Hybrid Cottonwoods Grown for Pulp
Bob Parker
Reviewed October 31, 2008

HYBRID COTTONWOODS—New Plantings
•  glyphosate
Rate  1.5 to 3.75 lb ae/A, depending on weed species
Time  Apply before planting, for site preparation.
Remarks  May be used to control weeds before planting. Prepare ground early to allow maximum weed emergence. Mix 1 quart or more nonionic surfactant per 100 gal of spray solution. Use surfactant with more than 50% ai. Surfactants with less than 50% ai require a higher surfactant volume per 100 gal of spray solution.
Caution  Using this product without surfactant may reduce performance.
Site of action  Group 9: inhibits EPSP synthase
Chemical family  None generally accepted
  paraquat (Gramoxone, Firestorm)
Rate  0.5 to 1 lb/A
Time  Apply before planting, for site preparation.
Remarks  May be used to control weeds before planting. Prepare ground early to allow maximum weed emergence. Good spray coverage essential for good weed control. Plant with minimal soil disturbance. Add 1 pint of nonionic surfactant or 2 pints of crop oil concentrate to 100 gal of spray mix. Apply in at least 20 gal/A spray volume.
Caution  Paraquat is a restricted-use herbicide.
Site of action  Group 22: photosystem I electron diversion
Chemical family  Bipyridilium
•  trifluralin (several trade names)
Rate  0.5 to 1 lb ai/A
Time  Apply before planting and thoroughly incorporate into top 2 to 3 inches of soil by cross-disking or rototilling.
Remarks  Apply to trash-free and clod-free soil. Rate of application depends on soil type and organic matter. Incorporate into soil within 4 hours of application using equipment that ensures thorough soil mixing. Will not control established weeds.
Site of action  Group 3: microtubule assembly inhibitor
Chemical family  Dinitroaniline
•  pendimethalin (Pendulum)
Rate  2 to 4 lb ai/A
Time  Apply after planting before weed seed germinates.
Remarks  Will not control established weeds. Apply after planting when soil around trees has settled. Rate applied determines the length of residual control.
Caution  Do not let pendimethalin contact developing roots, or roots may be stunted.
Site of action  Group 3: microtubule assembly inhibitor
Chemical family  Dinitroaniline
•  imazaquin (Scepter)
Rate  2 to 3.9 oz ai/A (2.8 to 5.6 oz/A product)
Time  Apply after planting but before tree buds start to swell.
Remarks  Clones may vary in their tolerance to imazaquin. Overhead moisture after application enhances weed control.
Site of action  Group 2: acetolactate synthase (ALS) inhibitor
Chemical family  Imidazolinone
•  oxyfluorfen (Goal, GoalTender, Galigan, or Oxiflo)
Rate  1 to 2 lb ai/A before bud break
Time  Apply after planting to dormant cottonwood cuttings.
Remarks  Direct spray to base of cottonwood tree. For best control, apply before weeds emerge. Adding 1 quart/100 gal spray mix of nonionic surfactant enhances control of emerged weeds.
Caution   Applications after bud break may injure crop.
Site of action  Group 14: protoporphyrinogen oxidase inhibitor
Chemical family  Diphenylether
•  2,4-D amine (several products registered)
Rate  0.24 to 1.43 lb ae/A (0.5 to 3 pints/A)
Time   May be applied before or after planting. Apply when weeds are actively growing, preferably before bud stage on annual weeds and at bud stage for perennials. Repeat treatments may be required for less susceptible weeds.
Remarks  Controls emerged broadleaf weeds. Glyphosate (Accord) may be tank-mixed in preplant applications to broaden weed control.
Caution   Do not let 2,4-D contact leaves or green tree bark. Avoid drift. Do not apply within 2 days after an overhead sprinkler irrigation or rain, and do not irrigate for 3 days after treatment.
Site of action  Group 4: synthetic auxin
Chemical family  Phenoxy acetic acid
HYBRID COTTONWOODS—Established Plantings
•  oxyfluorfen (Goal, GoalTender, Galigan, or Oxiflo)
Rate  1 to 2 lb ai/A before bud break
Time  Apply before bud break to dormant cottonwood cuttings.
Remarks  Direct spray to base of cottonwood tree. For best control, apply before weeds emerge. Adding 1 quart/100 gal spray mix of nonionic surfactant enhances control of emerged weeds.
Caution   Applications made after bud break may injure crop.
Site of action  Group 14: protoporphyrinogen oxidase inhibitor
Chemical family  Diphenylether
•  oryzalin (Surflan AS)
Rate  2 to 4 lb ai/A
Time  Apply to weed-free soil.
Remarks  Apply only to established plantings, which the Surflan label defines as those that have been transplanted into their growing location long enough that soil has firmly settled around the roots from packing and rain or irrigation; 0.5 inch of moisture (rain or sprinkler irrigation) needed to activate. Lower rate controls for 2 to 4 mo, higher rate for 4 to 8 mo.
Site of action  Group 3: microtubule assembly inhibitor
Chemical family  Dinitroaniline
•  diuron (Karmex DF, Direx, or Diuron)
Rate  0.8 to 2.4  lb ai/A
Time  Apply before weeds emerge or while they are small.
