Weed Control in Ornamental Bulb, Rhizome, Corm, and Tuber Crops
Daffodil, dahlia, gladiolus, iris, lily, narcissus, tulip, and peony
Tim Miller
Revised December 17, 2008

Crop rotations and cultural practices used to produce each crop influence the weed spectrum in cultivated or tilled fields. Annual weeds that grow and produce seed quickly or perennials such as yellow nutsedge that are distributed by cultivation and plowing often predominate in cultivated fields. Also, repeated use of the same or similar herbicide will select for resistant weed species, increase the number of tolerant weed biotypes, and change the population of soil microorganisms that degrade particular herbicides, resulting in shorter soil persistence.
Preventing weed shifts  Weeds that survive plowing, cultivation, repeated herbicide treatments, or other routine cultural practices must be eliminated before the tolerant species or biotypes become established. Combine a variety of weed control practices or treatments, rotate practices and herbicides, and spot treat with a hoe or registered herbicide when the weed first appears. Also, clean your equipment when moving from an infested field.
Weed management in annual cropping systems  Successful weed management in annual cropping systems requires a year-round approach using and rotating a combination of weed control practices throughout several years. Developing these strategies requires knowledge of specific weeds that infest your land. Identify and map major weed species and infested patches within each field. With an established point of reference and occasional observations, growers can evaluate weed shifts and adjust crop and weed management strategies before resistant weeds predominate.
Planning weed control options  After accurate identification, plan an efficient year-round weed and crop management program by listing previous rotations, possible herbicide residues, and effective control measures for each weed. Consider the strengths and weaknesses of each control method based on your experience, herbicide labels, and local experts including pesticide dealers, consultants, and Extension agents. Examine the information carefully and select a field, soil conditions, weed infestations, and appropriate control methods compatible with the crop you intend to plant.
Field preparation and planting  Eliminate perennial weeds before planting by designing a selective control program in the previous crop or by controlling the weed during a temporary fallow period. Canada thistle, for example, can be controlled with spot treatments of glyphosate (Roundup or similar product) in other crops, and S-metolachlor (Dual Magnum or Pennant Magnum) or halosulfuron (Permit or Sandea) reduces yellow nutsedge infestations in other crops. During field preparation, destroy all weedy vegetation and prepare a reasonably smooth surface for uniform herbicide application. Wet soils or delayed applications following the last soil disturbance often result in erratic weed control.
Early and midseason scouting  While assessing crop emergence, soil moisture, disease incidence, and other factors, evaluate the performance of the preplant or preemergence weed control treatments. Note field conditions where gravel or a low spot may have caused abnormal weed control or possible crop injury. About midseason, carefully map each field by identifying individual weeds including patches of perennial weeds. Determine whether additional control measures such as cultivation, application or spot spraying of a post-emergence herbicide, or hand-hoeing individual weeds will be necessary to achieve a quality product at harvest yet minimize the chances of allowing a weed shift.
Preharvest scouting and postharvest weed control  Verify the location of perennial weed infestations on your field map and note additional weed species since your midseason evaluation. Soon after harvest, destroy existing weeds and crop stubble to reduce unnecessary increases in pest populations. If perennial weeds were present, maintain optimum growing conditions so appropriate herbicides can be applied to actively growing weed foliage for maximum control. Consider factors such as timing of herbicide applications and stage of weed growth, herbicide persistence in the soil, crop rotations, and label restrictions for subsequent crops when selecting a postharvest herbicide for perennial weed control. Following the postharvest treatment, consider planting a rotation crop such as winter grains or other crops that require significantly different cultural practices.
Note  Herbicides must be applied at exactly the correct rate and time to selectively control weed growth with minimal chance for injury to the crops. Obtain more consistent results by reading the herbicide label and other information about the proper application and timing of each herbicide. Suggested rates listed in this guide are stated as pounds active ingredient per acre (lb ai/A) or pounds acid equivalent per acre (lb ae/A). For band applications over bulb row, reduce quantity of herbicide applied proportionately to the area within the row actually sprayed.
