Weed of the Week: wild buckwheat

Wild buckwheat, or polygonum convolvulus becomes a problem when later season rain interferes with post emergent spraying or arrives after spraying.

In those years, the weed can make seed and interfere with crops and harvestability.

Its wiry stems rise up through the crop seeking sunlight. Unless treated with glyphosate or another preharvest herbicide, it can make combining a chore for producers.

Buckwheat is one of several weeds showing resistance to Group 2 chemicals on the Prairies.

Resistance has been identified in Alberta, but there is a threat of increasing resistance across the western provinces because of the weed’s widespread presence and the extensive use of Group 2 products.

Wild buckwheat is an annual and each plant can potentially produce as many a 12,000 seeds per year, although typically they produce only 1,000 seeds per plant. This provides plenty of opportunity for herbicide resistance selection.

Most buckwheat seeds make a play for life within a year of creation, but research in the 1990s showed they can persist for several years. As a result, a single year of control might not be enough.

The small, triangular seeds can persist in farm-saved seed and require thorough grain cleaning to remove.

The weed first trails along the soil’s surface and then branches at its base, expanding and creating its own canopy until other plants are encountered. The plant will grow up to one metre tall and has heart-shaped leaves up to 75 millimetres in length with a smooth surface and edge.

Light green flowers appear without petals, but they have five conspicuous sepals.

The plant has a large, fibrous root system that can chase water and nutrients down to 80 centimetres, 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.

Although a mature buckwheat plant might produce 12,000 seeds in a season, only three percent typically germinates, according to research at Perdue University.

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

Research at the University of Saskatchewan showed that wild buckwheat seed 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.

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 of MCPA and moderately tolerant to 2,4-D.

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

Controlling buckwheat in broadleaf crops has been a challenge for producers. Chickpea, flax and sunflower growers can use a burn-off mix of glyphosate, carfentrazone and sulfentrazone to provide lasting control in those crops.

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 researchers say bromoxynil, clopyralid, dicamba, glufosinate and sulfonylurea products are the most effective. Using these herbicides or mixtures 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.

Tillage is effective: pre-seeding discing or cultivation that causes germination followed by harrowing to kill the seedlings.

Rotations that include several years of forage production will also provide control. As well, grazing will prevent the weed from reaching maturity and further seed production.

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

It can be confused with field bindweed until it flowers. Bindweed, another nasty weed, has white petals on its flowers and is a perennial.

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