Tackling troublesome weeds takes time and money-so before you apply, make sure you consider the prevention options as well as incorporating the most appropriate controls for your particular needs.

Weed control in the Pacific Northwest can pose challenges that are unique to the region, but the basics of control apply no matter where your nursery is located. When battling these insidious pests in fields of young plants, it’s critical to follow the kind of protocol that’s designed to take down weeds without risking collateral damage.

Weeds compete with newly planted nursery stock. Deep-rooted perennial weeds can reduce vigor and render a mature product unmarketable due to laws on transporting noxious weeds. Consequently, weeds are controlled with herbicides either broadcast-applied in closely spaced crops, or within the tree or shrub row for wider spaced crops, with alleys being mowed or flailed.

Senecio vulgaris, common groundsel
Photo courtesy of Steve Dewey, Utah State University; Bugwood.org

Weed Shifts

Repeated use of simazine was first demonstrated, in nurseries, to cause a shift within a natural population of common groundsel to a resistant or tolerant biotype. Routine cultural practices also cause such a weed shift. Examples include prostrate weeds that tolerate flailing, deep-rooted or easily spread perennials that tolerate cultivation, and weeds that resist repeated applications of the same or similar herbicide treatments. A significant weed shift noted over the last decade is the rapid increase in marestail or horseweed in many nurseries. Speculation for the cause of this included glyphosate resistance, but recent screening of populations of the Willamette Valley and the Columbia Basin did not find elevated levels of glyphosate tolerance.

Equisetum arvense, field horsetail
Photo courtesy of John Cardina, The Ohio State University; Bugwood.org

Preventing weed shifts

Weeds that survive cultivation, mowing or flailing, specific 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 fields, cultural practices, and herbicides; and spot treat with a hoe or registered herbicide when the weed first appears. Also, clean equipment when moving from an infested field.

Sod covers

In specimen tree plantings, manage either native or planted grasses in row middles to reduce soil erosion and improve traffic conditions during wet weather. In addition, soil moisture and nutrient retention are improved along with increased water infiltration rates for many sods. New sod varieties, including dwarf cultivars that respond to minimal management practices such as drought, low fertility or sublethal rates of postemergence herbicides, are being introduced into various horticultural cropping systems. Consult your local Extension agent, or request recent information about living mulches and their management.

Sonchus oleraceus, annual sowthistle
Photo courtesy of Chris Evans, Illinois Wildlife Action Plan; Bugwood.org

Managing weedy vegetation

Successful weed control in nurseries requires a comprehensive, year-round approach that uses and alternates a combination of weed control practices over several years. Developing these strategies requires knowledge of each weed and weed control practice. Weeds must be identified and information gathered about the effectiveness of each weed control practice.

Consider costs and select herbicide combinations that may be applied together or in split applications that control all weeds present in the nursery. Note the action of each herbicide or how the chemical works in the plant. Then tank mix and alternate use of these products to reduce the chance of developing resistant species or biotypes. Often a combination of mechanical, herbicidal—and sometimes hand-removal or spot-treatment with herbicide sprays or wipers—will provide the most effective year-round control.

Soil-active (preemergence) herbicides

Persistent, soil-applied herbicides can be applied to weed-free soil during winter when rain will activate the chemical; some compounds may be applied throughout the year. Apply lower rates on sandy soils having lower clay, organic matter or cation exchange capacities. Existing vegetation can be controlled by mixing a postemergence contact or translocated herbicide. Consult label for listed species and duration of expected control.

Cirsium vulgare, bull thistle
Photo courtesy of Loke T. Kok, Virginia Polytechnic Institute and State University; Bugwood.org

Postemergence herbicides

In nursery production, postemergence weed control requires precision. Postemergence treatments either selectively control susceptible weeds or are applied with selective equipment.

Frequent scouting to identify susceptible weeds and the correct stage of weed growth must be combined with appropriate weather conditions and labeled spray additives to maximize control. Consult labels for numerous precautions or information about crop or cultivar tolerances. Due to the multitude of nursery crops, always verify selectivity on a few plants before treating the entire block or field.

Pacific Northwest Weed Management Handbook

The online publication is the product of Oregon State University, and it’s designed as a quick and ready reference for weed control practices and herbicides employed in Idaho, Oregon and Washington. Research is conducted and recommendations are made by staff of OSU’s Agricultural Experiment Stations and the Extension Services of the three states involved; results from other states, as well as from the U.S. Department of Agriculture, also are incorporated. The Handbook’s authors and contributors make every effort to list only registered herbicides, and to ensure that the information conforms to product labels and company recommendations.

