Postemergence weed control in Christmas trees

Controlling weeds is a year-around process. Stay ahead of the weeds with these pointers.

A weed management plan for Christmas trees includes preemergence and postemergence herbicides, mowing, tilling, and hand weeding. Cover crops may be planted between trees to suppress weeds and reduce soil erosion. An effective weed control program is easy to implement, suppresses weeds throughout the growing season, and has no adverse effect on the trees and cover crops. Weed control should be an active process throughout the year. It is not possible to maintain fields weed free with only preemergence herbicides. Many of the problem weeds in tree plantings are perennials and are difficult to control with preemergence herbicides. Christmas tree weed control programs are further complicated by the range of Christmas tree species, various tree sizes and growth stages, many soil types, and limited number of registered herbicides.

Weed competition at any time during the year of establishment may suppress tree growth and result in tree death. In new tree plantings, preemergence herbicides should be the primary weed control method because the herbicides keep weeds from germinating and growing early in the season, when young seedlings are very susceptible to competition. Postemergence herbicide application, hand weeding, tillage, and mowing are effective for killing or removing emerged weeds, but weed competition may have suppressed tree growth already. Improperly applying postemergence herbicides may cause injury to young trees, so it is important to follow label instructions.

Postemergence herbicides target specific biological activities in plants. All plants have essentially the same physiological and biochemical processes. In most cases, selectivity is based on the degree of tolerance to a herbicide. Fortunately, conifers are sufficiently different physiologically from the angiosperms (monocots such as grasses and sedges, and dicots, which are broadleaves), that often there is a natural level of tolerance for herbicides. Some postemergence herbicides may be applied safely over the top of conifers. In other situations, the level of herbicide selectivity is narrower and the Christmas trees are only moderately tolerant of low doses of herbicides. If a herbicide is potentially toxic to Christmas trees, care must be taken to avoid herbicide contact with trees during periods of active tree growth and to apply the herbicides at less sensitive growth stages.

Recently transplanted seedlings and young trees are more sensitive to herbicides than trees established two or more years. In most cases, conifer trees of all sizes are less sensitive to herbicides if sprayed in early spring before budbreak or in the fall after new growth has hardened off. However, postemergence herbicides need to contact actively growing weeds to be effective, and by mid-September most weeds have flowered or become dormant. Some winter annuals, biennials, and perennials are susceptible to postemergence herbicides applied in the fall. These include several mustard species, goldenrod, quackgrass, and horseweed.

When making spring or summer postemergence applications, growers need to select herbicides that have good foliar or translocated activity and are safe on or near trees. In general, avoid broadcast applications of most herbicides between bud break (about May 1) and September 1. If applications are made during the summer, direct the herbicides to the base of the trees and to the area between the trees.

Several herbicides with different modes of action are registered for postemergence weed control in Christmas trees. Growers should consider crop stage, weed species, weed size, method of application, and placement (broadcast or directed) in selecting a postemergence herbicide for an application. An effective postemergence herbicide program should include two or three herbicides and modes of action to expand the weed control spectrum, avoid tree injury and reduce potential for weed resistance to herbicides.

The method of applying herbicides to Christmas trees is always a matter of concern. During the first three years after transplanting seedlings in the field, it is possible to use a boom sprayer for broadcast or directed (with drop nozzles) applications. A highboy sprayer can be used on level ground for a couple more years. As trees expand vertically and horizontally, it becomes more difficult to drive through the plantings. Growers may be tempted to use air-blast sprayers for application of preemergence and postemergence herbicides. However, air-blast sprayers were designed to apply fungicides and insecticides to trees and bushes, not to the area between the trees. Air-blast sprayers should not be used for preemergence herbicide application, and are not very effective for postemergence applications. Soil coverage with herbicides from a blast sprayer is inadequate and most of the herbicides end up on the side of the Christmas trees facing the sprayer.

After trees are too tall for a boom sprayer, growers should consider using backpack sprayers operated by laborers. Be sure to supply operators with personal protective equipment (PPE) to avoid self-contamination. A few herbicides are registered and safe for air application to Christmas trees.

The following herbicides are labeled for postemergence application to Christmas trees. Before applying postemergence herbicides, read the labels and carefully follow instructions to avoid tree injury.

Abbreviations used

BF            Balsam Fir

DF            Douglas Fir

FF            Fraser Fir

WF            White Fir

GF            Grand Fir

NF            Noble Fir

WP            White Pine

BS            Blue Spruce

NS            Norway Spruce

WS            White Spruce

AP            Austrian Pine

SP             Scotch Pine

Stinger 3L (clopyralid); there are generic clopyralid products available.
Species on Label: BF, FF, GF, NF, DF, BS, WP.
Rate: 4-10 fl oz (0.09-0.23 lb ai)/acre
When: Trees of any size, including year of transplanting; apply when susceptible weeds are at the 3-5 leaf stage; for Canada thistle, apply high rate from 5 leaf to bud stage.
Weeds Controlled: Legumes (clover, alfalfa, trefoil), composites (aster, burdock, daisy, goldenrod, common groundsel, horseweed, knapweeds, mayweed, ragweeds, thistles), nightshades, plantains, smartweeds, wild carrot.
Limitations: Do not exceed 8 fl oz (0.188 lb ai) on blue spruce; do not use an adjuvant or surfactant; do not apply with air blast sprayers; may be applied over the top of trees at any stage.
Mode of Action: Clopyralid is a synthetic auxin which disrupts plant hormonal balance.

