Knotweed Treatment Update: The Swimming Pool Studies. Tim Miller WSU NWREC

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1 Knotweed Treatment Update: The Swimming Pool Studies Tim Miller WSU NWREC

2 Swimming Pool Knotweed First thought of by Cathy Lucero Fill wading pools with potting soil (to make it easier to dig rhizomes) Transplant knotweed into pools in late summer, overwinter them, then allow to grow the next year until proper timing Dig up rhizomes and look at how well the herbicide translocates into the rhizomes (herbicide should inhibit shoot growth)

3 Pilot Trial

4 Swimming Pool Knotweed Trial Greenhouse transplants of Bohemian knotweed were moved to pools in September, % survival (!!) Plants overwintered in the field

5 Swimming Pool Knotweed Treated 2-year-old knotweed August 29 (full bloom) or October 14 (fall) Rate of 2.5 ml/plant (2.5% solution % surfactant) Foliar treatments with Rodeo or Habitat Injection with Rodeo Rhizomes dug up at 1 DAT or 3 DAT Placed in flats in the greenhouse After five weeks of growth, rhizomes were measured, shoots counted, and biomass of both determined

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9 Dug 1 DAT Dug 3 DAT Rhizomes and shoots from fall application at about four weeks after treatment

10 Total number NS Rhizome Number NS Foliar glyphosate Foliar imazapyr Inject glyphosate Nontreated 2 0 At Bloom Fall Treated August 29 and October 14; rhizomes in greenhouse for 5 weeks

11 g NS At Bloom Rhizome Biomass NS Fall Foliar glyphosate Foliar imazapyr Inject glyphosate Nontreated Treated August 29 and October 14; rhizomes in greenhouse for 5 weeks

12 cm NS Rhizome Length NS Foliar glyphosate Foliar imazapyr Inject glyphosate Nontreated At Bloom Fall Treated August 29 and October 14; rhizomes in greenhouse for 5 weeks

13 Total number 40 At-Bloom Treatments Shoot Number Foliar glyphosate Foliar imazapyr Inject glyphosate Nontreated DAT 3 DAT Treated August 29; rhizomes in greenhouse for 5 weeks

14 Total number Fall Treatments Shoot Number 1 DAT 3 DAT Treated October 14; rhizomes in greenhouse for 5 weeks Foliar glyphosate Foliar imazapyr Inject glyphosate Nontreated

15 g At-Bloom Treatments Shoot Biomass Foliar glyphosate Foliar imazapyr Inject glyphosate Nontreated 0 1 DAT 3 DAT Treated August 29; rhizomes in greenhouse for 5 weeks

16 g Fall Treatments Shoot Biomass Foliar glyphosate Foliar imazapyr Inject glyphosate Nontreated DAT 3 DAT Treated October 14; rhizomes in greenhouse for 5 weeks

17 Swimming Pool Knotweed Second round in Same treatments and measurements as in Treated August 27 (full bloom) or October 14 (fall) First set of data (full-bloom treatments) are presented here Fall-application rhizomes are currently in the greenhouse, to be measured next week

18 Rhizome Production Different in 2013 Overall rhizome length In the 2011 knotweed plants, rhizomes from both timings averaged 349 cm long (11.4 feet) In the 2013 plants, full-bloom plants averaged only 126 cm (4.1 feet) Overall rhizome number In 2011, knotweed plants from both timings produced 7.9 rhizomes per plant (including secondary rhizomes, or rhizome branches) In 2013, full-bloom plants only produced 3.6 rhizomes per plant

19 Total number 14 At-Bloom Treatments Shoot Number Foliar glyphosate Foliar imazapyr Inject glyphosate Nontreated DAT 3 DAT Treated August 27; rhizomes in greenhouse for 5 weeks

20 g At-Bloom Treatments Shoot Biomass Foliar glyphosate Foliar imazapyr Inject glyphosate Nontreated DAT 3 DAT Treated August 27; rhizomes in greenhouse for 5 weeks

21 Summary Seasonal growing conditions apparently play a very large role in knotweed rhizome production Plants in 2011 produced over twice the number of rhizomes that were nearly three times as long as rhizomes from plants grown in 2013 In 2011, nontreated knotweed rhizomes produced 65% fewer shoots in fall than at bloom, although shoots were present at both timings

22 Summary (cont.) In 2011, both glyphosate and imazapyr applied at bloom prevented knotweed shoot growth from rhizomes after 3 days; in 2013, only imazapyr stopped all shoot growth after 3 days Both glyphosate and imazapyr applied in the fall resulted in no knotweed shoot growth after 3 days (2011 only) Based on 1-day suppression data, it appears that imazapyr more quickly stops shoot production than does glyphosate Quicker translocation? Greater activity at lower rates?

23 Conclusions We can conclude that about 3 days is required for full translocation of imazapyr and glyphosate into knotweed rhizomes It appears that 2.5 ml of either product applied to foliage of a single 2-year-old knotweed plant is adequate to suppress shoot growth for up to 11.4 feet of rhizome It may be necessary to apply more glyphosate than imazapyr to achieve the same result

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