (Amaranthus retroflexus),
prostrate pigweed (Amaranthus blitoides),
tumble pigweed (Amaranthus albus),
waterhemp (Amaranthus rudis),
Palmer amaranth (Amaranthus palmeri)

Noxious weeds are by definition intrusive and out compete domesticated preferred species, ie., crops and ornamentals. This is a defintion from the US Federal Seed Act. Most "ag" agencies follow that terminology.
These five plants are defined as noxious weed on a state-by-state basis

New Mexcio: http://www.nmda.nmsu.edu/home/divisions/apr/noxious-weed-information/ NMDA is a state law.

That said, two points:

A.) This will let you determine which of the nasties, if any, these guys in the mug shots represent. Sort of a poor man's guide to pigweed:
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=27466
B.) Unless you are a Botanist and know how to use a taxonomic dichotomous key figuring these species out is kinda hard to do. I'm not sure what knowing the species is going to do for you. Especially in the case of the little ag blurb above not helping to ID some of the plants easily. Here is one answer to why :

Several Amaranthus species also exhibit strong phenotypic plasticity.

Some tall species that used to stand out 6 feet tall in fields, when they occur as individuals in lawn populations that have been mowed for many generations, faced extreme selection pressure to grow and flower really close to the ground. They do not look like their cousins out in the field. They sprawl (are decumbent) close to ground level, for example.

Dandelions (Taraxacum spp.) were the first species shown to do this. "lawn" populations of Dandelion have flower stems that are about 2cm long, even when grown in the lab. "field" populations have stem lengths of ~10-15cm.

The photos are Amaranths.

 

[BillTre]

@jim mcnamara,
Are the burrs a seed dispersal mechanism?

 

[jim mcnamara]

I think so, especially the ones with antrorse hairs (trichomes). Reverse pointing hairs that force the pod to burrow deeper into small mammal's fur with movement. Or into your socks. Extends the free ride for the seeds.

Ever see the South American grain quinoa in a market (usually where bulk grains and beans are sold)? It is dark and small BB pellet size. That seed is typical of Amaranthus as well. Same plant family. Those seeds disperse largely via rodent activity - like pack rat middens. They also disperse temporally in buried seed populations, distubance exposes them to sunlight & water.

https://en.wikipedia.org/wiki/QuinoaFWIW - the quinoa and Amaranthus seeds are covered with saponins which need extensive washing to remove the bitter taste - From saponins on them are also anti-nutrients. All of the seeds in the plant family are edible - I think.

https://www.aaccnet.org/publications/cc/backissues/1992/Documents/69_85.pdf