Weed of the Week: hemp-nettle

Hemp-nettle is the nastiest of the many species of the mint family found in Western Canada.

The pest, formally known as Galeopsis tetrahit, has been in Western Canada since at least the 1940s and became a significant problem in the late 1960s.

It likes damper regions and thrives in moist, black soils. However, it has recently been moving out of this area and can be found in low-lying areas in the thin black and dark brown soils.

The weed is native to Europe and Asia but is now found throughout Canada and the northeastern United States.

It has also been introduced to New Zealand and the Canary Islands.

Hemp-nettle is considered a noxious weed in Quebec, Manitoba, Alberta and Alaska.

It tends to germinate later in spring, when soil temperatures have increased, and is an aggressive competitor for nutrients and moisture.

The amount of yield loss depends on crop type and the emergence of the weed in relation to the crop. Studies have recorded yield losses of 20 to 40 percent in wheat, 25 in canola, 30 in oats and 85 in seedling alfalfa.

Leaves are simple, coarsely toothed and opposite. The stem is square, bristly and may have an enlarged area below each node.

Each ovary is comprised of four lobes that mature to become a hard, single-seeded nutlet.

It has a uniquely shaped cotyledon, making it easy to identify in the seedling stage. They are described as round to oblong, with a distinct notch at the tip, but the most distinctive characteristic is a pair of backward pointing lobes on the stem at the plant’s base.

Farmers used to rely on MCPA-K, alone or in combination with other products for control.

These products were effective but had to be applied in a narrow window of time in spring. Larger hemp-nettle can re-grow from axial buds where the leaves attach once the plant passes the four-leaf stage.

Effective control with a wider application window was achieved with the introduction of sulfonylurea chemistry and other Group 2 products. However, hemp-nettle has developed resistance to Group 2 products.

Unlike many other Group 2 resistant weeds, hemp-nettle has also developed Group 4 resistance. This was first confirmed in Alberta in 1998 and evolved resistance to Group 2 herbicides in 2006.

Different Group 4 resistant bio-types have been able to withstand dicamba, fluroxypyr and MCPA.

Herbicides containing fluroxypyr and MCPA ester have recently demonstrated good control, but caution should be used because of resistance potential.

Infinity, a newer product that contains Group 27 pyrasulfotole and Group 6 bromoxynil, is effective. Early applications will give the most consistent control.

Pixarro, another new product, contains halauxifen and has hemp-nettle on its label.

All these products are for use in cereals.

Tools such as Group 2 Odyssey and Pursuit or Viper, a Group 2 and 6 combination, provide effective control in pulses.

MCPA or MCPA-K are the only registered control options in flax, but the rates and crop stage timing are critical to avoid crop damage.

Producers should rotate herbicide groups and apply post-emergent products as early as crop staging allows.

Thom Weir, P.Ag, is an agronomist with Farmer’s Edge. He can be reached by emailing thom.weir@farmersedge.ca.

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