Kochia management on the Northern Plains and Prairies

Identification and biology

Kochia (Bassia scoparia, still listed as Kochia scoparia in many guides) is a summer annual broadleaf that grows upright to about 1.8 m (6 ft) in a competing crop and forms a bushy 1 m (3–4 ft) plant in the open. It uses the C4 pathway and roots deeply in drought. Its biology sets the management calendar:

• It emerges first. Most kochia emerges in a two- to three-week window very early in spring; NDSU lists its air temperature range as starting at about 4°C (40°F). Seedlings tolerate frost, and roughly 80% of the population is up before the crop. Smaller flushes continue through July.
• It tumbles. In fall the stem breaks at the base and the plant rolls with the wind, dropping seed along its path. A single plant produces 15,000 to 25,000 seeds, and dense patches far more.
• The seedbank is short. Manitoba Agriculture reports most seed dies within a year; NDSU puts viability at about 5% after one year and 1% after two. A small fraction can persist longer, especially when buried.
• It outcrosses. Pollen stays viable for up to 12 days and moves between neighbouring fields, which is how resistance travels even where tumbleweeds are stopped.
• It competes hard. Manitoba data show 21 plants/m² cutting wheat yield by about a third and 195 plants/m² by 73%.

Look-alikes

Seedlings are most often confused with Russian thistle, the other Prairie tumbleweed, which was surveyed alongside kochia in the AAFC resistance surveys, and with lamb's-quarters. Where a seedling is in doubt, your provincial or state weed guide has side-by-side photos; the control decision is the same for both tumbleweeds: residual first, small plants second.

Documented resistance

Stacked resistance is common: 40% of Saskatchewan populations surveyed in 2019 carried Group 2, 9 and 4 resistance together. Resistance mechanisms matter here. Glyphosate resistance in kochia comes from extra copies of the EPSPS gene, so a higher rate does not recover control, and populations tolerate roughly five times the rate that once worked.

Conditions that favour it

Kochia tolerates salinity, drought and heat and takes over saline rings, field approaches, sloughs, ditches and roadsides first. Chem-fallow and glyphosate-only burnoffs selected the resistant populations: Saskatchewan reports that every confirmed glyphosate-resistant population came from a field with chem-fallow in the rotation. Thin, late-seeded crops in wide rows give it the light it needs in May.

Scouting and when to act

Scout in the first week after the burnoff and again two weeks after the in-crop pass. Plants that survive two herbicide passes should be treated as resistant. Size is the threshold: the Canola Council's data on glufosinate (over 90% control in a 2013–2015 study) depended on plants being under 15 cm, which in canola means spraying at the crop's two-leaf stage. A commercial DNA test for glyphosate resistance is available in Manitoba; North Dakota's National Agricultural Genotyping Center has been developing a test for Group 14 resistance, and no commercial test yet exists for Group 4.

Management

Fall or early-spring residual. Because kochia emerges before most crops are seeded, a soil-applied residual is the one herbicide that meets the main flush. Options depend on the crop that follows and on registration in your province or state. Ethalfluralin granules (Group 3) are applied from about October 1 until the ground is frozen or snow-covered, or very early in spring, incorporated into the top 0.6–1.3 cm (1/4–1/2 in) and spread beyond the patch edge. Group 14 (sulfentrazone, flumioxazin) and Group 15 (pyroxasulfone) products carry fall or pre-plant uses ahead of several crops; NDSU's guidance is to apply fall residuals after soil temperature falls to 4–10°C (40–50°F), and in its 2025 Hettinger trial fall-applied pyroxasulfone gave 71–84% kochia control at 35 days after planting, with September applications holding better than October ones by midsummer. Metribuzin (Group 5) pre-emergence has been effective ahead of soybean, pea and lentil in North Dakota. Rates, crops and re-cropping intervals are on the label registered where you farm.

Pre-seed burnoff. Never glyphosate alone. Effective partners vary by crop and include bromoxynil (Group 6), Group 14 products where resistance has not appeared, topramezone (Group 27) ahead of some crops, dicamba (Group 4) where it still works, and paraquat (Group 22), which NDSU reports is still an effective burndown ahead of any crop.

In-crop. Glufosinate (Group 10) in glufosinate-tolerant canola and soybean remains effective on small plants. In cereals, Group 4 and Group 6 mixes still control susceptible populations. Glyphosate- or Group 2-dependent systems should not be relied on for kochia.

Tillage and cover. Manitoba surveys found 0.4 plants/m² under zero till against 16.3 under conventional tillage, so broad tillage is not the answer, but tilling or close-mowing a green patch before seed set is. Harrowing through a patch spreads debris and seed. Under-seeding sweet clover cut emergence by 80% or more the following year in Saskatchewan work, and perennial, salt-tolerant forages on saline areas remove the nursery. Rotations that mix early- and late-seeded crops, winter cereals and forages, with higher seeding rates and narrower rows, keep the canopy ahead of it.

Patches and borders. Hand-pull scattered resistant plants, mow or spot-till patches where yield is already gone, and manage approaches, ditches and well sites, because tumbleweeds start there. NDSU suggests six to eight rows of a tall crop such as corn or sunflower on the downwind border as a fence for rolling plants.

Harvest weed seed control. Kochia begins shedding seed at about 1,585 growing degree days; in Canola Council data about 90% of seed was still on the plant at a mid-September canola harvest but only 56% by late September. Seed destructor mills kill 90–99% of kochia seed that passes through them. Pre-harvest glyphosate plus saflufenacil 10 days before harvest reduced viable seed by only 28% on average, so it is a dry-down tool, not a seed-control tool.

When a spray pays and when it does not

A residual pays on any field with a kochia history, and it is the one herbicide spend that reliably works on a multiple-resistant population. A glyphosate-only burnoff does not pay where surveys say most populations are resistant; the partner is doing the work. An in-crop rescue on 30 cm plants selects survivors, and where a patch is already seeding in August a mower pays better than a sprayer.

Registration differs across the border

Products registered by EPA in the US are not automatically registered in Canada, and the reverse is also true. A Canadian label carries a Pest Control Products (PCP) registration number from Health Canada's PMRA, and the crops, rates, timings and re-cropping intervals can differ from the US label for the same active ingredient. Check the registered label for your province or state before planning a fall application.