Weed of the week: redroot pigweed

As the weather warms up, redroot pigweed can become a serious problem for some crops that aren’t strong competitors.

Worse yet, amaranthus retroflexus, a dicot weed in the Amaranthaceae family, is developing resistance, or appears to at least have become harder to control with Group 2 chemistries.

In Ontario it has become resistant to Group 5’s atrazine. So far, good news for lentil growers, it has not shown the same taste for metribuzin, Sencor. This may be due to better options for rotations of those crops and herbicides.

The plant’s ability to produce up to 100,000 seeds if left alone through the growing season, means it can produce a large infestation in a short time once soils warm.

It will tend to germinate after the crop is up, also avoiding spring burn-off applications. If the conditions are cold in spring and the seed bank inventory is high, it may even avoid germination until after post-emergent herbicide applications are made.

Those 100,000 seeds can lay dormant for up to five years, meaning a tough season half a decade ago can be a lingering weed threat.

The more open sky the weed can see the better it likes it. Flax and lentils are two crops that run into issues, as can sloughs that aren’t seeded or later planted.

Pigweed leaves are oval and have a notched tip when the plants are young and later develop a diamond shape.

The weed’s large, reddish taproot can scavenge moisture from deep in the soil’s profile. Lower stalks are thick and smooth, while upper areas and branches are rough and hairy.

Flower spikes carry densely packed green blossoms in July and August. The tiny seeds are round, black and shiny.

In China, the plant is cropped and used for production of a flour.

Herbicide products with residual characteristics and some soil-applied tools, such as sulfentrazone, Authority, or Authority Charge with added carfentrazone are effective in control for chickpeas, peas, flax, conventional soybeans and sunflowers. Sulfentrazone alone should applied at the higher rate.

Group 2 chemistries, applied with good timing are generally effective, especially if tank-mixed with, or part of, combination approaches with herbicides using other modes of action.

Bromoxynil and MCPA or 2-4D can be effective as a post-emergent product, depending on the crop.

Delayed post-emergent applications of herbicides can improve control, however that can present crop-staging issues, and control problems for other weeds.

Steel can be effective in providing less surface trash, through shallow applications of vertical tillage or with heavy-harrowing. This allows for warming of the soil earlier in the season, but also might negatively affect soil moisture.

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