Pigweeds (Redroot, Green and Smooth)
Table of Contents
Three types of pigweed - redroot, green and smooth - are common in southern Ontario, and are all often referred to as "redroot pigweed". They are similar in appearance and difficult to distinguish before the flowering stage. In areas where their distributions overlap, it is not uncommon to find all three species growing together in the same field.
Redroot pigweed (Amaranthus retroflexus L.) is the most widespread of the three species, being found throughout the United States, in all provinces of Canada except Newfoundland, and as far north as Kenora and Cochrane in Ontario. Green pigweed (Amaranthus powellii S. Wats.) is common in the western United States, and since 1940, has spread into the midwestern states, and southern Ontario. It is referred to as Powell amaranth in the United States. Smooth pigweed (Amaranthus hybridus L.) is abundant throughout the eastern United States, and since 1970, has spread into Ontario, occurring primarily in the southwestern counties.
All 3 pigweeds have a pink or red taproot, and a green to reddish stem which may be simple or branched and up to 1.5 m high. Upper stems and leaves of mature plants are sparsely hairy and have a rough texture. The leaves are alternate on the stem, long-stalked, and range from dull green to shiny or reddish green. The leaf blade is oval to diamond-shaped, but is usually broader at the base. The margins of the leaves are smooth. The tips of the leaves are pointed or sometimes slightly notched. The flowers are small, green and crowded into coarse, bristly spikes at the top of the plant, with smaller spikes in the leaf axils below. The tiny seeds are round and somewhat flattened, about 1 mm in diameter, and shiny black to dark brown.
The first leaves of redroot and smooth pigweed are rounded, whereas those of green pigweed are tapered and slightly pinched toward the end. (Figure 1) The upper stem of green pigweed is often less hairy than that of redroot or smooth pigweed. Redroot pigweed has a relatively short, thick, compact inflorescence, with the uppermost central spike extending only a short distance above the first branches of the panicle. (Figure 2a) Green pigweed has a longer and narrower terminal spike with fewer but longer branches. (Figure 2b) Smooth pigweed has a very narrow, often lax, terminal spike with numerous short lateral branches. (Figure 2c)
Hybrids between these species may occur. Hybrid plants are usually sterile and their flower spikes may be oddly shaped and remain green long after the seed heads of other pigweeds in the field have turned brown. Two other pigweed species occur in Ontario: tumble pigweed and prostrate pigweed. Neither of these has a terminal spike, bearing small green flowers only in the leaf axils. Both are smaller in stature and less common than redroot, green or smooth pigweed.
Figure 1a. Three week-old seedling of redroot pigweed.
Figure 1b. Three week-old seedling of green pigweed.
Figure 1c. Three week-old seedling of smooth pigweed.
Figure 2a. Mature inflorescences of redroot pigweed.
Figure 2b. Mature inflorescences of green pigweed.
Figure 2c. Mature inflorescences of smooth pigweed.
Redroot, green and smooth pigweed have an annual life cycle and reproduce only by seeds. Their seeds are small and germinate near the soil surface when average soil temperatures exceed 15°C. Light and high temperatures stimulate germination. Cultivating the soil at night rather than during the day can reduce but not eliminate germination of pigweed seeds. Flushes of emerged seedlings can occur throughout the growing season following rainfall or cultivation. Pigweeds are a problem in both conventional and reduced tillage or no-till fields.
Flowering usually begins in July and the seeds mature over a period of several months. In southwestern Ontario, seeds of redroot pigweed are the first to mature and those of smooth pigweed the last. Smooth pigweed often matures 2-3 weeks later than redroot pigweed, which may restrict its northern range expansion. Green pigweed tends to have faster germination and growth, and greater competitive ability than redroot or smooth pigweed. All 3 pigweed species can produce as many as 100,000 seeds per plant, although most plants growing in a crop produce between 10,000 and 30,000 seeds. Ungerminated seeds can live in the soil for as long as 40 years. The seeds are small and lightweight and are dispersed by wind and as contaminants of crop seeds or farm machinery.
Pigweeds are troublesome weeds for several reasons. They reduce crop yields through competition for light, water and nutrients. Economic thresholds for pigweed in soybeans and corn, emerging at the same time as the crop, vary from 5-15 plants per 10 m of row, depending on herbicide cost and crop price. Pigweed species occasionally accumulate nitrates in the stem and branches in concentrations high enough to poison livestock. Infestations in silage corn have been reported to cause severe illness or death in cattle. Pigweeds serve as alternate hosts for crop pests such as the green peach aphid, tarnished plant bug, European corn borer, flea beetle, cucumber mosaic virus, and strains of Fusarium and Rhizoctonia that attack sugar beet. Finally, pigweed pollen can cause allergic reactions in sensitive individuals.
Pigweeds are susceptible to most soil applied and postemergent herbicides recommended for general control of broadleaf weeds. Most herbicide labels do not distinguish between redroot, green or smooth pigweed with respect to efficacy or rate. Residual control is often desirable, because multiple flushes of germination can occur. Pigweed seedlings are easily controlled by cultivation or hoeing. Older plants are sometimes able to recover from partial damage if the root has not been completely severed.
A number of herbicide resistant pigweed populations have been identified in Ontario. Atrazine resistant populations of green pigweed and redroot pigweed were first reported in the early 1980's in southern Ontario. In 1998, populations of both green pigweed and redroot pigweed resistant to ALS inhibiting herbicides (Group 2), such as PURSUIT (imazethapyr) or PINNACLE (thifensulfuron-methyl), were confirmed in at least 7 counties. Atrazine resistant and ALS resistant populations of smooth pigweed occur in the eastern United States, but have not yet been reported in Ontario. A single population of green pigweed resistant to linuron (AFOLAN, LOROX) was discovered in 1999 in a carrot field near Keswick, Ontario. Resistant plants cannot be visually distinguished from susceptible ones. Rotating herbicides as well as crops, and following other integrated weed management practices will help prevent the spread of herbicide resistant pigweeds.
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