Weed of The Week: wild buckwheat

Wild buckwheat remains a pest risk for farmers across the Prairies.

Alberta farmers say it is their least favourite weed.

It can be tough to kill, degrades grain and oilseed samples, messes up harvests and will trip up the odd field scout.

The ropy pest can prompt grain buyers to refuse shipments of canaryseed if its triangular seeds are found in the sample.

Wild buckwheat, formally known as polygonum convolvulus, is an annual that typically produces 1,000 seeds, although mature plants are capable of 10 times that amount if left unchecked.

Producers who have reclaimed sloughs this year are reporting instances of the pest, despite seeing few examples in adjacent fields in recent years. Agronomists recommend that producers take steps to target those areas for scouting and control this season to prevent spread.

Most of the plant’s seeds will germinate the year after they are produced, but research has found they can persist for several years. As a result, a single year of effective control might not be enough to manage an infestation.

The seeds persist in farm-saved seed and require thorough grain cleaning to remove. They can be particularly difficult to deal with in flax.

The vine-like weed starts out trailing along the soil’s surface, branching and expanding and creating its own canopy of large, heart shaped leaves until other plants are encountered. The plant will grow up to one metre tall through the crop.

Light green flowers without petals appear, bearing conspicuous sepals.

Buckwheat has a fibrous root system that can chase water and nutrients 80 centimetres deep, which makes it crop competitive and drought tolerant.

Wild buckwheat can reduce cereal yields by up to 12 percent at a population of five plants per sq. metre, while flax yields can be reduced by 10 to 20 percent at five to 15 plants per sq. metre.

Seeds germinate all season, depending on moisture conditions. Most begin life in the top five cm of soil, but they have been known to successfully germinate as deep as 20 cm in deep tillage.

Research at the University of Saskatchewan found that wild buckwheat seeds planted between April 15 and July 15 took an average of 17 days to emerge, 28 days to reach the first and second true leaf, 31 days to get to third true leaf, 50 days to make its first vine and 61 days to flower.

Buckwheat can be tough to kill, even at spring burnoff, provided it gets a head start.

However, even with effective management ahead of the crop, it can get started later in the season and then fight crop for resources throughout the rest of the year.

In-crop spray sometimes has trouble dealing with the plant, which creates problems at harvest time.

It rises up through the crop in mid-season, seeking sunlight, and the tough, wire-like stems are often still green when the combines are ready to roll. Straight cutting header reels can become choked with the long ropy stems.

Buckwheat is one of several weeds showing resistance to Group 2 chemicals on the Prairies and was identified before 2010 as being resistant in parts of Alberta.

Widespread use of Group 2 chemistries has meant that the pest has had plenty of opportunity to select for resistance.

Most crops have an in-crop registered herbicide that will kill wild buckwheat early in the season or when it is small, but the weed is tolerant to MCPA and moderately tolerant to 2,4-D.

Group 2 resistance in wild buckwheat means multiple modes of action through tank mixes or combination products can be critical.

Controlling buckwheat in broadleaf crops can be challenging. Chickpea, flax and sunflower growers can use a burn-off mix of glyphosate, carfentrazone and sulfentrazone to provide lasting control.

Buckwheat control in its earliest stages can be done with glyphosate ahead of the crop and in herbicide tolerant crops. It can also be effectively managed with timely post emergent applications, which will allow crops to develop a canopy and reduce buckwheat growth.

The weed is somewhat tolerant of glyphosate, which means full rate applications are required for anything other than the earliest stages.

Perdue University researchers say bromoxynil, clopyralid, dicamba, glufosinate and sulfonylurea products are the most effective. Using these herbicides or tank mixes with these ingredients will ensure the most effective wild buckwheat control.

Clopyralid, dicamba and some sulfonylurea herbicides may persist in higher pH soils and provide ongoing control beyond the first half of the growing season.

The weed can also be controlled by discing or cultivating before seeding, which causes germination. That is then followed by harrowing to kill the seedlings.

Rotations that include forage production will also clean up an infestation issue. Grazing or hay removal will prevent the weed from reaching maturity and further seed production.

Wild buckwheat is also known as black bindweed, climbing bindweed and corn bindweed.

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