Weed of the Week: redroot pigweed

Redroot pigweed is tough to control in later seeded crops.

Amaranthus retroflexus, as it is known to botanists, is an annual weed that has developed resistance to herbicides in Groups 2 and 5. More than 100,000 seeds per season gives the weed plenty of opportunity to select for genes that resist applications of everything but steel.

A post emergent application of herbicides can usually control pigweed in crops that develop good canopies early in spring. However, pigweed can be tough to kill and it will require a full rate application if it is allowed to grow for a month before application.

The weed thrives in hot conditions and is able to scatter a lot of seed. As a result, the ground will often be a thick carpet of seedlings where it has been allowed to reach maturity.

Redroot pigweed needs warm soil for seeds to germinate, so it often pops up after seeding before in-crop herbicide applications. Seeds remain viable for up to 40 years when buried in the soil.

The weed will sometimes germinate later in the growing season after early applications of post emergent products. This can be a serious problem in crops such as flax and lentils.

Products with some residual control are particularly useful when controlling the weed.

Redroot pigroot quickly develops a large, fleshy red root, which chases moisture and nutrients well, making it dangerous in dry conditions.


The weed sets seed in July and August as tall flower spikes with densely packed blossoms appear. Seeds are round, black and shiny.

Leaves are oval with a notch in the end early in the season but develop a diamond shape later in life.

Lower stalks are heavy and wide with a smooth surface, while upper areas and branches are rough with sparse, coarse hair.

The plant can sneak through the application season because of its rapid growth and season-long germination period, growing to a point where it is can be tough to control.

If allowed to reach maturity, it creates future problems while drastically reducing yields in most crops.

Redroot pigweed also acts a host plant for crop pests such as green peach aphid, tarnished plant bug, European corn borer, flea beetle, mosaic virus and strains of fusarium and rhizoctonia.

It is toxic to livestock, particularly hogs, accumulating oxalates to as much of 30 percent of its dry weight. As a result, larger infestations can’t be grazed or included in greenfeed or silage.


People can also suffer from this plant’s effects in the form of an allergic reactions to its pollen.

Resistance to Group 2 herbicides is limited to Manitoba, Ontario and North Dakota, while tolerance to Group 5 is found only in Ontario and some U.S. states.

Group 5 products include atrazine, which is likely where resistance formed. Group 2 products, such as the imazethapyr or thifensulfuron-methyl ALS inhibitors, are also known to have little effect on some populations. In vegetable production, the weed has also shown resistance to linuron.

A tank mix with other groups is recommended if Group 2 herbicides are used.

Delaying post-emergent applications of broadleaf products until soil temperatures are higher than 20 C can be an effective strategy to control new seedlings.

Vertical tillage or heavy harrows in the fall or early spring will hasten soil warming, which can help ensure earlier weed germination to coincide with crop development.