Grass and Herb Control for Conifer Plantation Establishment

Planting conifers on fields with heavy grass and weeds usually reduces plantation survival and growth. A moderately dense stand of grass in an open field in western Oregon can be expected to remove virtually all available moisture in the top 12 inches of soil by the end of June. Removing the grass and weeds gives seedlings much of the water that the grass would have transpired. Tillage may be used, but mechanized equipment must be able to negotiate the terrain. The advantage of herbicides is that a single treatment can give complete weed control during the first season and may give partial weed control in later seasons.

The method chosen should apply herbicide evenly for uniform weed control near planted seedlings. Many products can be applied over the top of existing conifer seedlings for release applications when done in a calibrated broadcast manner. Hand application equipment or aircraft such as helicopters are the only types of equipment suitable for application on rough terrain. Tractor-mounted sprayers equipped to spray a strip of herbicide down the plantation row have had some success on flat ground. When doing this, it is quite important that a wide enough strip be sprayed on each side of the planted row so that lateral roots do not draw down moisture supply too rapidly. In most situations, a strip at least 4 ft wide should be adequate for some habitat improvement for seedlings.

If spot spraying, it is important to calibrate backpack sprayers and use flat fan type nozzles, such as a Spraying Systems 9502E. This will make a uniform swath, about 5 ft wide when held 4 ft from the ground. As you pass over a seedling, a quick on-off will create square patches with uniform dosage very quickly. Using a solid-cone nozzle in a circular motion around the tree risks overdosing in the center and possible conifer damage. Treating spots less than 3 ft in diameter has not generally proven very satisfactory. Large scale broadcast spraying with a backpack sprayer can be accomplished using a calibrated mini-boom set up with multiple nozzles or waving wand techniques with adjustable cone nozzles or off center nozzles like OC-12 or similar.

2,4-D ester (Weedone LV 4 or LV 6 and many others)

Rate 1 to 2 lb ae/a (33 to 66 oz/a LV 4 or 23 to 46 oz/a LV 6 product)

Time Apply preplant or post plant before conifer budbreak when susceptible plants are developing.

Remarks 2,4-D controls a large spectrum of broadleaved weeds and woody plants on forest sites including thistles, false dandelion, woodland groundsel, sow thistle, alder, manzanita species and many others. Amine salt formulations can also be used, but ester products perform better in early spring and on woody species. For best herbaceous control apply in spring while plants are small and before conifers break bud. Douglas-fir, hemlock, and spruce are tolerant. True firs, redwood and cedars can show injury at high rates. Larch and ponderosa pine can show variable injury. 2,4-D can be a good foliar herbicide tank mix partner with atrazine or sulfometuron, depending on the conifers treated.

Caution Consult label to avoid exceeding maximum yearly and single application rate.

Site of action Group 4: synthetic auxin

Chemical family Phenoxy acetic acid

atrazine (several trade names)

Rate 3 to 4 lb ai/a (3 to 4 quarts/a of liquid or 3.3 to 4.4 lb/a of 90% active product)

Time Apply preplant or postplant before conifer budbreak, when weeds are less than 1.5 inches tall.

Remarks Since atrazine is soil active and requires rainfall to activate it, apply in the spring on high rainfall sites or spring or fall on high elevation, high snowfall sites. Atrazine is best at controlling a wide variety of germinating grasses and broadleaf weeds when they are small. All conifer species appear tolerant to atrazine at labeled use rates. A foliar herbicide may be needed as a tank mix partner to achieve the best weed control such as clopyralid, 2,4-D, or glyphosate.

Caution Restricted-use herbicide. Applicators must be licensed to buy and apply atrazine. A closed system must be used for loading atrazine for aerial application at rates higher than 3 lb ai/a. Special buffer requirements exist for applications around certain streams in forests. Check label and state forestry regulations.

Site of action Group 5: photosystem II inhibitor

Chemical family Triazine

clopyralid (Transline and others)

Rate 0.19 to 0.49 lb ae/a (8 to 21 oz/a product)

Time Apply any time of the year, but especially in the spring when susceptible weeds are present and developing.

