Weed Control in Tree Plantings

W-1097 (Revised)

June 2004

Joe Zeleznik, Extension Forester

Rich Zollinger , Extension Weed Specialist

The information in this guide provides a brief review of weed control methods plus a summary of herbicide uses for many of the tree species grown for conservation purposes in North Dakota.

The information is based on federal and state herbicide labels, research at North Dakota Agriculture Experiment Stations and other information from the North Dakota Department of Agriculture.

Establishing and maintaining healthy trees in the northern Great Plains can be a challenge. Average annual rainfall varies from 13 to 20 inches across North Dakota, and moisture is a critical factor for survival and establishment of newly planted trees and shrubs. More than 2.5 million trees are planted each year in conservation plantings.

In North Dakota, several million dollars are spent on trees each year for conservation and landscape plantings. This value does not consider
site preparation costs and annual maintenance. Once established, field windbreaks can increase crop yield by up to 33 percent, and shelter and landscape trees reduce home cooling and heating costs and increase property values. Because weeds lower aesthetic value and seriously compete with trees, weed control is critical for tree survival, establishment and growth.

 

ALWAYS READ AND FOLLOW LABEL DIRECTIONS

Instructions for registered uses of herbicides are given on container labels.

The label is the final guide and should be followed strictly.

The information in this guide only applies to North Dakota because many herbicide uses are allowed only by supplemental or specific labeling for North Dakota.

Persons are required to have the necessary supplemental or specific labeling in their possession at the time of application.

Because labels can and do change frequently, applicators should ensure they are following the most recent label.

This publication is provided for your information.

North Dakota State University or its officers or employees make no claims, representations or guarantees as to product performance nor accept responsibility for results from using herbicides.

Use of trade names in this publication does not constitute an endorsement nor recommendation by NDSU or by the NDSU Extension Service over other similar products with the same active ingredient.

What is a Weed?

A weed is a plant growing where it is not wanted, competing with more desirable plants for water, nutrients, sunlight and space. Any vegetation
growing between tree rows and between trees within the row will reduce available moisture and nutrients in the soil. Weeds compete with trees by quickly developing root systems in the top several inches of soil, which reduce water and nutrient availability to tree roots. Bromegrass and quackgrass form prolific root and rhizome systems that form dense mats. These grasses severely compete with trees for water and nutrients even when the trees are taller than the weeds and competition for light is not a factor. Tall and vining weeds reduce growth of small trees
by blocking sunlight that would reach tree leaves. Examples of tall weeds include common cocklebur, burdock, common ragweed, marshelder and volunteer sunflowers. Vining weeds include field bindweed and wild buckwheat. Other weeds that are particularly detrimental to trees include deep-rooted species such as sweet clover, alfalfa and switchgrass.

Understanding growth cycles and the most vulnerable growth stage of weeds is important for effective control strategies.

Annual weeds are plants that germinate from seed in the spring of the year, grow quickly, flower and set seed during the growing season, and die. Early weed control is critical to reduce competition with trees and to prevent weed/seed production. Control becomes increasingly difficult as weeds develop past the seedling stage.

Biennials require two growing seasons to complete their life cycle. They usually form rosettes the first year and reproduce the second year by bolting or growing a flower stalk. Controlling the rosettes the first year is the easiest and most effective way to control biennials.

Perennial weeds are plants that grow from roots, rhizomes, stolons, tubers, tillers, or seed in the spring, then flower and set seed. Unlike annual plants, they do not die but continue to grow and spread each year through specialized roots and stems. The ability to reproduce from rhizomes makes smooth bromegrass one of the most competitive weeds, and leafy spurge and Canada thistle the most invasive weeds, in
North Dakota tree plantings. Control can be difficult, requiring greater and more consistent efforts than
for annual weeds.

Mechanical Weed Control

Mechanical weed control, such as cultivation or mowing, is one of the oldest agricultural practices. The object of mechanical weed control is to limit competition by uprooting, separation of the green stem and leaves from the root system, or a total maceration of the weed plant. This activity either kills the weed completely or causes sufficient injury to render the plants noncompetitive.