In order to maintain currency, the handbook is updated regularly. Although the authors strive to provide the most up-to-date information, it is not intended to be a complete guide to weed control or herbicide use, and mention of trade names does not imply endorsement.

Remember: Always comply with the label. Regulations and chemical listings are subject to change, so before you apply, be sure you’re armed with the most current information.

The complete and current Pacific Northwest Weed Management Handbook can be located at http://pnwhandbooks.org/weed.

Use of select herbicides

The accompanying tables provide information specific to the use and performance of individual herbicides. Some of this information is based on observations and research of university extension and research faculty in the Pacific Northwest—Idaho, Oregon and Washington—and the narratives apply only to that region.

Some of these products may not be labeled in your particular area of the country, but if you’re considering their use, make sure all labels are designated Section 3 EPA without exclusions. When in doubt, consult your local Extension agent.

Note:Herbicides must be applied at the correct rate and time to selectively control weed growth. For band applications under tree rows, reduce quantity of herbicide applied proportionally to the area within the row actually sprayed.

This article was adapted, with permission, from the article, “Weed Control in Field-grown Nursery Stock,” a part of the Pacific Northwest Weed Management Handbook. The article can be accessed at http://pnwhandbooks.org/weed/horticultural/nursery-greenhouse-and-bulb-crops/weed-control-field-grown-nursery-stock.

Ed Peachey is Associate Professor/Senior Research, Weed Sciences, in Oregon State University’s Department of Horticulture. He can be reached at peacheye@hort.oregonstate.edu. Sally Benson is editorial director for American Nurseryman; she can be reached at sbenson@mooserivermedia.com.

FIELD-GROWN NURSERY STOCK-Preplant Incorporated and Preemergence


DCPA (Dacthal)

Rate 10.5 to 12 lb ai/A

Time Apply any time to weed-free soil; must be activated with a minimum of 0.33 to 0.5 inch of irrigation or rainfall water, 3 to 5 days after treatment.

Remarks Applications west of the Cascades usually perform poorly. In other areas, can be applied immediately after planting liners. Inhibits cell division or mitosis in roots and shoots.

Caution Do not use on seedbeds.

Site of action Group 3: microtubule assembly inhibitor

Chemical family Phthalic acid


dichlobenil (Casoron 4G)

Established stock only

Rate 4 to 8 lb ai/A (100 to 200 lb/A granular product)

Time Apply midwinter immediately before a cold rain, to reduce volatility and to enhance weed suppression.

Remarks Weigh and distribute exact quantities over precisely measured areas to ensure accurate applications. Avoid applying near tree trunks to reduce possible girdling.

Caution Do not apply for 6 months while roots develop on liners, or until 4 weeks after transplanting stock. Oregon results over 9 years suggest that perennial weeds can be suppressed with 4-, 3-, and 2-lb ai/A rates applied during 3 consecutive years. Grazing livestock is prohibited. Inhibits cellulose and cell wall formation.

Site of action Group 20: inhibits cell wall synthesis Site A

Chemical family Nitrile


dithiopyr (Dimension)

Rate 0.5 lb ai/A

Time Apply to bare ground, before target weeds germinate.

Remarks For best weed control, apply to soil that is free from clods, weeds, and debris such as leaves.

Site of action Group 3: microtubule assembly inhibitor

Chemical family Pyridine


EPTC (Eptam 7E or Eptam 20G)

Rate Consult labels

Time Apply 2 weeks before transplanting balled and canned stock (only), or any time after transplanting or growth on established plants begins in spring.

Remarks Use lower rate and incorporate with nursery cultivator or rototillers to 3 inches for annual weeds, and higher rates to 6 inches for perennial weeds. Inhibits shoots of emerging seedlings.

Site of action Group 8: lipid synthesis inhibitor but not an ACCase inhibitor

Chemical family Thiocarbamate


flumioxazin (SureGuard 51WDG)

Rate 0.25 to 0.38 lb ai/A (8 to 12 oz/A SureGuard)

Time Apply before or after weeds emerge. Kills existing annual weeds if weeds are less than 2 inches.

Remarks Can be used safely on established and dormant conifers. Also can be applied to established deciduous crops (check label) with directed sprays as long as contact with foliage and green bark is avoided. Injury to linden (Tilia spp.), which has a thin, green bark when young, has been noted in several Oregon nurseries. Payload is similar to SureGuard and is labeled for use to maintain bare ground and non-crop areas.

