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Weed control in tree plantings
by Bob Hartzler

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April 9, 2002 -  Controlling weeds in new tree plantings is an important step in successful establishment.  If left uncontrolled, weeds compete with young seedlings for moisture, sunlight and nutrients.  Competition with weeds is a primary cause of tree seedling death.  A variety of control tactics are available to  manage undesirable vegetation, and a combination of strategies generally provides the most consistent control.  Successful weed management begins well before the trees are planted.  Unfortunately, too many people wait until the new seedlings arrive, or after they're in the ground, to consider how they will deal with the weeds that inevitably will invade the planting.

Site preparation - Weed management in new tree plantings begins with proper site preparation the fall before planting.  The objective is to provide new seedlings with a 3 to 5 ft vegetation-free area, either a strip or circle.  If planting into a perennial sod, the existing vegetation can be killed either with tillage or systemic herbicides.  Herbicides applied in the fall will provide more consistent control than spring applications.  Glyphosate (Roundup, Touchdown, many others) is the most effective herbicide for perennial sods.  The rate required is dependent upon the type of vegetation (fescue, brome, bluegrass, etc.)  and is specified on the herbicide label.  Control can be improved by mowing the area in August - early September to eliminate existing foliage and treating the regrowth when at least ten inches of new foliage is present.  The addition of 2,4-D to glyphosate may improve control of certain broadleaf perennials (e.g. dandelions).

Persistent perennials (Canada thistle, field bindweed, etc.) can not be eliminated with a single herbicide application or tillage operation.  Control operations in areas with these weeds should begin soon enough to allow the regrowth to be treated a second time before planting.  A variety of options are available for these situations.   Make sure products used to control these weeds do not have a long residual that could damage trees planted in treated areas.  For example, the Tordon 22K label states not to plant susceptible broadleaf crops in treated areas for 36 months after application.

If the area is infested with annual weeds, efforts should be made to prevent seed production the year prior to tree planting.  Preventing weed seed set will greatly reduce the number of weeds needed to be controlled in the first few years after planting.  Weeds should be mowed or sprayed with an appropriate herbicide before flowering to insure that seed production is stopped.

After planting - Weeds should be controlled for the first 3 to  5 years after planting.  If the site was prepared properly, the amount of effort needed to manage weeds should decrease as the trees become established and compete more effectively with weeds.  Several options are available, the best method is dependent upon site characteristics, nature of weed infestation, size of planting, and labor availability.

Cultivation or hand weeding can be effective if labor is available.  It typically requires 3-5 operations per year to control weeds mechanically.  To avoid damaging roots, do not cultivate closer than 6-12" from the seedling or deeper than 3".

Mowing is relatively ineffective as a weed management tool since it fails to eliminate competition for water and nutrients by low-growing weeds and grasses.  In addition, there is a high risk of injuring the stems of seedlings through contact with the mower.

Mulches, both organic and inorganic, can be used to control weeds in new plantings.  Mulches control weeds by preventing new weed seedlings from receiving sunlight.  In addition, mulches conserve soil moisture by reducing evaporation from the soil.  Existing vegetation needs to be eliminated before spreading the mulch.  Organic mulches (sawdust, wood chips, bark, straw) should be spread at least 3-4"  thick to effectively control weeds.  If straw is used, it should be raked away from the seedlings in the fall since rodents may nest in the straw and feed on the bark.  A top dressing of nitrogen may be required to replace nitrogen used in decomposition of the organic mulch.

Landscape fabrics are a good option for certain settings.  Material should be selected that allows good water penetration and blocks sunlight.  Fabric that is at least 4 oz in weight should be used so that it will last several years.  A key to success with landscape fabric is to fasten it securely so that it will not be blown by the wind.  The edges should be sealed with soil and staples at least 6" long be used to secure the fabric.  Another option is to spread an organic mulch on top of the fabric.

Herbicides  Chemical control of weeds may be the best option for large plantings, but herbicides must be carefully selected and applied with precision to avoid injuring the new plantings or sensitive plants in adjacent areas.  Appropriate equipment is required to accurately apply the herbicide and the sprayer must be correctly calibrated.  The two primary options available for chemical weed control in plantings are preemergence and postemergence herbicides.  

Most herbicides that are registered for use on new tree plantings have a relatively large margin of safety to trees.  However, if the chemical is not applied as directed on the label or if the trees are stressed from environmental conditions the seedlings may be injured.  Common causes of injury are use of excessive rates due to misapplication or use of a product on a species not listed on the label.  Follow all label precautions and avoid applications to seedlings that are in poor health.  In addition, no herbicide is effective against all weeds.  Products should be selected based on what weeds are present.  A combination of herbicides may provide the most effective control.