Mechanical weed control is quite effective for control of annual weeds, especially in the seedling stage. However, cultivation may bring weed seeds to the soil surface where they can germinate. Seed dormancy may be broken by exposure to light during tillage or by other changes in the environment around the seed. Therefore, shallow cultivation is encouraged to reduce the number of dormant weed seeds brought to the surface. Repeated tillage may be required throughout the growing season as new weeds emerge. Cultivation actually propagates and spreads a perennial weed problem throughout the tree planting when rhizomes, stolons, tillers, tubers or roots are spread by the tillage tool. Chemical weed control through use of the proper herbicide(s) is the most effective method to control perennial weeds.

After young trees are established and growing, mechanical cultivation deeper than two to three inches can damage tree roots resulting in stunted growth. Tractors and equipment can injure lower branches, root collars or tree trunks, providing entry wounds for insect and disease pests. Excessive tillage can expose the soil to wind erosion that can cause sandblast damage to young trees, but a cover crop between tree rows is a solution to eliminate sandblasting. Excessive tillage also can increase soil erosion.

Tillage practices adjacent to suckering shrubs must be modified as the planting matures. Many shrubs begin suckering (sending up new shoots from the root system) within two to five years after planting. Tillage should be modified to prevent damage or removal of these newly emerged shrub stems. Tillage may need to be done intermittently or performed farther from the row to encourage healthy and vigorous sucker production.

Mechanical weed control implements

A variety of hand tools and tractor-mounted implements are available for mechanical weed control. The size of the tillage implement should
be determined by the particular tree planting, width of the rows and spacing in the row. Equipment used in yards must be smaller than that used in nursery and tree plantations.

Hoes and hand cultivators are some of the oldest tools used for cultivation of trees and crops. Although effective, especially in closely spaced
tree plantings, these methods are laborious and time-consuming in larger tree plantings. Over the years, many designs of hand cultivators have been developed for home gardens and most of these work equally well around trees.

Line trimmers have become increasingly popular with homeowners for grass trimming around trees. When line trimmers are used near trees, plastic tree guards should be placed around the base of young trees and trees with thin bark. These guards protect the tree from accidental wounding and possible girdling of trees. Bark and cambium on larger trees are often severely damaged by careless line trimmer usage. Perennial weeds will develop multiple stems after existing stems are removed by any mechanical tool or implement.

Equipment is an important consideration when planning row spacing of a new tree planting. As trees grow, close cultivation can cause mechanical injury to roots. Cultivation should be used no closer than the edge of the tree crown. To minimize root damage, cultivation depth should be consistent.

In yards, use of a rototiller around trees should be limited to soil preparation for tree planting. Rototillers should not be used around larger trees
but can be effective in providing weed control for young plantings. However, caution is necessary to avoid extensive root damage to the tree. Do not allow the tines or the drag bar of a walk-behind tiller to penetrate deeper than two inches. Tractor-mounted tillers provide more uniform depth control than walk-behind tillers, but require greater caution to prevent damage to trunks, root collars and branches. These same precautions apply to larger, farm-scale rotary tillage implements.

A specialized small rotary tiller has been effective for weed control between trees within the rows (e.g., Weed Badger^®). The cultivator is mounted on a retractable arm to allow movement in and out of the tree row. Several models can be mounted on a tractor or small skid-steer loader. The arm is controlled manually or reacts to a contact bar; i.e., when a tree is contacted the arm automatically pulls out of the row. The contact bar does not work when seedlings are small or weeds are tall. Many conservation districts and private individuals use these machines for in-row weed control services. A small rigid-shank field cultivator works well between tree rows in young windbreaks and forest plantations.

Mulches

Mulches can be either organic material or artificial fabric sheets. They are placed around the trees to serve as physical barriers to weed emergence. Mulches also prevent sunlight from reaching weed seeds or small plants. Decreased weed competition means more moisture and nutrients for tree growth. Mulches conserve soil moisture by preventing evaporation from wind and they keep soils cooler through shading during the growing season. Also, mulches reduce rainfall runoff and increase water infiltration into the soil. Mulches also maintain warmer soil temperature later into the autumn by insulation, allowing for an extended period of root growth for the tree.

Organic mulches include straw, woodchips, chopped corn cobs, grass clippings, various composts, and other organic material. These are
most effective when placed around the tree to the edge of the tree crown. A minimum three-foot circle of mulch is recommended around individual trees, and the diameter of the circle should be increased as the tree grows. Organic mulch must be maintained at three to four inches deep to prevent weed emergence. Sufficient depth must be maintained because control of weeds through mulch may be more difficult than controlling weeds without mulch. Finer-mulch materials are effective at thinner depths while a greater thickness is needed for coarse materials
to prevent weed emergence.

