PPDL Picture of the Week
February 29, 2016
Legleiter, Weed Science Program Specialist, Department of Botany and
Plant Pathology, Purdue University
The term pigweed is often used to lump together all species
within the Amaranthus genera. Within the
state of Indiana the most prevalent and common Amaranth is smooth or redroot
pigweed (Amaranthus retroflexus). Although two new amaranth species are
beginning to become predominate in Indiana: Common Waterhemp (Amaranthus Rudis) and Palmer amaranth (Amaranthus palmeri). Common waterhemp and Palmer amaranth have become
prevalent species in many areas of Indiana and are anticipated by Purdue weed
scientist to continue to spread throughout Indiana.
The identification of these individual species can prove to
be essential as waterhemp and palmer amaranth are much more aggressive and likely
to be herbicide resistant as compared to the redroot pigweed populations that have
traditionally existed in Indiana. So
when scouting your fields this spring rather than simply glancing at that weed
in your field and generalizing it as a
“pigweed”, take a closer look and determine just exactly what species
you have. The following characteristic
can help you distinguish the three amaranth species.
Redroot pigweed: Stems will have hairs present especially
towards the growing point of younger plants.
Leaves will appear to be rough in texture and are oblong to oval in
hairs will be absent on all parts of the plant. Leaves appear to have a waxy upper surface
and are much more lance to linear in shape than that of either redroot pigweed
or palmer amaranth. The petiole of a
waterhemp plant will be shorter than the leaf blade attached to it.
Palmer amaranth: Also will have an absence of hairs on all
plant parts. Petioles of palmer are much
longer than the other two species and can be as long or longer than the actual
leaf blade. Leaves will be lance to
diamond shaped with a much wider apex than waterhemp leaves and less rounded
than redroot pigweed. When viewed from
the top, palmer plants will have a rosette-like appearance that is similar to
that of a poinsettia plant.
It can be very difficult to distinguish these
characteristics with isolated plants in the field. Make sure to become familiar with these characteristics
and take along weed id guides and pictures to assist yourself in the field. The petiole length is the only characteristic
that consistently distinguishes waterhemp and palmer amaranth from each other.
Further ID pictures, and management can be found in the
following Purdue publication: Palmer Amaranth
Biology, Identification, and Management (http://www.extension.purdue.edu/extmedia/WS/WS-51-W.pdf)