Remarks All conifer species appear tolerant to clopyralid at any growth stage. For control of bull thistle and small woodland groundsel, use at least 8 oz/a product. In late spring higher product rates up to 21 oz/a may be needed to control larger plants. Canada thistle is best controlled at bud stage in late spring/early summer with rates at 16 oz/a product or higher. Additional sensitive weeds include other thistles, clovers, hawkweeds, yellow starthistle, and vetches. Clopyralid can be an effective tank mix herbicide with soil active herbicides. Grasses and many other broadleaved weeds are tolerant.

Site of action Group 4: synthetic auxin

Chemical family Pyridine

flumioxazin (SureGuard, Payload, and others)

Rate 0.255 to 0.3825 lb ai/a (8 to 12 oz/a product)

Time Apply preplant or over established seedlings before conifer budbreak.

Remarks Federal supplemental labels allow for use in Oregon, Washington, and Idaho on conifer reforestation sites. This label must be in the possession of the user at the time of application. Flumioxazin controls many broadleaved weeds and certain grasses both pre- and postemergence. Apply before budbreak over seedlings established for at least one year. Douglas-fir, Sitka spruce, grand fir, noble fir, ponderosa pine and lodgepole pine are tolerant. Other species may be treated with local experience. For site preparation wait 3 months after treatment to plant seedlings. The addition of a tank mix foliar herbicide such as glyphosate may be needed to effectively control existing plants for site preparation.

Caution Do not mix with any adjuvant for conifer release.

Site of action Group 14: protoporphyrinogen oxidase inhibitor

Chemical family N-phenylphthalimide

glyphosate (Rodeo, Roundup Custom and many others)

Rate 0.75 lb ae/a (0.75 quarts/a product) for release; 1.5 to 3 lb ae/a (1.5 to 3 quarts/a product) for site preparation.

Time Apply preplant or over established Douglas-fir seedlings before conifer bud swell or budbreak in the spring or after buds set in late summer or fall.

Remarks There are many glyphosate products labeled for forestry use. Products that have no surfactant in the formulation may be safer over conifers for release in the spring. To control many grasses and broadleaved weeds, apply in the spring over Douglas-fir that has been established for at least one year. Additional surfactant may increase risk of injury. Glyphosate controls existing plants growing on the site and provides no residual soil activity. For site preparation, apply any time plants are actively growing. Use higher rates and add additional surfactant per label recommendations. For directed spraying around seedlings anytime most product labels allow a 1-2 percent concentration of product in water sprayed to cover vegetation. Avoid contact with conifer foliage or stems.

Caution Douglas-fir can be injured by glyphosate as buds swell and budbreak occurs in the spring. Apply to dormant seedlings to mitigate risk of injury.

Site of action Group 9: inhibits EPSP synthase

Chemical family None generally accepted

hexazinone (Velpar L, Velpar DF)

Rate 1 to 3 lb ai/a (2 to 6 quarts/a of Velpar L; 1.33 to 4 lb/a of DF formulation)

Time Apply preplant or post plant before conifer budbreak.

Remarks Hexazinone is a soil active product; apply in mid-late spring on high rainfall sites or spring or fall on high elevation, high snowfall sites. Too much precipitation can dilute or negate effectiveness. Hexazinone controls a wide variety of germinating and established grasses and broadleaved weeds. Some woody plants like manzanita and Ceanothus species can be killed or partially controlled on drier sites. Best results occur when they are small. For herbaceous weed control over conifers, rates less than 2 lb ai per acre have been adequate and less damaging. Rates above 2 lb ai/a can injure or kill conifers and are best suited to established ponderosa pine and Douglas-fir where shrub control is wanted on drier sites. Not all conifer species are tolerant. Cedars, redwood, sequoia, larch, white pine, and sugar pine can be severely injured or killed. A suitable foliar herbicide may be needed as a tank mix partner to achieve the best weed control.

Caution Newly planted seedlings, especially container seedlings, can be more sensitive to hexazinone and can be injured or killed. Use lower rates on coarse soils and apply after soils have settled around the roots to help mitigate risk of injury.

