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Weed Watch: Distinguishing Tall Waterhemp from Redroot Pigweed

By Clark Brenzil, PAg, Provincial Specialist Weed Control, Regina

June 2020

Saskatchewan producers are no stranger to pigweeds. There are several common pigweeds, such as redroot, prostrate and tumble. Powell amaranth or green pigweed is likely more common in Saskatchewan than most believe, but most people often mistake it for redroot pigweed because they bear a close resemblance. In addition, there are new pigweeds approaching that we need to watch for.

Pigweeds are native to North America, typically in the southern regions of the continent. Several have moved northward and now infest Saskatchewan. Pigweeds can be quite problematic in warmer regions of Canada as it can grow up to 10 feet high in row crops, under the right conditions. In Saskatchewan, pigweeds are not overly competitive due to their typically high-heat requirement for germination. Quick growth of typical Saskatchewan crops fill inter-row spaces, challenging late-emergence pigweeds. However, with the increase interest in row-crops, pigweeds may increase in interest.

Redroot pigweed resistant to Group 2 herbicides was first reported in Saskatchewan in 2010. But herbicide-resistant pigweeds did not register in surveys until the most recent in 2014/15, where it was present in 10 per cent of pigweed samples collected or 0.5 per cent of all fields in the survey. Outside of Saskatchewan, herbicide resistance is more common in the pigweeds with redroot pigweed reported resistant to Groups 2, 5, 6, 7, and 14, resistant biotypes of Powell amaranth are reported to Groups 2, 5 and 7, prostrate pigweed biotypes are reported to Group 2, and 5 and tumble pigweed is reported resistant to Group 5.

 redwood pigweed water hemp seedlings
Figure :1 Redroot pigweed (L) and waterhemp (R): note hairs on redroot pigweed vs. the hairless waterhemp.
waterhemp seedling cotyledons
Waterhemp seedling with cotyledons pointed and mottled.
(Photos: Tammy Jones, Manitoba Agriculture)

Tall waterhemp (Amaranthus tuberculatus) is a new pigweed that comes ready made with resistance to many different herbicide groups. It is currently threatening to migrate north to Saskatchewan. Tall waterhemp is reported in Bottineau and Renville counties of North Dakota, immediately south of the southeastern portions of Saskatchewan. Manitoba has reported five new sites in 2019. Most of Manitoba’s cases are in the Red River Valley region.

Waterhemp can be distinguished from redroot or green pigweed in the seedling stage because it has pointed cotyledons that have a mottled appearance (Figure 1 inset), while redroot pigweed cotyledons have parallel sides and a blunt, rounded tip. Waterhemp seedlings also have no hairs on the stem or leaves, while redroot and green pigweed have hairs that make them feel rough to the touch (Figure 1).

Waterhemp plant
Figure 2: Waterhemp leaves

Waterhemp leaves are also longer and thinner than redroot pigweed. Redroot pigweed leaves are roughly twice as long as they are wide and somewhat diamond shaped, but waterhemp leaves are roughly three to four times longer than wide and more oval shaped or gradually taper to a point. Waterhemp leaves have a waxy (shiny) leaf surface whereas other pigweeds have a dull or granular surface (Figure 2).

When approaching maturity, redroot pigweed produces flower/seed heads from the axils or crotches of the leaves and then has a terminal 'clump' at the top of the stem. Green pigweed has a slightly more elongated clump than redroot pigweed. Both redroot and green pigweed have both male and female reproductive structures within the same flower.

Waterhemp, on the other hand, has separate male and female plants, meaning that a male plant has to be in relatively close proximity to a female plant for seed to be produced. This separation of the sexes produces a great range of genetic diversity, which has contributed to the high level of herbicide-resistant and multiple-resistant populations. Flowering structures on waterhemp present as greatly elongated spikes at the top of the plant (Figure 3), making for a fairly distinctive appearance in late-summer. Waterhemp plants also branch profusely, with flower spikes at the end of each branch. Redroot pigweed seed heads will prickle when grabbed whereas waterhemp seed heads do not have prickles.

Pile of waterhemp plants
Figure 3: Tammy Jones of Manitoba Agriculture with a
pile of pulled waterhemp plants, illustrating the elongated
flowering structures and the variety of colours.
(Photo Tammy Jones, Manitoba Agriculture)
Colour is not a good indicator of weed identity for waterhemp as there is a broad range of colouring from no red to entirely red plants.

Saskatchewan Agriculture will be working with the Plant Health Officers from the Saskatchewan Association of Rural Municipalities (SARM) to conduct a monitoring exercise for waterhemp in southeastern Saskatchewan later in 2020. The study aims to see if they can find waterhemp in farm fields or industrial sites. Should they find something that is suspicious, the producer will be contacted to alert them and ask to enter the field to investigate further, and remove any suspicious plants.

Catching this weed, that has developed resistance to all herbicide groups, when there are only a limited number of plants is critical to making sure that it does not spread further into Saskatchewan. If you find a weed that you suspect might be waterhemp please contact Clark Brenzil, Provincial Specialist, Weed Control at 306-787-4673 or by email.

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