AgEBB-MU CAFNR Extension

Green Horizons

Volume 21, Number 1
January 2017

Forest Management

Weed Control in New Tree Plantings

By Hank Stelzer | MU Forestry Extension

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. 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. That means last fall. "Great, Hank!" you are probably muttering to yourself. "I'm reading this in January. NOW what do I do?"

Well, you still have options. It is just that your range of choices is a bit more limited and they require that you pay a little more attention to what you are doing. The best method depends upon site characteristics, nature of weed infestation, size of planting, and labor availability.

Before we begin, let's set the record straight. Mowing is not weed control. It fails to eliminate competition for water and nutrients by low-growing weeds and grasses. In fact with grasses, since they grow from the base of the plant, mowing actually stimulates their growth. In addition, there is a high risk of injuring the stems of seedlings through contact with the mower.

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

Mulches, both organic and inorganic, can be used to control weeds in new plantings. They control weeds by preventing new weed seedlings from receiving sunlight. Plus, 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 another 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 it will last several years. The edges should be sealed with soil and, staples at least 6" long should be used to secure the fabric. Another option is to spread an organic mulch on top of the fabric.

Chemical control of weeds is yet another option and 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 objective is to provide each new seedling with a three-to-five-foot, vegetation-free area as either a strip or circle.

Tree planting can be a fun family activity in the spring. To ensure that legacy "takes root', one must have an effective weed control program.

Most herbicides 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, 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. In addition, no herbicide is effective against all weeds. Products should be selected based on the weeds that are present. A combination of herbicides may provide the most effective control.

Pre-emergence herbicides are used to control weeds as they germinate. They have little, if any, activity on established weeds.

Spring applications should be made 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.

Another important consideration is pre-emergence herbicides should not be applied until after the soil has settled in and around the planting hole. This is because rainfall can move the herbicide into the planting hole and damage the roots.

Simazine/Princep (simazine). Simazine is used to control both grass and broadleaf weed species. Tolerance of seedlings vary so it is important to read the label to determine if it is appropriate for the site and species.

Pendulum (pendimethalin). Pendimethalin is a strong inhibitor of roots, so care must be taken to not apply the material until soil is settled. 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.

Oust (sulfometuron methyl). Oust has a lower margin of safety to many deciduous species than other pre-emergence options. It can be used on conifers (but not intended for use in Christmas tree plantations) 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. In hardwood plantings, oust must be applied before seedlings break dormancy in order to avoid injury. It should not be applied on poorly drained soils or soils with a pH greater than 7.

Post-emergence 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 pre-emergence and post-emergence 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 pre-emergence activity, and it generally does not provide as long of control as the other pre-emergence 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.

Fusilade (fluazifop). Fusilade (similar products include Envoy and Vantage) 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.

Glyphosate products are very effective, 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 used that minimizes the spray reaching the seedling.

In closing, effective weed management 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 it is appropriate for the intended setting.

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