Site of action Group 5: photosystem II inhibitor

Chemical family Triazinone

indaziflam (Esplanade F)

Rate 0.73 to 1.46 oz ai/a (3.5 to 7 oz/a product)

Time Apply preplant as a site preparation treatment or after planting and before conifer budbreak or after bud set.

Remarks Indaziflam is a preemergent herbicide that controls certain broadleaf weeds and grasses such as downy brome (cheatgrass). It requires at least 0.25 inches of rainfall to activate. Emerged plants at the time of spraying will not be controlled and may require an additional herbicide active on those species. Best results can be expected on clean sites free of existing vegetation and slash. Conifer species tolerant to broadcast, over the top applications include Douglas-fir, ponderosa pine, sugar pine and coast redwood. White and red fir may be injured or killed. The previous listed conifers plus western larch can be planted into areas sprayed as a site preparation. Other conifer species can be treated or planted if prior experience indicates tolerance.

Caution Total Esplanade F rate applied in a 12 month period must not exceed 10 oz/a. For aerial applications, a 25 foot buffer must be established around lakes, streams, rivers, marshes, estuaries and fish ponds. Do not apply to frozen or snow covered soil or soil with standing water except as stated on the label.

Site of action Group 29: cellulose biosynthesis inhibitor

Chemical family Alkylazine

penoxsulam + oxyfluorfen (Cleantraxx)

Rate 0.031 to 0.046 lb ai/a penoxsulam + 1.47 to 2.21 lb ai/a oxyfluorfen (3 to 4.5 pints/a product)

Time Apply preplant for site preparation or post plant before conifer budbreak between early fall and late winter.

Remarks Available for use through Special Local Need labels for Oregon and Washington. These labels must be in possession of the user at the time of application. Provides both preemergent and some early postemergent control of certain broadleaf weeds like woodland groundsel and false dandelion, as well as grasses. Germinating seeds of woody plants like manzanita and Ceanothus spp. (deer brush, snowbush, and squaw carpet) may also be controlled. Activation by at least 0.5 inches of rainfall within 21 days of application is needed. Best results occur when applications are made prior to weed germination or when weeds are less than 4 inches tall. Postemergent applications require additions of a surfactant, and when applied over conifers, shown to be tolerated by those conifer species. Tolerant conifer species include Douglas-fir, ponderosa pine, sugar pine, incense cedar, white fir and grand fir. Other conifers may be treated with prior experience indicating tolerance. Any soil disturbance after application such as tree planting or machine traffic can disturb the treated layer in the soil and reduce effectiveness. For site preparation, split applications of 2.25 pints/acre in the fall and spring along with additional tank mix herbicides have shown best results.

Caution Buffers of 25 feet must be maintained around lakes, streams and other water sources and commercial fish ponds. For aerial applications, buffers of 150 feet must be maintained around non-targeted vegetable fallow beds, crops, or desirable vegetation. Do not graze or feed treated vegetation to domestic livestock. Do not treat frozen or snow covered ground or irrigation ditches. Maximum application rate is 4.5 pints per acre per year. Check individual state labels for additional precautions.

Site of action (penoxsulam) Group 2: acetolactate synthase (ALS) inhibitor; (oxyfluorfen) Group 14: protoporphyrinogen oxidase inhibitor

Chemical family (penoxsulam) Triazolopyrimidine; (oxyfluorfen) Diphenylether

sulfometuron (Oust XP and others)

Rate 1.5 to 3 oz ai/a (2 to 4 oz/a product)

Time Apply preplant or post plant before conifer budbreak.

Remarks As a soil active product apply in spring on high rainfall sites or spring or fall on high elevation, high snowfall sites. Sulfometuron controls a wide variety of germinating and some established grasses and broadleaved weeds. Applications made to small or less established weeds result in best control. Some species are poorly controlled such as bull thistle, bedstraw, marestail, and woodland groundsel (Senecio sylvaticus). Additional tank mix herbicides such as clopyralid may be needed to achieve best results. Most conifer species like Douglas-fir, hemlock, spruce, true firs (Abies spp.), and pines are tolerant. Ponderosa pine, cedars, and true firs may show delayed budbreak and shoot growth in the first year.

Caution Use lower rates on cedar species (see label).

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

Chemical family Sulfonylurea