Organic mulches are commonly added to improve soil structure by increasing organic matter content. This practice is effective in heavy clay soils, where added organic matter improves porosity, which in turn increases water permeability and oxygen exchange. In light sandy soils, organic matter improves water- and nutrient-holding capacities of the soil. Organic mulches also provide slow release of nutrients to encourage growth.

Caution should be taken when using fresh sawdust or woodchips around trees. Fresh sawdust and chips from certain trees may contain chemicals that inhibit growth of other trees. Sawdust and woodchips of redwood, cedar, Douglas-fir, larch, black walnut and spruce have inhibited tree growth when used as fresh mulch.

Another problem with fresh woodchips and sawdust is that nitrogen is depleted from the soil as the wood decomposes. Nitrogen loss is only temporary until the initial decay processes have occurred. Woodchips and sawdust can be kept in an outside pile for at least a year so exposure to the environment will leach the harmful chemicals and begin the decay process. When fresh woodchips are used as mulch, ammonium nitrate at one-fourth pound per 100 square feet should be applied.

Artificial mulches, especially various grades and colors of plastic mulches, were first used around trees in home landscaping to control weed growth and conserve soil moisture. Solid plastic sheets did not allow sufficient oxygen exchange between the tree roots and open air. Newer forms of plastic mulches are manufactured with an interwoven or spun fiber to allow more porosity for better water and oxygen exchange. The weed barrier mulches currently being used in windbreak and conservation plantings are sold as three- by three-foot or four- by four-foot squares, or in rolls 4, 6 and 12 feet wide by 300 to 500 feet long.

An artificial weed barrier is normally installed with a specialized machine that unrolls the fabric over the top of the newly planted trees. People or weights are needed to temporarily anchor the starting end of the fabric. Coulters or moldboards open a trench under the edges of the fabric; press wheels push the fabric to the bottom of the newly formed trench; and covering devices place soil on top of the fabric edges to anchor them in place. A person follows behind the machine to cut a short slit at each tree location and pull the tree through the opening. The slit can be C-, J-, L- or X-shaped, leaving one or several flaps of fabric, and should be large enough to allow future growth of the tree without girdling it. A rock, brick or long staples should be placed between trees during application to help hold the weed barrier down.

Several words of caution are necessary about the use of weed barrier fabrics.

1. During initial application, trees should not be left under the weed barrier for more than five minutes, especially when temperatures are above 70 degrees. Tree foliage can burn or completely dry out if the trees are not quickly pulled through the weed barrier fabric.

2. As trees grow, the openings in the fabric must be enlarged, to avoid girdling the trees. Newer fabric types may overcome this drawback.

3. Organic matter and soil should not be allowed to accumulate on the fabric as weeds can become established here, destroying the effectiveness of the fabric.

4. Where suckering from shrubs is desired, consider weed control other than fabric, as fabric reduces or eliminates suckering in many shrub species.

Cover Crops

Cover crops are commonly used in fruit orchards, Christmas tree plantings and other forest plantations to reduce muddy conditions and allow access between tree rows for harvest and maintenance activities. When planted between tree rows, they can reduce the spread of invasive weed species by minimizing their establishment. If weeds do become established, cover crops can control them by shading and competing for water and nutrients at critical times of growth. Cover crops planted between tree rows allow better rainwater penetration into fine-textured soils. More importantly, cover crops can reduce soil erosion and eliminate the drying effects from tillage. However, a weed-free zone should be maintained within the rows or for three feet around individual trees to minimize competition for water and nutrients.

Annual cover crops (three- to six-feet tall) such as flax, corn or sunflower, can be used as temporary small-scale windbreaks to protect sensitive tree species from drying winds. Tall cover crops collect snow for winter cover and add moisture to soils. They reduce wind erosion and sandblasting on light-textured soils. However, despite these benefits, annual cover crops may compete for water, reducing tree growth.

Cover crops are particularly beneficial when used between tree rows that are mulched, especially with synthetic weed barrier mulches, because they eliminate the need for close cultivation and possible disturbance of the mulch. Close tillage can damage synthetic weed barrier mulches by snagging or tearing the fabric. When patches of perennial weeds become established, spot treatment with an approved herbicide may be necessary. As tree canopies expand, the cover crop will gradually be shaded out and weed competition reduced.

