Thistle, Canada (Cirsium arvense)-nonselective and/or non-cropland control

aminocyclopyrachlor + chlorsulfuron (Perspective)

Rate 1.8 to 3.2 oz/a aminocyclopyrachlor + 0.7 to 1.3 oz/a chlorsulfuron (4.5 to 8 oz/a of product)

Time Apply to actively growing plants in spring.

Remarks Adjuvants can be used; these include methylated seed oils 0.5 to 1% v/v, nonionic surfactants at 0.25 to 1% v/v, and crop oil concentrates at 1%v/v. Can be applied using an invert emulsion rather than water.

Caution Even low rates can kill nontarget tree and shrub species, so avoid application within a distance equal to the tree height of the sensitive species. Do not allow spray to drift off target. Can injure several grass species including bromes, as well as basin wildrye.

Site of action (aminocyclopyrachlor) Group 4: Synthetic auxin; (chlorsulfuron) Group 2: ALS inhibitor

Chemical family (aminocyclopyrachlor) Pyrimidine carboxylic acid; (chlorsulfuron) Sulfonylurea

aminopyralid (Milestone)

Rate 1.25 to 1.75 oz ae/a (5 to 7 fl oz/a Milestone)

Time Apply in the spring to plants in the prebud stage of growth or in the fall to plant regrowth.

Remarks A nonionic surfactant at 1 to 2 quarts per 100 gal of spray enhances control under adverse environmental conditions.

Caution Do not allow drift to desirable vegetation. Many forbs (desirable broadleaf plants) can be seriously injured or killed. Do not exceed 7 fl oz/a Milestone per year.

Site of action Group 4: synthetic auxin

Chemical family Pyridine

chlorsulfuron (Telar)

Rate 1.125 oz ai/a (1.5 oz/a)

Time Apply postemergence. For best results, apply to thistles in the bud-bloom stage or to fall rosettes.

Remarks Do not apply to frozen ground. Constantly agitate while mixing in spray solution. Add 0.25% v/v nonionic surfactant to the spray mixture.

Caution Avoid contact with sensitive crops. For non-cropland use only. Chlorsulfuron can persist in soil; if land is to return to cropland, allow sufficient time for product to dissipate. Powdery, dry soils and light, sandy soils should not be treated if rain is not likely after treatment.

Site of action Group 2: acetolactate synthase (ALS) inhibitor

Chemical family Sulfonylurea

clopyralid + 2,4-D amine (Curtail) or clopyralid (Stinger or Transline)

Rate Consult labels. Rate depends on use site.

Time Apply to actively growing thistle after most basal leaves emerge but before bud stage.

Remarks Lower rate for in-crop cereal grain application, higher rate for fallow, postharvest, non-crop, and Conservation Reserve Program (CRP) applications. See label for specifics. CRP applications are for established grass only. For best results, wait at least 20 days after application before disturbing treated areas (cultivation, mowing, fertilization with shank-type applicators) to allow thorough translocation. Apply in enough total spray volume to ensure good coverage.

Caution Consult label for crop rotation restrictions before using these products. Several crops may be injured up to 4 years after application. Do not transfer livestock from treated areas to sensitive broadleaf crop areas without first grazing 7 days on untreated pasture.

Site of action (both) Group 4: synthetic auxin

Chemical family (clopyralid) pyridine; (2,4-D) phenoxy acetic acid

dicamba (Banvel, Rifle, or Clarity)

Rate 2 lb ae/a. Spot treatment: use mixtures of 2 to 4 lb ae dicamba per 100 gal of water.

Time May be applied any time during the growing season.

Remarks Higher rates may be required in high-rain areas. Soil residues are normally for one winter in western Oregon. Apply dicamba to growing thistles; uptake is through both foliage and root system. May mix with glyphosate to apply in fall.

Caution Avoid drift to sensitive crops. Dicamba will damage or kill clovers in pastures.

Site of action Group 4: synthetic auxin

Chemical family Benzoic acid

diflufenzopyr + dicamba (Overdrive)

Rate 0.26 to 0.35 lb ae/a (6 to 8 oz/a)

Time Apply in spring to the rosettes.

Remarks Add a surfactant, either nonionic or methylated seed oil, to the spray mix.

Caution Avoid drift to sensitive crops. Will kill legumes.

Site of action (diflufenzopyr) Group 19: inhibits indole acetic acid transport; (dicamba) Group 4: synthetic auxin

Chemical family (diflufenzopyr) semicarbazone; (dicamba) benzoic acid


Rate Broadcast: 1.5 to 2.25 lb ae/a; wiper: 10 to 33% solution. Hand-held and high-volume equipment: 2% solution

Time Apply when thistles are actively growing but past the bud growth stage. Fall applications must be before the first killing frost. Thistles that were mowed or tilled and have rosettes at least 6 inches wide in late summer or fall can be suppressed with 0.75 lb ae/a glyphosate plus 0.5 to 1% nonionic surfactant applied in 3 to 10 gal/a water.

Remarks Wait 3 days for maximum root translocation of glyphosate before tillage.

Caution Glyphosate controls grasses as well as other vegetation in treated areas.

Site of action Group 9: inhibits EPSP synthase

Chemical family None generally accepted

picloram (Tordon)

Rate In broadcast or boom sprayers, apply 1 lb ae/a. Mixtures normally used for spot treatments include 1 lb ae per 100 gal of water.

Time Control is best if applied to actively growing thistle after most leaves emerge but before bud stage.

Remarks Picloram is both a foliar and soil active herbicide. It will not kill perennial grasses when used according to the label.

Caution Most formulations are restricted-use herbicides. Do not contaminate water. Potatoes, beans, and many other broadleaf crops are very sensitive to picloram. Do not use in diversified cropping areas. Do not graze dairy animals on treated areas within 2 weeks after application. For rates exceeding 0.5 lb ae/a (1 quart/a), apply only as a spot treatment not to exceed 25% of a landowner's acreage in any particular watershed in a single season.

Site of action Group 4: synthetic auxin

Chemical family Pyridine

triclopyr + clopyralid (Redeem R&P)

Rate 2.5 to 4 pints/a

Time Apply from rosette to bud stage to actively growing thistle.

Remarks Add a nonionic surfactant at surfactant manufacturer's recommended rate. Apply in at least 10 gal/a water by ground.

Caution Do not exceed 4 pints/a per year. Do not allow drift to desirable vegetation. Note label restrictions on overseeding or reseeding.

Site of action (both) Group 4: synthetic auxin

Chemical family (both) pyridine