Remarks  Labels may differ among products. Use lower rates on coarse soil. Rain or overhead irrigation is necessary to move the herbicide into soil. If weeds are present at time of treatment, add a surfactant at 1 to 2 quarts/100 gal of solution.
Caution   Do not use on gravelly soils or on any soil with less than 0.5% organic matter. Sprinkler irrigation might injure trees in coarse soils. Do not apply to stressed trees; injury may occur. Do not apply to frozen soils. Do not plant treated areas to any other crop for 2 yr after last application.
Site of action  Group 7: photosystem II inhibitor
Chemical family  Substituted urea
•  terbacil (Sinbar)
Rate  0.8 to 1.6 lb ai/A
Time  Apply in late winter or early spring before weeds emerge or while they are small.
Remarks  Use lower rates on coarse soil. Rain or overhead irrigation is needed to move herbicide into soil. If weeds are present at time of treatment, add a surfactant at 1 to 2 quarts/100 gal of solution.
Caution   Do not use on gravelly soils or on any soil with less than 0.5% organic matter. Sprinkler irrigation might injure trees in coarse soils. Do not apply to stressed trees; injury may occur. Do not apply to frozen soils. Do not plant treated areas to any other crop for 2 yr after last application.
Site of action  Group 5: photosystem II inhibitor
Chemical family  Uracil
•  trifluralin (several trade names) 
Rate  1 to 4 lb ai/A incorporated into soil. Consult appropriate label for rates. Adjust rate according to weed pressure.
Time  Before weeds emerge.
Remarks  Apply as a directed spray to soil between rows and beneath trees. Apply to trash-free and clod-free soil. Incorporate into soil within 4 hr of application using equipment that ensures thorough soil mixing but will not injure the trees. Will not control established weeds.
Site of action  Group 3: microtubule assembly inhibitor
Chemical family  Dinitroaniline
•  pendimethalin (Pendulum)
Rate  2 to 4 lb ai/A
Time  Apply to weed-free soil.
Remarks  Will not control established weeds. Rate applied determines length of residual control.
Site of action  Group 3: microtubule assembly inhibitor
Chemical family  Dinitroaniline
•  imazaquin (Scepter)
Rate  2 to 3.9 oz ai/A (2.8 to 5.6 oz/A product)
Time  Apply before weed seeds germinate to control the broadest spectrum of broadleaf weeds.
Remarks  May be applied over the top of actively growing trees; however, clones may vary in their tolerance to imazaquin. Overhead moisture after application enhances weed control.
Caution  Refer to label for rotational crop restrictions.
Site of action  Group 2: acetolactate synthase (ALS) inhibitor
Chemical family  Imidazolinone
•  2,4-D amine (several products registered)
Rate  0.24 to 1.43 lb ae/A (0.5 to 3 pints/A)
Time  Apply when weeds are actively growing, preferably before the bud stage of growth on annual weeds and at bud stage of growth for perennials. Repeat treatments may be required for less susceptible weeds.
Remarks  Special local needs label WA-980013. To control emerged broadleaf weeds.
Caution   Do not let 2,4-D contact leaves or green tree bark. Avoid drift. Do not apply within 2 days after overhead sprinkler irrigation or rain; do not irrigate for 3 days after treatment.
Site of action  Group 4: synthetic auxin
Chemical family  Phenoxy acetic acid
•  dichlobenil (Casoron 4G and CS)
Rate  2 to 4 lb ai/A (4 to 8 lbs/A 50 DF, 1.4 to 2.8 gal/A CS)
Time  Apply to soil surface from mid-November through mid-February to trees established at least 6 mo.
Remarks  Spot treatments to control emerged weeds may be made up to mid-May. Rain required to move herbicide into soil.
Caution   Do not use on trees established less than 6 mo. Do not let granules stick to tree leaves or accumulate at soil line around base of stem.
Site of action  Group 20: inhibits cell wall synthesis Site A
Chemical family  Nitrile
•  sulfometuron (Oust)
west of Cascades in Washington only
Rate  0.375 to 0.94 oz ai/A (0.5 to 1.25 oz/A product)
Time  Apply to soil surface of dormant trees established for at least one growing season.
Remarks  Rain or overhead irrigation required to move herbicide into soil. 
Caution   Do not let spray contact green bud or leaf tissue; injury may result. Limit first use to determine selectivity on specific clone. Do not apply to trees under stress from low fertility, disease, insect damage, severe weather conditions, or water-saturated soils; injury may occur.
Site of action  Group 2: acetolactate synthase (ALS) inhibitor
Chemical family  Sulfonylurea
•  clopyralid (Stinger, Transline, or Clopyr Ag)
Rate  0.125 to 0.25 lb ae/A (0.33 to 0.66 pint/A)
Time  Apply to established plantings, or to new plantings only after they are well established with several inches of new healthy growth, as a banded or directed spray.
Remarks  Consult label for broadleaf weeds that clopyralid can control. Multiple applications may be made as long as total does not exceed 0.5 lb ae/A (1.33 pints/A).
Caution  Avoid drift. Rain or irrigation within 6 to 8 hours of application may reduce efficacy. Carefully follow all rotation crop restrictions and other precautions.
Site of action  Group 4: synthetic auxin
Chemical family  Pyridine