Site of action Group 14: protoporphyrinogen oxidase inhibitor

Chemical family N-phenylphthalimide


flumioxazin (BroadStar 0.25G)

Rate 0.375 lb ai/A (150 lb/A BroadStar)

Time Apply to weed-free containers.

Caution Foliage of desirable crops must be dry at the time of application. Wet foliage will trap granules on leaf surface and burn foliage.

Site of action Group 14: protoporphyrinogen oxidase inhibitor

Chemical family N-phenylphthalimide


isoxaben (Gallery 75 DF)

Rate 0.495 to 0.99 lb ai/A (0.66 to 1.33 lb/A)

Time Apply in late summer to early fall, in early spring, or immediately after cultivation. Apply to debris-free soil surface. Remarks Identify weeds and adjust rates according to charts on label. Activate with 0.5 inch water or shallow cultivation before weeds emerge. Chemical is stable enough when left on soil surface for 21 days.

Caution Do not apply to newly transplanted crops until media settle.

Site of action Group 21: inhibits cell wall synthesis Site B

Chemical family Benzamide


isoxaben + trifluralin (Snapshot 2.5TG)

Rate 2.5 to 5 lb ai/A (100 to 200 lb Snapshot/A)

Time Apply to soil that is free from weeds and debris.

Remarks Soil must be settled with water and free from cracks after transplanting. Activate within 3 days using 0.5 inch of water or shallow cultivation before weeds begin to emerge. Follow label instructions for repeat treatments.

Caution Do not apply to unrooted liners or cuttings, bedding plants, or new-planted ground cover. Site of action (isoxaben) Group 21: inhibits cell wall synthesis Site B; (trifluralin) Group 3: microtubule assembly inhibitor

Chemical family (isoxaben) benzamide; (trifluralin) dinitroaniline


napropamide (Devrinol 2G or Devrinol 50DF)

Rate 4 lb ai/A (8 lb/A Devrinol 50DF)

Time Consult label for crops listed. Apply November to February and irrigate if no rain falls within 2 to 3 days; or apply March through October and irrigate within 24 hours to wet soil 2 to 4 inches deep.

Remarks Shallow mechanical incorporation improves performance. Use higher rates if weed infestations are severe or broadleaf weeds predominate. Inhibits root growth.

Site of action Group 15: inhibits very long chain fatty acid synthesis

Chemical family Acetamide


norflurazon (Solicam)

Established stock only

Rate 3 lb ai/A

Time Apply in fall or spring to bare soil after nursery crops are established. Remarks Rain must be adequate before weeds begin to emerge. Read label thoroughly about site, soil, crop rotation and mixing precautions. Inhibits yellow pigment formation, bleaching green chlorophyll.

Caution Do not apply to cherry, grape or caneberry nurseries.

Site of action Group 12: bleaching; inhibits carotenoid biosynthesis

Chemical family Pyridazinone


oryzalin (Surflan AS)

Rate 2 to 4 lb ai/A (2 to 4 quarts/A Surflan AS)

Time Apply any time to bare soil. Activate within 21 days with 0.5 inch water. Wait at least 90 days before repeating application. Apply only to established plantings.

Remarks Inhibits cell division or mitosis, primarily in roots.

Site of action Group 3: microtubule assembly inhibitor

Chemical family Dinitroaniline


oxadiazon (Ronstar G or Ronstar 50WSP)

Rate 2 to 4 lb ai/A (100 to 200 lb/A Ronstar G)

Time Apply uniformly with granular applicator any time to weed-free soil and when plants are dry. Remarks Irrigate with 0.25 to 0.33 inch water immediately after applying; do not incorporate mechanically. Weeds in the pink family (chickweed, mouseear chickweed, pearlwort) are resistant. Controls weed seedlings by contact action during emergence.

Site of action Group 14: protoporphyrinogen oxidase inhibitor

Chemical family Oxadiazole


pendimethalin (Pendulum 2G, Pendulum WDG or Corral 2.68G)

Rate 1.5 to 2 lb ai/A (75 to 100 lb/A Pendulum 2G)

Time Apply at planting or before weed seed germinates. Soil should be loose and free from established weeds.

Remarks Water within a few days to activate herbicide before weeds emerge. Inhibits mitosis in roots and shoots.

