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Pigweed Identification in Arizona (April 4, 2018)
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Pigweeds are some of the most common summer annual broadleaf weeds in the low deserts.
Palmers Amaranth is the first weed in Arizona to been documented as being resistant
to Glyphosate. Although they are often lumped together, there are 4 different species
of pigweed that are common here and more than 10 species that occur as weeds in
California and Arizona. Their growth habits and response to herbicides are similar.
It is easy to identify them by physical characteristics but one species of pigweed
can hybridize with another and become less distinguishable.
Palmer Amaranth (Amaranthus palmeri) is probably the most
common pigweed species found in this region. It is very aggressive and fast growing
and can become 6 feet tall or higher if uncontrolled. It has one thick stem and
several lateral branches. The leaves are lance shaped, hairless and have distinctive
white veins on the underside. It has flowering tassels that become stiff and spiny.
This species has become resistant to Glyphosate in many parts of the county.
Pigweed (Amaranthus retroflexus) is probably the second
most common pigweed species. It is shorter and the seed heads are smaller, in clusters
and have stiff spine-like scales. It has leaf hairs on the margins and the veins
are often reddish. The lower stems are often reddish. This species will hybridize
with Palmer Amaranth and become less distinguishable.
Pigweed (Amaranthus albus) is very different from Palmers
or Redroot. It grows lower to the ground and has many branches that turn upright.
The leaves are much smaller and narrower. The numerous stems are light green rather
than red. The seed heads are small, spiny and at the base of the leaves rather than
in long terminal spikes. When mature, the branches are sticky, stiff bristles that
break off at the ground and tumble with the wind.
Prostrate Pigweed (Amaranthus blitoides) is very similar
to Tumble Pigweed but the stems are more prostrate, grow close to the ground and
form mats. The stems and leaves are smaller and reddish rather than light green.
Weed Control
Tickes/J.Pena
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For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.
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