redroot pigweed (Amaranthus retroflexus)
Amaranthus retroflexus var. salicifolius I. M. Johnston
CONFIRMATION STATUS: Pending confirmation.
TAXONOMY: The currently accepted scientific name of rough pig-
weed is Amaranthus retroflexus L. It is a member of the Pigweed Family
(Amaranthaceae). This native to central and eastern North America, is a
successful invasive species and has effectively colonized a wide range of
habitats on all inhabited continents. Its variability is extremely wide;
usually the species is easily recognized and its identification causes no
specific problems. There are no generally recognized subspecies, varieties,
or forms. Infraspecific entities described within Amaranthus retroflexus
are mostly ecologic variants of little or no taxonomic value. Some author-
ities recognize two varieties: the common var. retroflexus, with bracts
about 1.5-2 times as long as tepals, and a more rare var. delilei (Richter
& Loret) Thellung (=Amaranthus delilei Richter & Loret), with bracts
1-1.5 times as long as tepals, a distinction very difficult for most causal
observers to differentiate.
Rough pigweed is quite similar in appearance to another common weed,
Amaranthus hybridus (slender amaranth), and prefers similar habitats
(perhaps those that are slightly drier). Rough pigweed tends to be shorter
and stouter in its growth habit than slender pigweed. The flowering spikes
of rough pigweed are usually more stout and bristly than those of slender
pigweed; the flowering spikes of rough pigweed are whitish green while in
bloom, while the flowering spikes of slender pigweed are more green
(because its sepals are less conspicuous). Using a 10x hand lense, it is
possible to observe that the sepals of rough pigweed often have flattened
tips, while the sepals of slender pigweed are more pointed at their tips.
Furthermore, the floral bracts of rough pigweed are up to 6 mm. in length,
while those of slender pigweed are up to 4 mm. in length. Red forms of
rough pigweed apparently don't occur; reddish pigweeds are usually
slender pigweed, or less often a cultivated Amaranth that has escaped
Occasional forms morphologically intermediate between Amaranthus
retroflexus and taxa of the Amaranthus hybridus aggregate (e.g., Amar-
anthus powellii and Amaranthus hybridus, in the strict sense) are known
both in the Americas and the Old World. Usually such plants are treated
as hybrids; in many cases they are probably just extremes of the natural
variability of Amaranthus retroflexus.
NATIVE STATUS: Native, United States; Introduced, Canada.
GENERAL BOTANICAL CHARACTERISTICS:
Habit: Rough pigweed is an adventive summer annual about 1-3' tall that
is either sparingly branched or unbranched. The central stem is stout, round,
light green, and more or less covered with white hairs; it also has fine long-
itudinal veins that are white.
Leaves: The alternate leaves are up to 6" long and 4" across (excluding the petioles), becoming smaller in the upper half of the central stem. They are
cordate-ovate or elliptic and smooth or slightly undulate along the margins.
The base of each leaf is rounded or wedge-shaped, while its tip is rounded
and blunt. The lower surface of each leaf is usually pubescent, while the
upper surface is less pubescent or hairless.
Flowers: The central stem terminates in a stout panicle of spikes with
whitish green flowers. This terminal inflorescence is up to 6" long (rarely
longer in large plants). There are also shorter axillary panicles of flower-
ing spikes or simple spikes that develop from the axils of the middle to
upper leaves. The flowering spikes are bristly in appearance from the
crowded flowers and pointed bracts. Rough Pigweed is usually mono-
ecious with separate pistillate (female) and staminate (male) flowers on
the same plant. The pistillate flowers have 5 pale white sepals, an ovary
with 3 styles, and no petals. The staminate flowers have 5 pale white se-
pals, 5 stamens, and no petals. The sepals are oblong and about 3 mm. in
length; their tips are either short and pointed or flattened. At the base of
each flower, there are one or more green bracts about 3-6 mm. long. These
bracts have long pointed tips. The blooming period occurs from late sum-
mer to early fall and lasts about 1-2 months. Cross-pollination of the flow-
ers is by wind.
Fruit/Seeds: Each pistillate flower develops a single seed in a membran-
ous bladder (utricle). This utricle splits up to release the seed. Each small
seed is dark brown or black, flattened, and circular; it has a smooth and
Roots: The root system consists of a short stout taproot that is usually tint-
ed red. This plant spreads by reseeding itself.
REGENERATION PROCESS: Rough pigweed regenerates from seed
each year. Plants can result from germination of newly released seed
(cross-pollination of the flowers is by wind), or from germination of seed
carried over in the seedbank from previous years. Seeds may germinate
any time soil moisture is adequate during the growing season.
HABITAT TYPES: Rough pigweed, originally from South America, is
found in a wide range of habitats. It can be found along the banks of rivers,
lakes, and streams, disturbed habitats, agricultural fields, cropland, fallow
fields, farm lots, gardens, orchards, fence rows, gravelly areas along roads
and railroads, and waste areas. It also inhabitats dry areas of disturbed
prairies, disturbed hill prairies, and prairie ravines. Highly disturbed areas
are preferred. It can be found from sea level along shores and mountainous
regions up to 7,500 feet.
Rough pigweed, an early successional species, extracts more nitrogen from
and grows faster on the nitrogen-poor soils of recently abandoned fields
than mid- and late successional species. It is often a dominant species dur-
ing the first growing season in abadoned fields and waste areas. During
the 1934 "dustbowl" drought, rough pigweed grew thickly where wind-
blown dust had covered considerable portions of prairies in Kansas and
Nebraska. Rough pigweed, normally not found in prairies, became widely distributed when released from their usual competition with grasses. How-
ever, with the end of drought and the return of grasses, rough pigweed
nearly disappeared in many prairies.