Some cover crops and their residues contain allelopathic chemicals that suppress growth of certain plants. Allelopathic cover crops include oats, rye, fescue grasses and hairy vetch. The best way to take advantage of allelopathic cover crops is to control the top growth by mowing or herbicide treatment and retaining the vegetation as a mulch over the soil surface. Allelopathic weed suppression may vary from 30 to 75 days after destruction, depending on the plant species and amount of biomass produced.

University of Minnesota researchers found that a late summer-early fall seeding of rye between five- to eight-foot-tall nursery trees provided control or suppression of many weed species. The rye was sprayed with glyphosate the following summer before seed head formation (early- to mid-June). The rye residue formed an organic mulch that provided weed control and moisture conservation through the rest of the summer.

Seeding a cover crop

Prior to seeding a cover crop, the soil should be lightly cultivated and packed. Seed with a grass or grain drill designed for seeding lightweight seed. If a drill is not available, broadcast the seed by hand or with a spreader, harrow or rake, and pack for good soil contact with the seed. Use the highest rates in Table 1 for broadcast seeding.

Warm-season grasses establish better when seeded in mid- to late-spring (May 10-June 25) while cool-season grasses can be sown in the spring (before May 20) or late summer (August 10-September 10). Fall sowing allows cool-season grasses to establish and grow more rapidly with fall moisture and cool temperatures. Most annual crops need to be reseeded annually to maintain a ground cover, but some, like black medic, may reseed themselves. Several cover crop species, their various growth characteristics, seeding rates and planting depths are presented in Table 1.

Timing, depth of seeding, seeding rate, firmness of seed bed and variety selection all are critical to a successful seeding. Depending on the cover crop, seeds should be planted ½- to 1½-inches deep. A poor stand will allow weeds and unwanted grasses (bromegrass, quackgrass, etc.) to establish, which defeats the purpose of cover planting.

Cover crop species provide the benefits of cover with limited competition to tree growth. If the cover becomes aggressive, a herbicide may be applied selectively to achieve the desired amount of cover. If a cover crop is used in tree plantings without artificial weed barrier mulch, weed control must continue within the tree row.

Table 1.Cover crops: Growth characteristics, seeding rates and planting depths.

		Pounds of
	No. of	Pure Live	Sowing	Sowing
Cover Crop	Seeds/lb.	seed/acre	Depth	Date	Bunch	Sod	Annual	Perennial
		(inches)

 

Warm season grasses
Blue grama `Bad River'
Bouteloua gracilis (cultivar)	825,000	2-5	0.25-0.5	May 10- June 25	X			X
Side-oats grama `Pierre'
`Killdeer' `Butte' Bouteloua
curtipendula (cultivar)	191,020	8-15	025-.5	May 10 - June 25	X			X
Buffalograss `Bowie'
`Titonka' `Cody'
Buchloe dactyloides (cultivar)	56,000	3-6	0.25-0.5	May 10 - June 25		X		X
Cool season grasses
Perennial ryegrass
Lolium perenne	18,000	3.5-7	0.25-0.5	Before May 20	X		X
				Aug. 10 - Sept. 10
Crested wheatgrass
`Parkway' `Ruff' `Hycrest'
Ephraim' `Nordan'
Agropyron cristatum (cultivar)	175,057	10-25	0.25-0.5	Before May 20	X		X
				Aug. 10 - Sept. 10
Legumes
Black medic Medicago lupulina	300,000	6-12	0.25-0.50	Before May 20	X		X
				Aug. 10 - Sept. 10

Chemical Weed Control

Herbicides are widely used for weed control in tree plantings because they provide selective and rapid control of weeds. Some are especially effective on hard-to-control perennial weed species such as quackgrass, bromegrass and Canada thistle where tillage and other control options are limited. A herbicide must be applied at the proper growth stage for effective control. The herbicide selected will vary with application preference, weed type and species or growth stage of the tree. Problem weeds should be controlled before site preparation.

Many herbicides are labeled for use in tree plantings. Some herbicides can be applied directly over the trees, while others must be applied as a directed spray to minimize spray contact or to avoid the trees entirely.

Herbicides are applied in various stages of tree growth. Correctly identify the existing weeds and use herbicides designed to control those weeds. Several publications to help identify weeds correctly are available through the NDSU Extension Service. Select herbicides that are safe for the desired trees and shrubs and apply at the optimum stage for effective weed control.

