Washington State University

Weed Science (subpage)

September 2010 Weed Counts

Weed population density data for September, between the first and second mint harvests, is presented below. The most prevalent weeds were mustards (tall hedge mustard and tumble mustard) at both the June and September sampling dates. In September, additional weeds included annual grasses (green foxtail, barnyardgrass, and large crabgrass) and redroot pigweed. Weed densities varied by herbicide treatment and/or irrigation treatment and there were only a few instances in which there was a significant herbicide by irrigation treatment interaction on weed density.

In each of the following tables, means within a column followed by the same letter are not significantly different at the P=0.05 level.

See results of the June 2010 weed counts at this link.


Kochia population density in September was significantly affected by herbicide treatment with the lowest numbers of kochia occurring in flumioxazin and sulfentrazone treated plots across all irrigation levels. Kochia density was greatest in the lowest irrigation treatment, similar to results in June.

Irrigaton Level No/m2   Herbicide No/m2
Deficit Irrig. 0.29 a Untreated 0.13 ab
Med. Dry 0.08 b Sinbar 0.18 a
Medium 0.02 b Chateau 0.02 b
Med. Wet 0.01 b Spartan 0.01 b
Full Irrig. 0.03 b    


Prickly Lettuce

Prickly lettuce population density was not affected by irrigation treatment and all three herbicides reduced prickly lettuce densities compared to the nontreated checks.

Herbicide No/m2*
Untreated 0.184 a
Sinbar 0.002 b
Chateau 0.023 b
Spartan 0.023 b
*Averaged over all irrigation levels; irrigation amount not significant.


Tall Hedge Mustard

Tall hedge mustard population density was affected by both irrigation level and herbicide treatment. Unlike spring counts, highest densities of tall hedge mustard were in the two treatments receiving the least irrigation, possibly due to establishment of many new seedlings with little crop competition following the irrigation of all plots (including deficit irrigated plots) after the first cutting. Terbacil controlled tall hedge mustard well across all irrigation levels. Flumioxazin partially controlled the weed, while sulfentrazone failed to control tall hedge mustard.

Irrigaton Level No/m2   Herbicide No/m2
Deficit Irrig. 117.9 a Untreated 141.0 a
Med. Dry 118.8 a Sinbar   13.3 c
Medium 73.7 b Chateau   49.6 b
Med. Wet 37.9 c Spartan 129.5 a
Full Irrig. 68.6 bc    


Russian Thistle

Russian thistle population density was greatest in the lowest irrigation treatment and greatest in the plots receiving 100% irrigation. Similar to June results, terbacil failed to control Russian thistle whereas sulfentrazone completely controlled Russian thistle and flumioxazin was intermediate.

Irrigaton Level No/m2   Herbicide No/m2
Deficit Irrig. 0.28 a Untreated 0.13 ab
Med. Dry 0.11 b Sinbar 0.17 a
Medium 0.07 bc Chateau 0.09 b
Med. Wet 0.02 c Spartan 0.01 c
Full Irrig. 0.02 c    


Common Lambsquarters

There was a significant herbicide by irrigation level interaction on common lambsquarters population density. In nontreated checks, common lambsquarters density increased as irrigation level decreased. Terbacil and sulfentrazone nearly eliminated lambsquarters across all irrigation levels, whereas flumioxazin partially reduced lambsquarters counts.

  Herbicide Treatments
  Untreated Sinbar Chateau Spartan
Deficit Irrig. 1.55 a 0.0 e 0.0 e 0.08 e
Med. Dry 0.48 c 0.0 e 0.01 e 0.15 de
Medium 0.72 b 0.0 e 0.0 e 0.14 de
Med. Wet 0.33 cd 0.04 e 0.01 e 0.07 e
Full Irrig. 0.20 de 0.04 e 0.02 e 0 e


Redroot Pigweed

Redroot pigweed was much more prevalent in September than in June as it is a later-emerging species requiring warmer soil temperature for germination. Pigweed counts were not significantly impacted by irrigation level, probably a result having irrigated the entire plot area uniformly once after the first peppermint harvest. Terbacil and sulfentrazone applied in February failed to reduce pigweed density in September whereas flumioxazin reduced numbers across all irrigation levels. Terbacil is normally somewhat weak on pigweed and terbacil-resistant pigweed may have been present. Sulfentrazone was likely degraded to levels below biological activity needed for pigweed control due to the long period of time between herbicide application in February and pigweed germination in July.   

Herbicide No/m2*
Untreated 1.29 a
Sinbar 1.24 a
Chateau 0.25 b
Spartan 1.27 a
*Averaged over all herbicide treatments; herbicide treatment not significant.


Grass Weeds

Annual grass weeds were also present in all plots after the first cutting and the entire plot area was treated with sethoxydim August 17, 2010 to control grasses. Grass counts in September were similar over all herbicide treatments and counts increased as irrigation rate decreased. Less peppermint competition was present in low irrigation treatments to compete with late-season grass weeds. In addition, poor grass control is often observed with postemergence graminicides when weeds are water-stressed.

GRASS WEED (green foxtail, barnyardgrass, crabgrass) COUNTS SEPTEMBER 2010
Herbicide No/m2*
Deficit Irrig. 8.45 a
Med. Dry 4.08 b
Medium 0.58 c
Med. Wet 0.87 c
Full Irrig. 0.76 c
*Averaged over all irrigation levels; irrigation amount not significant.


See results of the June 2010 weed counts at this link.

Weed Science subpage

This page is designed to provide additional information pursuant to the Weed Science component of the overall project discussed on the Washington State University website http://hopmintstress.wsu.edu .

It is not designed to be viewed or utilized independently of that website.

View June 2010 weed counts.

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Washington State IPM Coordinator, WSU Irrigated Agriculture Research & Extension Center, Prosser WA 99350, 509-786-9287