Preemergence herbicides are used to control weeds as they germinate.  Most preemergence herbicides have little activity on established weeds.  On new plantings preemergence herbicides should not be applied until after the soil has settled.  On established plantings, most products can be applied either in the fall or spring.  Spring applications should be applied early enough to ensure that rain moves them into the soil profile before weed emergence begins.  If weeds become established before rain activates the herbicide, a shallow cultivation can be used to kill the emerged weeds and move the herbicide into the soil profile.
    Simazine/Princep (simazine) -
Simazine is used to control both grass and broadleaf weed species.  Delay applications until soil has settled.  Simazine has no activity on established weeds so it must be applied to a weed-free soil surface.  Tolerance of seedlings vary so it is important to read the label to determine if it is appropriate for the site.  Product labels vary in the species cleared for use.  For example, Princep sold by Norvartis only lists conifer species, whereas Simazine distributed by Agriliance has a variety of conifer and deciduous species listed.

    Pendulum (pendimethalin) - Pendulum can be applied to seedlings after they have become well rooted.  Pendimethalin is a strong inhibitor of roots, so care must be taken to not apply the material until soil is settled and cracks are not present that will allow the herbicide to reach the seedling roots.  The product is cleared for a wide range of tree species and has a large margin of safety on established plants.  Pendulum is effective against annual grasses and certain small-seeded broadleaf weeds.  Combinations with simazine provide broad spectrum control.

 Surflan (oryzalin) - Oryzalin is in the same chemical family as pendimethalin and is used in a similar fashion with the same precautions.  Combinations with simazine provide broad spectrum control.

   Kerb (pronamide) - Kerb is applied in the fall to control winter annual and certain perennial grasses.  Kerb should be applied when the likelihood of temperatures exceeding 55˚ F is low but before soil freezes.  It will provide little control of weeds that emerge the following spring.

    Oust (sulfometuron methyl) Oust can be used on conifers and selected deciduous plantings.  Although it has some activity on emerged weeds, more consistent results will be obtained by applying the product in early spring before emergence begins.  On deciduous plantings Oust must be applied before seedlings break dormancy in order to avoid injury.  Oust should not be applied on poorly drained soils or soils with pH greater than 7.   Oust is not intended for use on Christmas tree plantings.  Oust has a lower margin of safety to many deciduous species than other preemergence options.

Postemergence herbicides  are used to control established weeds.   Some can be applied over-the-top of seedlings, whereas others need to be directed to minimize contact with trees.  Good coverage of weeds is often required, and timely applications to small weeds will provide more effective control than applications to large weeds.
Goal (oxyflourfen) - This product has both preemergence and postemergence activity.  It can be sprayed over the top of conifer species, but should not be sprayed during bud break or before needles harden off.  Applications on deciduous species must be directed to minimize contact with foliage or green bark.  Disturbance of the soil following application will diminish preemergence activity, and Goal generally does not provide as long control as the other preemergence herbicides.

    Transline (clopyralid) - Clopyralid is a growth regulator herbicide that is safe for use on several woody species.  Only use on plants listed on the label to avoid injury.  Transline is especially effective on Canada thistle and other plants in the Composite (sunflower) family.  Clopyralid is also sold under the trade name Stinger.

    Postemergence grass herbicides - A number of similar products (Envoy, Vantage, Fusilade) can be used to control emerged grasses in tree plantings.  These products have little or no residual activity.  All products require the addition of a spray additive to improve absorption into the foliage of target weeds.

    Roundup and other glyphosate products  - Glyphosate products can be used to spot treat weeds in plantings, but they must be applied in a way to minimize contact with seedlings.  Trees can be covered with buckets or a shielded sprayer can be constructed that minimizes the risk of spray reaching the seedlings.

Summary   Effective management of weeds is critical in the first few years after planting trees.  For large plantings herbicides are often the most economical choice, but on smaller plantings mulching or cultivation may provide an effective alternative.  Mulches have the added benefit of conserving moisture, often a limiting factor in the establishment of trees.  Before purchasing any herbicide, read the label to ensure that is is appropriate for the intended setting.

Acknowledgement:  Much of the information in this article came from the bulletin:  Grass and Weed Control for Tree and Shrub Seedlings.  Iowa Department of Natural Resources, State Forest Nursery.  February 2002.

 Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.