Site of action Group 3: microtubule assembly inhibitor

Chemical family Dinitroaniline


prodiamine (Barricade 4L or RegalKade)

Rate 0.65 to 1.5 lb ai/A (21 to 48 fl oz/A Barricade)

Time Apply to weed-free site.

Remarks Follow with 0.5 inch rain or irrigation, or shallow incorporation. Caution Do not exceed maximum rate on label in a 12-month period. Do not apply to recently planted red maple (Acer rubrum) liners; this species is not labeled, and severe girdling has been observed in several Oregon nurseries.

Site of action Group 3: microtubule assembly inhibitor

Chemical family Dinitroaniline


pronamide (Kerb)

Established stock only

Rate 1 to 2 lb ai/A

Time Apply in fall before leaves drop or soil freezes. Or apply from early to midwinter west of the Cascades to soil relatively free of plant residues.

Remarks Requires soil moisture from rain or irrigation to activate; do not incorporate mechanically. Use lower rates for annual grasses and light-texture soils; higher rates for perennial grasses and heavier soils. Control of established grasses is slow. Degraded by microorganisms in warmer weather. Use on nursery stock established at least 1 year and seedlings that are at least 1 year old. Inhibits root growth.

Caution A restricted-use pesticide.

Site of action Group 3: microtubule assembly inhibitor

Chemical family Benzamide


S-metolachlor (Pennant Magnum)

Rate 1.24 to 2.48 lb ai/A (1.3 to 2.6 pints/A Pennant Magnum)

Time Apply any time to weed-free soil as a spray directed to base of ornamentals transplanted at least 10 days.

Remarks Activate herbicide with light irrigation. Use lower rates on sandy soils and reduced weed infestations. Gives about 60 days control, less under very wet conditions. Suppresses yellow nutsedge when applied preemergence and activated with adequate water.

Caution Do not use on seedbeds or unrooted cuttings. Inhibits seedling roots and/or shoots. Site of action Group 15: inhibits very long chain fatty acid synthesis

Chemical family Chloroacetamide


simazine (Simazine 4L and others)

Established stock only

Rate 2 to 3 lb ai/A

Time Apply at least 1 year after transplanting to weed-free soil and activate with rain or sprinkler irrigation.

Site of action Group 5: photosystem II inhibitor

Chemical family Triazine


trifluralin (4HF or 10G products)

Rate 4 lb ai/A

Time Apply to established nursery crops before weeds germinate. Incorporate with 0.5 inch of irrigation or rain within 3 days of application. Inhibits cell division or mitosis, both in roots and shoots.

Site of action Group 3: microtubule assembly inhibitor

Chemical family Dinitroaniline


FIELD-GROWN NURSERY STOCK- Preemergence Premixes

benefin + oryzalin (XL 2G)

Rate 4 to 6 lb ai/A (200 to 300 lb/A XL 2G)

Time Apply before weeds germinate. Incorporate immediately with shallow cultivation (1 to 2 inches) or overhead water.

Remarks Precisely calibrate application equipment, preferably at half rates to enable two perpendicular passes for greater accuracy. Inhibits cell division or mitosis, primarily in roots.

Site of action (both) Group 3: microtubule assembly inhibitor

Chemical family (both) dinitroaniline


oxyfluorfen + oryzalin (Rout)

Rate 100 lb/A (2% + 1% formulation, respectively)

Time Apply uniformly, either in spring before weeds germinate or to weed-free soil. Remarks Requires 0.5 inch water immediately after application to activate and to wash product from leaves. A contact-type herbicide; inhibits cell division or mitosis in roots and shoots, respectively.

Caution Do not apply to wet foliage or plants with whorls.

Site of action (oxyfluorfen) Group 14: protoporphyrinogen oxidase inhibitor; (oryzalin) Group 3: microtubule assembly inhibitor

Chemical family (oxyfluorfen) diphenylether; (oryzalin) dinitroaniline


oxyfluorfen + pendimethalin (OH2 Ornamental Herbicide)

Rate 100 lb/A (2% + 1% formulation, respectively)

Time Apply uniformly using a rotary or drop-type spreader, either immediately after transplanting new stock or to weed- free soil.

Remarks Requires 0.5 to 1 inch water immediately after application to activate and to wash product from leaves.

Caution Do not incorporate mechanically. A contact herbicide that inhibits cell division (mitosis) in roots and shoots.

