Knapweeds (Centaurea spp. and Acroptilon repens)

The following herbicides usually control treated plants. Plants often regrow, so plan annual applications for several years. Control of regrowth and of new seedlings is much better if a competitive crop or sod is established. A perennial grass is the logical choice because, except for glyphosate, the herbicides listed here will not kill established grasses.

2,4-D

Rate 1 to 2 lb ae/a (4 to 8 lb ae/a for Russian knapweed)

Time Apply at the early stage of flower stem elongation (late April to early May).

Remarks Treatment will control only plants emerged at time of spraying.

Caution Avoid drift to sensitive crops.

Site of action Group 4: synthetic auxin

Chemical family Phenoxy acetic acid

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 to 1.75 oz ae/a (4 to 7 fl oz/a Milestone). Rate of application will depend on knapweed species to be controlled.

Time Consult label for opintsimum timing. Diffuse and spotted knapweed: apply to actively growing plants in fall or in spring from rosette to bolting growth stages. Russian knapweed: apply in spring and summer to plants from bud to flowering stage; in fall, to dormant plants.

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

clopyralid (Stinger or Transline)

Rate 0.25 to 0.5 lb ae/a (0.66 to 1.33 pints/a). Labeled rates vary with crops.

Time Up to the bud stage of knapweeds.

Remarks Results are best if applied to actively growing weeds. See labels for registered sites.

Caution Consult label for crop rotation restrictions before using Stinger. Several crops may be injured up to 4 years after application.

Site of action Group 4: synthetic auxin

Chemical family Pyridine

clopyralid + 2,4-D amine (Curtail)

Rate 2 to 4 quarts/a Curtail

Time Apply after most rosettes emerge but before flower stem elongates.

Remarks Lower rate for in-crop cereal grain application; higher rates for fallow, postharvest, and Conservation Reserve Program (CRP) applications. Consult label for specific directions. CRP applications for established grass only. Apply in enough total spray volume to ensure good coverage for diffuse and spotted knapweed.

Caution Consult label for crop rotation restrictions before using product. Several crops may be injured up to 4 years after application.

Site of action Group 4: synthetic auxin

Chemical family (clopyralid) Pyridine; (2,4-D) Phenoxy acetic acid

diflufenzopyr + dicamba (Overdrive)

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

Time Apply to rosettes.

Remarks Add a surfactant 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

glyphosate

Rate 3 lb ae/a

Time Apply to actively growing knapweed when most plants are at bud stage.

Remarks Glyphosate kills many knapweed plants but also kills grass that might compete with new knapweed seedlings. Russian knapweed is not controlled. When using glyphosate, follow by seeding with a locally adapted grass.

Site of action Group 9: inhibits EPSP synthase

Chemical family None generally accepted

imazapic (Plateau)

Rate 0.188 lb ai/a for Russian knapweed

Time Apply in fall or early winter after Russian knapweed has grown old.

Remarks Use 1 quart/a methylated seed oil as the adjuvant. Selective to most native grasses. Higher rates may suppress seed of some cool-season grasses.

Caution Before using, note crop rotation restrictions.

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

Chemical family Imidazolinone

picloram (Tordon)

Rate 0.25 to 0.5 lb ae/a (1 lb ae/a for Russian knapweed)

Time Apply in late spring before or during flower stem elongation.

Remarks A selective treatment that, at the suggested rate, will not damage perennial grasses. Treatment made in bud stage may not prevent seed production in the year of application. However, seed germination is markedly reduced.

Caution Most formulations are restricted-use herbicides. Do not contaminate water or use in diversified crop areas. Potatoes, beans, and most other broadleaf crops are sensitive to picloram.

Site of action Group 4: synthetic auxin

Chemical family Pyridine

triclopyr + clopyralid (Redeem R&P)

Rate 1.5 to 2 pints/a (2.5 to 4 pints/a for Russian knapweed)

Time Apply from rosette to early bolt stage when weeds are actively growing. Russian knapweed should be in early bud to early flower growth stage.

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