Weed Control in Tall Bearded Iris; A Field Study with Six Herbicides ...

ORNAMENTALSNORTHWESTARCHIVESSept.-Oct. 1987Vol.11, Issue2Pages 19-20Richard Regan OSU ExtensionHort. Agent, Salem, ORRay William, OSU Extension WeedSpecialist, Corvallis, ORRay Schreiner, Schreiner’s Gardens, Inc.Salem, ORWEED CONTROL IN TALL BEARDED IRIS: A FIELD STUDY WITH SIXHERBICIDES, 1986.Herbicides are used for weed control when producing tall bearded iris rhizomes. Althoughcultivation is used, preemergence herbicides control many of the problem weeds. Herbicidesreduce the amount of hand hoeing and subsequent damage to the shallow rhizomes. Registeredherbicides for this use are limited and phytotoxicity is often a concern. Six materials,metolachlor, napropamide, oryzalin, oxadiazon, oxyfluorfen, and simazine, being considered foruse in tall bearded iris were tested for phytotoxic effects and for their effectiveness in controllingindigenous weeds.Procedures: A field trial was established in the Salem area of the Willamette Valley, 1985.Treatments, described in Table 1, consisted of various rates or combinations of metolachlor,simazine, oryzalin, napropamide, oxadiazon, oxyfluorfen + oryzalin, and an untreated checkapplied over-the-top one day after planting the tall bearded iris cultivar "Stepping Out." Irisrhizomes were planted on September 9, 1985, at a depth of 1/2 to 3 inches in a Woodburn siltloam. The number of rhizomes in each plot ranged between 4 to 10 and averaged 6.4.Combination treatment plots received separate applications of oryzalin in the fall, simazine in thewinter, and napropamide in the spring.Treatments were applied September 19, 1985 to dry soil surfaces as either a spray or granularbroadcast. Spray materials were applied in a water carrier at the rate of 60 gallons per acre with aCO 2 backpack sprayer using two flat fan type 8003 nozzles on an 18 inch boom, while thegranular materials were spread uniformly over the plot with a glass quart shaker. All treatmentsreceived 0.5 to 1.0 inches of irrigation 20 hours after application. Air temperature was 66 degreesF and soil temperature was 67 degrees F. Combination treatment plots received additionalapplications of simazine on January 8, 1986, and napropamide on April 18, 1986. Theseadditional treatments receive 0.2 to 0.5 inches of rainfall 2 to 6 days following application.Individual plot size was 3 x 6 feet and each treatment was replicated four times in a randomizedcomplete block experimental design. Fluazifop-butyl was applied to all plots in March at the rateof 0.12 lb ai/A to control grass weeds. No other herbicides were applied to the trial site. Theuntreated check was hand weeded. Standard cultural practices for iris production were followedfor the crop rotation.Annual ryegrass, curly dock, and wild carrot weed control ratings were recorded on November11, 1985, 47 days after treatment applications. When the November 11, 1985 weed control ratingwas done, oryzalin was the only herbicide applied in the treatments where simazine and

