Washington State University

Weed Science

Water Levels and Weeds

The weed science portion of our research sought to evaluate the effect of deficit irrigation on herbicide performance and weed seed longevity in soil.

Herbicide Performance

On February 23, 2010, we applied four herbicide treatments to blocks within our experimental peppermint fields. The treatments included the herbicides flumioxazin, sulfentrazone, and terbacil, each in a tank mix with paraquat, and paraquat alone, which served as our untreated control. When irrigation water became available in April, line source irrigation treatments were imposed across the test blocks, with five varying levels of water applied. After the first harvest of mint in July, the entire field was irrigated at full capacity in an attempt to prevent death of peppermint in the extreme deficit irrigation plots. The varying levels of irrigation were then imposed for the remainder of the season.

The number of weeds emerged in each plot were counted (by species) June 10 and September 21, 2010 to determine the impact of irrigation level and herbicide treatment on weed population density. Results are posted at the links provided (June results, September results) and illustrated in general below.

Peppermint was harvested July 27 and October 18, 2010.

Visible results of four herbicide treatments

Top Left: Sulfentrazone failed to control many weed species. Top Right: Terbacil controlled most weeds across all irrigation levels. Bottom Left: Flumioxazin controlled most weeds, but not quite as well as terbacil. Bottom Right: Control plot is weedy at all irrigation levels.

Weed Seed Longevity and Viability

Weed seed longevity in three irrigation treatments was evaluated by burying weed seed in nylon mesh packets on February 17, 2009 in native spearmint and December 15, 2008 and November 3, 2009 in peppermint. One hundred seeds of redroot pigweed and of prickly lettuce and fifty seed of Western salsify were buried 2 cm deep in nylon mesh packets and placed within each of three irrigation treatments (100%, 60%, and dry). Each weed species and irrigation regimen was replicated 4 times, except for Western salsify, which was replicated 8 times. Weed seed packets were recovered after 9 and 12 months of burial and weed seed was removed and counted. All intact seed was placed on germination paper in Petri dishes in a dark germination cabinet at 60º F (prickly lettuce and salsify) and 95º F (redroot pigweed) for two weeks. Germinated seed was counted and removed every 2 to 3 days. Results are posted here.




The Weeds

  • Kochia
  • Prickly lettuce
  • Tall hedge mustard
  • Russian thistle
  • Horseweed
  • Tumble mustard
  • Grass weeds
    • Green foxtail
    • Barnyardgrass
    • Crabgrass
  • Redroot pigweed
  • Lambsquarters
  • Western salsify

The Herbicides

  • Flumioxazin (Chateau)
  • Sulfentrazone (Spartan)
  • Terbacil (Sinbar)
  • Each tank-mixed with paraquat
  • Paraquat alone served as control

The Crops

  • Native spearmint
  • Peppermint

The Assessments

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