ORNAMENTAL BULB, RHIZOME, CORM, AND TUBER CROPS—Preplant Incorporated
•  EPTC (Eptam 7E) 
dahlia and daylily only
Rate  5 lb ai/A
Time  Apply preplant and incorporate immediately 2 to 3 inches deep for annual weeds and 6 inches deep to suppress growth of certain perennial weeds.
Remarks  Rotate fields to reduce degradation by biotypes of microorganisms.
Site of action  Group 8: lipid synthesis inhibitor but not an ACCase inhibitor
Chemical family  Thiocarbamate
•  trifluralin (Treflan EC, Treflan 5G)
established gladiolus and dahlia only; 5G on bulbous iris, tulip, and narcissus
Rate  Consult labels.
Time  Apply 0.5 to 1 lb ai/A of 4 lb/gal formulation depending on soil type. Incorporate 2 to 3 inches deep with power-driven rotary equipment; be careful not to disturb the crop. Apply 4 lb ai/A of 5% granular formulation to newly planted gladiolus; activate immediately with overhead irrigation.
Remarks  Gladiolus corms smaller than 1 inch diameter may be injured.
Site of action  Group 3: microtubule assembly inhibitor
Chemical family  Dinitroaniline
ORNAMENTAL BULB, RHIZOME, CORM, AND TUBER CROPS—Preemergence
•  oryzalin (Surflan AS)
bulbous iris, narcissus, hyancinth, and tulip
Rate  0.75 to 1.5 lb ai/A
Time  Apply in the fall 2 to 4 wk after planting and final hilling but before weeds emerge. Can reapply at 0.75 lb ai/A in early spring (late February to early March) to weed-free soil.
Remarks  Herbicide is stable on soil surface up to 21 days but must be activated to be effective. A single rain or sprinkler irrigation of 0.5 inch or more is necessary to activate.
Caution  Do not apply to tulip varieties that have emerged 0.75 to 1 inch. Plow deeply before planting any crop.
Site of action  Group 3: microtubule assembly inhibitor
Chemical family  Dinitroaniline
•  bensulide (Prefar 4-E) 
field flowers, bulbs, and ornamentals
Rate  Consult label
Time  Apply fall through spring to weed-free soil.
Remarks  See label for rates for grass species. Can persist up to 18 mo. See label for replanting sensitive crops. Inhibits seedling root growth.
Caution  Do not exceed 25 lb ai/A per year. 
Site of action  Group 8: lipid synthesis inhibitor but not an ACCase inhibitor
Chemical family  Organophosphorus
•  benefin + oryzalin (XL 2G)
bulb iris, tulip, and narcissus
Rate  1.5 to 3 lb ai/A
Time  Apply before weeds germinate and incorporate immediately with shallow cultivation (1 to 2 inches) or with overhead water.
Remarks  Application equipment must be calibrated precisely. Although registered for commercial plantings, product may perform erratically along ridges where granules may not be distributed evenly.
Caution  Do not apply to tulips emerged more than 0.75 inch.
Site of action  (both) Group 3: microtubule assembly inhibitor
Chemical family  (both) dinitroaniline
•  sethoxydim (Segment)
dahlia, gladiolus, and iris only
Rate  0.28 to 0.47 lb ai/A (2.25 to 3.75 pints/A)
Time  Apply at optimum growth stage listed on label. 
Remarks  Identify susceptible grasses and add 2 pints/A nonphytotoxic crop oil concentrate to improve leaf absorption. Control often is erratic on grasses stunted or stressed from drought, high temperatures, or low fertility. Resistant grasses include annual bluegrass and all fine fescues, but quackgrass can be suppressed.
Site of action  Group 1: acetyl CoA carboxylase (ACCase) inhibitor
Chemical family  Cyclohexanedione
•  diuron (Diuron 80WDG)
bulb iris and narcissus; western Washington only
Rate  3.2 lb ai/A (4 lb/A)
Time  Apply once after planting but no later than 4 wk before bulbs emerge.
Remarks  Diuron is most effective when applied before weeds emerge; it will not consistently control large emerged weeds.
Caution  Do not replant treated areas to any crop within 1 yr after last application. Do not use on sand or loamy sand soils.