napropamide were to be added at a later date. Leaf area was rated on June 3, 1986, 257 days afterapplication by visual examination. Rhizome fresh weights were measured on June 17, 1986, 271days after treatments were applied. Iris leaves were removed just above the rhizome growing tipwith a sharp knife before weighing.Results and discussion: The average efficacy of weed control for fall populations of annualryegrass, curly dock, and wild carrot is shown in Table 1. Annual ryegrass control in all plotscontaining metolachlor, oxadiazon, and oxyfluorfen + oryzalin was good. Simazine and oryzalinwere not effective at the lowest rate although control did improve with an increase in rate.Control of annual ryegrass was not satisfactory at any rate of napropamide. Metolachlor,simazine, oxadiazon, and oxyfluorfen + oryzalin were very effective in controlling curly dock.Satisfactory control was obtained only at the highest rates of oryzalin and napropamide. Wildcarrot control with simazine, oxadiazon, and oxyfluorfen + oryzalin was very good. Theremaining materials did not give satisfactory control.Phytotoxicity was observed in iris by reductions in leaf area and rhizome fresh weight. Iris inaffected plots had reduced leaf areas caused by slight to severe leaf tip burn and reduced growth.The highest rate of simazine significantly reduced rhizome fresh weight and caused severe leaftip burn. Although slight to moderate tip burn was observed at the lower rates of simazine,rhizome fresh weight was not affected. Slight to moderate growth reduction was observed inplots containing oryzalin, napropamide, oxyfluorfen + oryzalin, and the combination treatments.Metolachlor at the highest treatment rate affected leaf area because of slight leaf tip burn.Growth reduction of iris was very slight in oxadiazon plots. No significant difference wasobserved in rhizome fresh weight other than the highest rate of simazine.Table 1. Weed control and effect on tall bearded iris "Stepping Out" with six herbicides,1985-86.Weed Control Rating 2 Phytotoxicity to Irisannual curly wildryegrass dock carrot3 leafareareduction4 rhizomefreshTreatment )(lb ai/A)Untreated check 0.0 0.0 0.0 0.0 135.0a 5metolachlor 8E (2.0) 6.8 8.8 0.8 0.1 134.8ametolachlor 8E (4.0) 7.8 7.9 6.0 0.0 113.4abmetolachlor 8E (8.0) 8.2 9.9 6.5 1.5 139.9asimazine 80WP (1.6) 2.8 8.5 9.1 0.2 146.6asimazine 80WP (3.2) 8.9 10.0 9.5 0.6 144.3asimazine 80WP (6.4) 6.2 9.5 8.9 3.0 79.8boryzalin 4.OAS (2.0) 2.8 5.0 3.8 0.5 123.7aboryzalin 4.OAS (4.0) 4.8 9.0 4.8 2.0 115.9aboryzalin 4.OAS (8.0) 7.4 6.5 5.0 2.5 111.4abnapropamide 50WP (2.0) 3.5 3.2 4.5 0.2 121.5abnapropamide 5OWP (4.0) 5.2 7.0 6.2 1.1 113.3abnapropamide 50WP (8.0) 5.0 7.5 5.5 1.4 132.1a

oxadiazon 2G (4.0) 8.8 9.9 8.6 0.5 120.6aboxyfluorfen (2%) 3G (3.0)+ oryzalin (1%)8.9 10.0 9.5 1.8 124.Oaboryzalin 4.OAS (2.0)simazine 8OWP (1.6 6 )napropamide 5OWP (4)4.2 4.2 2.2 1.4 116.6aboryzalin 4.OAS (4.0) +simazine 8OWP (1.6 6 ) +napropamide 5OWP (4)5.0 8.0 6.5 1.8 99.9ab1 Planted on September 18, 1985. All treatments were applied on September 19, 1985.2Average weed control rated on November 11, 1985, 47 days after applications on a scale of 0(no control) to 10 (complete control).3Leaf area rated on June 3, 1986, 257 days after applications on a scale of 0 (no reduction) to 10(complete reduction, no leaf growth).4 Mean iris rhizome fresh weight in grams measured on June 17, 1986, 271 days afterapplications.5 Numbers within columns followed by the same letter are not significantly different at the 5%level using the Duncans Multiple Range.6 Combination treatment plots received additional applications of simazine on January 8, 1986and napropamide on April 18, 1986.Pesticide Use - Due to constantly changing laws and regulations, no liability for the suggested use of chemicals inthis Newsletter is assumed by the ONW Newsletter. Pesticides should be applied according to label directions on thepesticide container.Permission to Reprint material appearing in the ONW Newsletter is granted with the request that you credit thesource: Ornamentals Northwest Newsletter, date, volume, issue, page numbers. Do not excerpt or reprint in such amanner as to imply the author's endorsement or criticism of a product or concept.Nondiscrimination - The information in the Ornamentals Northwest Newsletter is provided with the understandingthat no discrimination is intended and that listing of commercial products implies no endorsement by the authors.Criticism of products or equipment is neither intended nor implied.