Site of action (oxyfluorfen) Group 14: protoporphyrinogen oxidase inhibitor; (pendimethalin) Group 3: microtubule assembly inhibitor

Chemical family (oxyfluorfen) diphenylether; (pendimethalin) dinitroaniline


FIELD-GROWN NURSERY STOCK- Preemergence and Postemergence

atrazine

Conifers only including Douglas-fir, Grand fir, Nobel fir, Sitka spruce and pine

Rate 2 to 4 lb ai/A

Time Apply October through March when trees are dormant.

Remarks Can be applied after small weeds emerge. Requires rain or sprinkler irrigation to activate. Apply only once per year.

Caution A restricted-use pesticide.

Site of action Group 5: photosystem II inhibitor

Chemical family Triazine


oxyfluorfen (Goaltender)

Rate 0.5 to 2 lb ai/A (1 to 4 pints/A)

Time Apply to deciduous, field-grown shade trees listed on label; direct the applications toward the soil and tree base. Apply to 2-0 conifers in winter dormancy fall through winter. After transplanting, irrigate with 0.5 to 0.75 inch water or rain for preemergence control of annual grass and broadleaf weeds. Do not exceed 2 lb ai/A per year.

Remarks Postemergence applications control susceptible broadleaf weeds up to 4 inches tall. Goaltender is a contact herbicide. It inhibits cell division (mitosis) in roots and shoots.

Site of action Group 14: protoporphyrinogen oxidase inhibitor

Chemical family Diphenylether


FIELD-GROWN NURSERY STOCK-Postemergence

asulam (Asulox)

Certain juniper and yew species only

Rate 3.34 lb ai/A

Time Apply postemergence treatment only when weeds are between the stages of early seedling and early seed head formation.

Remarks Use at least 20 gal water. Do not use a surfactant. Controls crabgrass, barnyardgrass and horseweed (Conyza canadensis). Inhibits cell division or mitosis.

Caution Do not reapply within 12 months. Labeled for a limited number of ornamental species; consult label before use.

Site of action Group 18: inhibits DHP synthase step

Chemical family Carbamate


bentazon (Basagran T/O)

Do not use on sycamore or rhododendron

Rate 0.75 to 1 lb ai/A

Time Postemergence control of broadleaf weeds, Canada thistle, yellow nutsedge and musk thistle.

Remarks Direct sprays toward actively growing weed foliage at weed size described on label. Add 2 pints/A crop oil concentrate to enhance activity (see label). Two treatments 7 to 10 days apart may be required for Canada thistle or yellow nutsedge. Temperatures below 55ºF, drought or rain within 8 hours will reduce activity. Inhibits photosynthesis.

Caution Do not apply within 1 year of harvest. Do not exceed 2 lb ai/A per season.

Site of action Group 6: photosystem II inhibitor

Chemical family Benzothiadiazole


clethodim (Envoy Plus)

Rate 0.09 to 0.24 lb ai/A (12 to 32 oz/A Envoy)

Time Apply postemergence to actively growing annual or perennial grasses as listed on label. Remarks For selective postemergence grass control. Consider environmental and plant growth conditions that affect leaf uptake; see label.

Caution Do not exceed 64 fl oz/A per season.

Site of action Group 1: acetyl CoA carboxylase (ACCase) inhibitor

Chemical family Cyclohexanedione


clopyralid (Lontrel Turf and Ornamental)

Rate Consult labels

Time Apply when weeds are small and actively growing. For Canada thistle, apply to rosette stage but before bud stage.

Remarks Apply over the top of labeled crops or as spot treatment to well established plantings. Caution Do not exceed 1.3 pints/A per season. Avoid drift or treatment near susceptible crops or unlabeled and desirable plants.

Site of action Group 4: synthetic auxin

Chemical family Pyridine


fluazifop (Fusilade II or Ornamec Over-the-Top)

Rate 0.25 to 0.375 lb ai/A (16 to 24 oz/A Fusilade II)

Time Apply to actively growing grasses or within 7 days after irrigation with 0.25% nonionic surfactant.

Remarks For selective postemergence grass control. Identify grasses and adjust rates for susceptibility and stage of growth as on label. Note lists of ornamentals and associated instructions regarding topical versus directed sprays. Results often are erratic if grasses are stressed from lack of vigor, drought, high temperature, or low fertility. More mature grasses and quackgrass can be controlled but may require two applications. Annual bluegrass and all fine fescues resist treatment. Do not tank-mix with other pesticides. Inhibits fatty acid production, cell membranes, and new growth.

Caution Do not apply within 5 days of other pesticide treatments. Grazing is prohibited.

Site of action Group 1: acetyl CoA carboxylase (ACCase) inhibitor