Lontrel 3L (clopyralid) (same active ingredient as Stinger); for use in nurseries and ornamentals
Species on Label: BF, FF, GF, NF, DF, SP, WP, BS, NS, WS.
Rate: 4-21 fl oz (0.09-0.49 lb ai)/acre           
When: Apply over the top of trees at any stage.
Weeds Controlled: (see Stinger above)
Limitations: Multiple applications may be made; do not exceed 21 fl oz per acre per year; adjuvants normally are not needed and may increase crop injury. Do not apply by aircraft.
Mode of Action: Synthetic auxin.

Turret 5.5L ester (2, 4-D)
Species on Label: DF (over the top); all others: directed application only.
Rate: 10-21 fl oz (0.42-0.90 lb ai)/acre
When: Dormant trees, prior to bud break.
Weeds Controlled: Herbaceous broadleaf weeds and some woody perennials.
Limitations: Do not spray over the top of pines or true firs. Apply before bud break. Do not apply to diseased or stressed seedlings.
Mode of Action: Synthetic auxin; plant hormone disruption

Garlon 3A (triethylamine salt of triclopyr)
Species on Label: Garlon may be applied to all conifer species; BF, FF, BS are less susceptible to injury than DF and WP.
Rate: 2-5 pint (0.75-1.75 lb ai)/acre
When: Late summer or early fall after conifer terminal growth has hardened off.
Weeds Controlled: Most herbaceous and woody broadleaves.
Limitations: Do not apply to newly seeded grass alleys. Do not apply to legume cover crops; apply to trees established for at least one full year.
Mode of Action: Synthetic auxin; plant hormone disruption.

Goal Tender 4 SC (oxyfluorfen) (also Goal 2XL)
Species on Label: FF, GF, NF, DF, SP, WP, BS, NS; others
Rate: Seedbeds: Preemergence: 0.5 to 2 pt (0.25-1 lb ai)/acre
Postemergence: 0.5-1.0 pt (0.25-0.5 lb ai)/acre
Trees in field and containers: 2-4 pt (1-2 lb ai)/acre
When: Seedbeds: apply preemergence before conifer emergence and 5 weeks after emergence. Container-grown conifers and conifer transplants in field: before bud break and after foliage has hardened off. Conifers may be transplanted from seedbeds and sprayed immediately if buds have not expanded yet.
Weeds Controlled: Preemergence- most annual grasses and broadleaves; postemergence- most small annual broadleaves.
Limitations: Maximum of 4 pt (2 lb ai)/acre/year; do not apply inside greenhouses; do not apply to conifers under stress.
Mode of Action: PPO inhibitor; disrupts plant biochemical pathways.

Roundup Pro 4L (4 lb isopropylamine salt of glyphosate/gallon; 3 lb/gallon glyphosate acid equivalent (ae)); generic glyphosate is available.
Species on Label: True firs, Douglas fir, pine, spruce, other conifers
Rate: 1-4 pt (0.375-1.5 lb ae)/acre directed to weeds between rows of trees.
When: Apply after new growth has hardened off. Avoid contact with new tree growth.
Weeds Controlled: Annual and perennial grasses; herbaceous broadleaves; sedges.
Limitations: Glyphosate applied at the labeled rate of 0.5 to 2 qt (0.375-1.5 lb ae/acre) kills many annual and perennial green plants. Glyphosate at 1 lb ae/acre or less usually is not sufficient to kill woody plants and vines. Some woody plants can be killed with multiple applications and higher rates. Glyphosate will not kill field horsetail (Equisetum arvense). Addition of a surfactant increases weed control activity, but also increases potential crop phytotoxicity. Several formulations of glyphosate contain adjuvants and are more phytotoxic to Christmas trees than the 4L formulation, and should not be used on Christmas trees. Apply glyphosate with a hand boom or wiper applicator.
Mode of Action: Glyphosate inhibits the shikimic acid pathway. No other commercial herbicides affect this pathway.

Envoy Plus 0.97L (clethodim; same as Select Max); Fusilade DX 2L (fluazifop-p)
Species on Label: All true firs, Douglas fir, pines, spruces.
Rate: Envoy Plus: 12-32 fl oz (0.09-0.24 lb ai/acre)
Fusilade DX: 16-24 fl oz (0.25-0.5 lb ai/acre)
When: Apply to actively growing grasses; young grass is easier to kill than mature grass.
Weeds Controlled: Most annual and perennial grasses. For annual bluegrass control, use Envoy Plus at high rate; for other perennials, use Fusilade DX at high rate.
Limitations: always include nonionic surfactant (NIS) at 0.25% by volume (2 pt/100 gallons) or crop oil concentrate (COC) at 1% by volume (1 gallon/100 gallons). For improved activity on large annual grasses and perennial grasses include ammonium sulfate (AMS) at 17 lb/100 gal of spray solution. Do not exceed 64 fl oz Envoy Plus or 72 fl oz/acre/year of Fusilade. Wait 14 days between applications. No activity on sedges or other non-grass species. Perennial grasses may require multiple applications for complete kill.
Mode of Action: ACCase inhibition (lipid synthesis inhibition).

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