Site of action  Group 7: photosystem II inhibitor
Chemical family  Substituted urea
•  fenoxaprop (Acclaim Extra)
daylily, iris, and peony only
Rate  0.06 to 0.17 lb ai/A (13 to 39 fl oz/A)
Remarks  Apply adjusted rates, depending on number of grass tillers (see label). Ensure thorough coverage to actively growing, susceptible grasses. Repeat after grass begins active growth, but do not exceed 120 fl oz/A per season.
Site of action  Group 1: acetyl CoA carboxylase (ACCase) inhibitor
Chemical family  Aryloxyphenoxy propionate
•  glyphosate (several products)
iris, tulip, and narcissus
Rate  Consult labels.
Time  Apply to actively growing weeds at least 2 wk before bulbs emerge.
Remarks  Adding a surfactant improves activity. Consult label for specific rates, times of application, and perennial species controlled.
Caution  Do not exceed 8 lb ae/A per year.
Site of action  Group 9: inhibits EPSP synthase
Chemical family  None generally accepted
•  paraquat (Gramoxone Inteon)
iris, narcissus, and tulip; Washington only
Rate  0.47 to 0.94 lb ai/A
Time  Apply as a broadcast application prior to bulb emergence.
Remarks  Supplemental label for Washington. Foliage emerged at application will be severely injured. Add a nonionic surfactant or crop oil concentrate as label specifies, taking care to avoid anionic formulations that form insoluble precipitates.
Caution  A restricted-use herbicide. Identify grasses and adjust rates for susceptibility and stage of weed growth as on label. Do not apply more than twice per season.
Site of action  Group 22: photosystem I electron diversion
Chemical family  Bipyridilium
•  S-metolachlor (Pennant Magnum)
lily, iris, narcissus, and tulip
Rate  Consult labels.
Time  Apply after final hilling operation, before bulbs or weeds emerge.
Remarks  Apply in a minimum carrier of 10 gal/A. Use lower rates on coarse soils low in organic matter and higher rates on fine-texture soils high in organic matter. Allow soil to settle around bulbs after planting before applying herbicide.
Caution  S-metolachlor is a restricted-use pesticide in Washington, to protect groundwater.
Site of action  Group 15: Inhibits very long chain fatty acid synthesis
Chemical family  chloroacetamide
•  napropamide (Devrinol 50DF)
lily, iris, narcissus, and tulip
Rate  4 lb ai/A (8 lb/A)
Time  Apply after October 15 or final hilling operation, before bulbs or weeds emerge. In spring, apply before summer annual weeds germinate; do not apply to tulips showing more than 2 inches top growth.
Remarks  Apply in a minimum carrier of 10 gal/A. Use a lower rate on coarse soils low in organic matter. Allow soil to settle around bulbs after planting before applying herbicide.
Site of action  Group 15: inhibits very long chain fatty acid synthesis
Chemical family  Acetamide
ORNAMENTAL BULB, RHIZOME, CORM, AND TUBER CROPS—Postemergence
•  sethoxydim (Segment)
dahlia, gladiolus, and iris only
Rate  0.28 to 0.47 lb ai/A (36 to 60 fl oz/A)
Time  Apply at optimum growth stage listed on label.
Remarks  Identify susceptible grasses and add 2 pints/A nonphytotoxic crop oil concentrate to improve leaf absorption. Control often is erratic on grasses stunted or stressed from drought, high temperatures, or low fertility. Resistant grasses include annual bluegrass and all fine fescues; quackgrass can be suppressed.
Site of action  Group 1: acetyl CoA carboxylase (ACCase) inhibitor
Chemical family  Cyclohexanedione
•  fenoxaprop (Acclaim Extra)
daylily, iris, and peony only
Rate  0.06 to 0.17 lb ai/A (13 to 39 fl oz/A)
Time  Adjust rates depending on number of grass tillers (see label).
Remarks  Ensure thorough coverage to actively growing, susceptible grasses. Repeat after grass begins active growth but do not exceed 120 fl oz/A per season.
Site of action  Group 1: acetyl CoA carboxylase (ACCase) inhibitor
Chemical family  Aryloxyphenoxy propionate