Site preparation. Prior to planting trees, the site is usually prepared by one of two methods: 1) a nonselective herbicide is applied to kill existing vegetation, followed by a preplant incorporated or pre-emergent herbicide to provide residual weed control; or 2) tillage is used to remove existing vegetation and a preplant or pre-emergent herbicide is used to provide residual weed control.

After planting. During the first season after planting, spot treatment of hard-to-kill perennial weeds may be required. Spray only the weed and avoid drift or spraying young tree bark and foliage. A hand sprayer with a shield or a pail placed over nearby trees will help prevent spray drift damage. Table 2 includes information on mixing and spraying with handheld sprayers.

Fall and early spring application. Weed control during the following growing season can be achieved by applying residual granular herbicide in the fall after trees are dormant or in early spring before trees break dormancy.

In mature trees. Grass weed (bromegrass, quackgrass, etc.) competition can reduce tree growth and vigor. A translocated herbicide applied selectively can remove competition of unwanted weed plants.

In native river stands. Water quality and runoff are major concerns. Some herbicides have a restricted use classification because of water quality concerns, and many general use herbicides cannot be applied in open water. Use only herbicides that are labeled
for use near water.

Herbicide Application with Hand-Held Sprayers

Hand-held sprayers are often used for spot treating patches of weeds or for treating small areas in tree plantings. Spray coverage should be uniform but not to the point of runoff. Calibrating a hand-held sprayer can be difficult because of irregular size and dimension of the area to be sprayed and difficulty in attaining a uniform spray coverage with a single nozzle. The following calibration information may be useful in determining the proper amount of spray volume to mix for an area of known dimension.

Calibrate hand-held sprayers by 1) spraying a known area using water, 2) measuring the amount of water applied, and 3) calculating the number of gallons applied per acre (gpa). For example, 1.5 gallons on 1,000 sq. ft. is the same as 65 gallons per acre.

          (43,560 sq. ft. per acre / 1,000 sq. ft.) x 1.5 gallon = 65 gpa.

The desired rate in lb/A or pt/A is used to calculate the amount of herbicide to add to the spray solution. If 3 pt/A is desired:

          3 pt/A / 65 gpa = 0.046 pt. or 0.74 fl. Oz. or
          1.5 tablespoons per gallon of spray solution
          (16 fl. oz. = 1 pt.; 2 tablespoons = 1 fl. Oz.).

When calibration of a hand-held sprayer is not possible and the herbicide used is safe to the environment and non-target plants, a volume of
50 to 70 gpa can be assumed. However, the actual volume applied can vary considerably with the type of sprayer, spray pressure and technique of the applicator, so calibration is strongly encouraged.

Some herbicide labels specify a percent solution for use in hand-held sprayers. Table 2 provides mixing instructions to obtain solutions of varying percent concentrations on a volume/volume basis.

Table 2. Calibration table.

     Desired                                   Concentraion of herbicide (%)
     solution
      volume           0.5                1.0                     1.5                     2.0                    5.0
        (gal)              ————— Amount of herbicide to add — fl. oz. —————

Prior to Weed Emergence

Postemergence

Postemergence - Directed Spray Only

 C = Control; S = Suppression; N = No control; Blank = No information available.

Table 5. Susceptibility of annual broadleaf weeds to herbicides.
 

C = Control; S = Suppression; N = No Control; Blank = No information available.

Mustard species include wild mustard, flixweed, field pennycress, shepherd's purse, and tansy mustard.
 

Table 6. Susceptibility of perennial broadleaf weeds to herbicides.
 

 C = Control; S = Suppression; N = No Control; Blank = No information available.

Table 7.Herbicide names and formulations.

			Concentration
Product	Company	Common name	and formulation
Casoron	Uniroyal	dichlobenil	4% G
Finale	Bayer	glufosinate	1 lb/gal EC
Fusilade II	Syngenta	fluazifop-P	2 lb/gal EC
Gallery	Dow AgroSciences	isoxaben	75% DF
Goal	Dow AgroSciences	oxyfluorfen	2 lb/gal EC
Ornamec	PBI Gordon	fluazifop-P	0.5 lb/gal EC
Pendulum	BASF	pendimethalin	60% WDG
Poast	BASF	sethoxydim	1.5 lb/gal EC
Princep	Syngenta	simazine	4 lb/gal L, 90% DF
Prowl	BASF	pendimethalin	3.3 lb/gal EC