Rough pigweed seeds are an important part of the seedbank in many hab-
itats, even when plants are almost absent. As an initial community spec-
ies, rough pigweed is restricted to particular conditions or circum- stances -
it needs bare, disturbed sites in order to establish. Fire, plowing, or garden-
ing techniques which clears away competing vegetation can allow the
establishment of rough pigweed. Rough pigweed seeds are very small,
and easily blown by the wind from off-site sources. Some seeds survive
cattle digestion, and can be carried by animals to burned areas, cleared
fields and recently cleared waste areas. The seeds can remain viable in the
soil for 30 years or more.
SITE CHARACTERISTICS: This plant is typically found in full sun,
moist to dry conditions, and a soil containing loam, clay-loam, or gravelly
material. Its size can vary significantly according to moisture levels and the
fertility of the soil. It strongly prefers highly disturbed areas, one reason it
is often found in construction sites, home gardens, and any area where the
soil is significantly modified by human intervention. The seeds can remain
viable in the soil for 30 years or more.
SEASONAL DEVELOPMENT: Rough pigweed normally begins growth
in late spring and matures in late summer or early fall and lasts about 1-2
months. It has a wide range of blooming times - in the Great Plains from
July to October, in the central and northeastern United States and adjacent
Canada from August to October, in southern California from June to No-
vember, in Montana from June to October, in North Dakota and Wyoming
from July to September, and in the Carolinas from July until frost.
GENERAL DISTRIBUTION: Rough pigweed is found throughout
North America, from Canada to Mexico, and from the Atlantic to the
Pacific coasts. It is also found throughout much of the rest of the world,
including Europe, South America, Eurasia, and Africa. It is a native of
tropical America. Rough pigweed is difficult to eradicate when once
established. A survey of weeds in spring annual crops throughout Man-
itoba over a 4-year period showed 83 weed species. Rough pigweed was
the third most common dicotyledonous weed. In Kansas, it was the most
abundant forb weed in the seedbank.
SKY MEADOWS DISTRIBUTION: To be determined.
IMPORTANCE AND USES: Insects rarely visit the wind-pollinat-
ed flowers. The caterpillars of the skipper Pholisora catullus (com-
mon sootywing) feed on the foliage, as do the caterpillars of several
moths, including Holomelina aurantiaca (orange holomelina), Hy-
menia perspectalis (spotted beet webworm moth), Spilosoma congrua
(agreeable tiger moth), and Spoladea recurvalis (Hawaiian beet web-
worm moth). Sometimes Disonycha spp. (flea beetles) chew little holes
in the leaves. The seeds of pigweeds are very popular with granivorus
birds as a source of food during the fall and winter. Pigs and cattle eat
pigweeds readily, although the foliage can cause bloating and other
symptoms of nitrate poisoning if an excessive amount of the foliage is
eatened. Deer and rabbits eat pigweeds to a limited extent.
Rough pigweed's palatability rating is fair for cattle and horses, and good
for sheep. In Minnesota, rough pigweed was as palatable to sheep as oats
(Avena sativa); also, rough pigweed harvested from late June to mid-July
showed a nutrient composition and digestibility for sheep equivalent to
that of high-quality alfalfa (Medicago sativa). Rough pigweed contains
adequate minerals to meet the requirements of ruminants. However, it must
be utilized at relatively early stages of maturity. Rough pigweed affects the
kidneys of swine and cattle when animals consume large quantities of fresh
material for 5 to 10 days. Cattle have developed perirenal edema and toxic
nephrosis after ingesting rough pigweed. The toxicant has not been iden-
tified, although oxalates and/or phenolics have been suspected. Addition-
ally, rough pigweed accumulates nitrates, which causes poisoning in most
livestock species when ingested in large quantities either fresh or in hay.
The excess nitrates cause cattle to bloat. In the Midwest, pigs have been
poisoned by rough pigweed growing under drought stress. In drought con-
ditions, rough pigweed accumulation of nitrates accelerates. In Nebraska,
cattle consumed immature leaves and tops of rough pigweed in fields seed-
ed to big bluestem (Andropogon gerardii var. gerardii). By mid-July of
the first year of the study, nitrate concentration had reached toxic levels
(10,000 ppm). In the second year, nitrate levels exceeded the toxic level
at the beginning of the grazing season (nitrate concentration is highest in
rough pigweed just before bloom). Sheep in Texas were maintained for
varying lengths of time on rough pigweed pasture supplemented with
wheat and alfalfa hays. Scaled quail in Texas made use of rough pigweed
seeds. When available, seeds averaged 0.9 percent of food eaten. In the
highest recorded use, rough pigweed seeds made up 3.6 percent of food
eaten. Percent use was greater than rough pigweed presence. Fortunately,
rough pigweed is probably unpalatable when mature because of the stiff,
spine-like bracts in the flower clusters. In addition to nitrate problems, the
calcium to potassium ratio in rough pigweed is such that it should not be
fed as the sole ration.
Rough pigweed was used for a multitude of food and medicinal purposes
by many Native American groups. This plant is still eaten as a vegetable in
different places of the world. Young leaves of rough pigweed are used as
salad greens when the plant is only a few inches tall, before the stem be-
comes woody. Fresh young plants are also used as a potherb. Seeds are
edible whole or ground into meal. However, because of its nitrates prob-
lems, it should be used in moderation, particularly when taken from
nitrate-fertilized areas (if boiled, the water should be tossed after boiling).
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