Planning your weed control programs

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

This article is a reprint at the suggestion of new Landscape Alert team member Mike Marshall. The original article was published on March 19, 2004, by Rob Richardson.

The first step in planning a weed management program is to decide exactly what you need out of the program. This can be done by understanding the production system and knowing what weeds to expect. Most growers will have thorough knowledge of their production methods, but many will not have the same comprehension of weeds. Knowing what weeds to expect requires knowledge of the past weed history in each field and knowledge of weed identification and biology. The best single resource for weed identification in the field is Weeds of the Northeast. This book is available through MSU Extension (inventory number E-2666) for $33.50. Call 517-353-6740, or order on the Internet at:

Selecting weed management practices

Once you have an idea of what weed pressure to expect, the next step is to select the weed management practices that will be used. These practices may include cultivation, mowing, herbicides, synthetic fabrics, mulches and other methods. A combination of methods will usually provide the best and most sustainable weed management. Here are some things to consider:

Can I cultivate? Cultivation can be effective for controlling many weeds, especially perennials. However, the benefits of cultivation should be considered against the drawbacks, such as crop root damage and increased soil erosion.

Is mowing possible? Mowing will be more useful in Christmas trees than in nursery settings. However, labor and equipment maintenance can make this a costly method. Mowing does offer the benefits of weed suppression and reduced weed seed production.

Are synthetic fabrics an option?These materials can be placed on the soil to prevent weed establishment. Most of these are fairly expensive, but this cost may be warranted in landscape beds, under containers at nurseries, or on the floors of production houses.

Will mulches be effective? It is very difficult to obtain enough quality mulch to use on a large-scale basis. However, mulches may be good options for landscape beds and limited production of Christmas trees or ornamental trees and shrubs. Mulches can reduce annual weed establishment and reduce the amount of herbicide used. Take caution as poor quality mulch may spread weeds like mugwort or Canada thistle.

How much do I hand weed?Hand weeding is very effective, but it is also very expensive. Reducing use of this component by using other methods will usually provide economic benefits. However, hand weeding at appropriate timings or at low weed thresholds will reduce weed seed production and may reduce the total amount of hand weeding needed during the year.

What other methods are available? Many weed control methods may be implemented. Some of these include flaming, use of stale seedbeds, geese, goats, etc. Flaming (controlled fire) could be utilized for controlling weeds around deciduous trees after bark is formed, but it is probably not a good option for conifers or for use during dry weather. Stale seedbeds can be created by cultivating and preparing seedbeds and then waiting. After weed emergence, the soil is then cultivated or sprayed with herbicides to kill the weeds. This can reduce weed populations and will limit further weed emergence in areas where the soil is not disturbed. The use of livestock to control weeds has been done in the past, but livestock may not differentiate between weeds and crops.

What herbicides do I need? Proper herbicide selection depends on many factors including cost, timing of application, spectrum of control, residual activity, possible crop injury, cultural practices, method of application, and others. Some products will control grasses, but not broadleaves or will control broadleaves, but not grasses. Preemergence herbicides generally do not control perennial weeds. Many postemergence herbicides do not have residual activity. Granular herbicides may have greater crop tolerance, but uniform coverage is more difficult. All of these factors (and more!) should be part of the decision making process.

General guidelines
With all crops, weeds need to be managed for the growth stage and time of year in which they compete with the crop.

Christmas trees. Application of residual herbicides in the fall may be more effective for controlling weeds that overwinter than spring applications. Weeds meeting this description include marestail (horseweed), hoary alyssum, spotted knapweed, bull thistle and other annual or biennial weeds. In the spring, preemergence herbicides should be applied before conifer bud-break. With most preemergence products, applications should be made to bare ground or in combination with a postemergence product, such as glyphosate. Herbicides can be applied in strips or around trees to reduce the area treated. In the summer, weeds escaping preemergence applications can be managed by mowing or with postemergence herbicides. Stinger and the post grass herbicides may be applied over-the-top of many conifer species, while most other products should be directed to the trees, spot applied, or applied with shielded sprayers. In late summer or early fall, woody perennial weeds should be managed. The most effective method will be with herbicides applied to the foliage, basal bark applications, or cut stem methods.

Nursery sites.As with Christmas trees, fall herbicide applications may result in the best control of overwintering weed species. In the spring, herbicides should be applied before the crops begin to actively grow. However, preemergence herbicides may not be applied in covered houses so proper timing on some containers may be difficult. Most preemergence herbicides will not provide season-long control, so plan on an additional application later in the season or other methods of control. The post grass killers may be used over-the-top of most crops, and postmergence broadleaf control options may be available for conifers or large deciduous tree species. Liverwort, algae, moss, and other primitive plants will be most susceptible to control during the hottest and driest part of the year, and reducing or eliminating the weed populations this time of year will help to ease problems during the cool season.

Landscapes.In general, use of synthetic fabrics plus mulch will control most weeds in a visually pleasing manner. Broadspectrum postemergence herbicides can then be used to control any weeds that grow through or on these barriers. In other areas, it will probably be useful to apply a preemergence herbicide in early spring and at least one other application later in the year. Broadspectum postemergence herbicides may be used to control anything that the preemergence herbicides do not. Many of the preemergence herbicides are available in sprayable or granular forms. Herbicides are also available that will kill grasses postemergence, but not damage broadleaf ornamentals. In late summer or early fall, use spot applied herbicides to kill any perennial weeds that have escaped.

Future weed management considerations
Take note during the growing season of areas where noxious weeds are present. In these areas additional steps may need to be taken in the future. Fumigation, rotation to fallow ground or smother crops, and crop rotation may be needed. Fumigation will control many noxious weeds in addition to other pests. While expensive, this can be a successful method for eradication of certain species. Rotation to fallow ground can allow growers to plow or use non-crop herbicides to control noxious weeds. This is a good option to reduce the population of weeds that cannot be controlled in the current crop. Smother crops are those that will out-compete weeds for resources and reduce the weed populations. Both smother crops and crop rotation can allow growers to utilize additional herbicides or weed control methods that could not be used in the previous crop. It is also important to make note of any “dirty” areas of production that can contribute to weed proliferation. Make sure media, mulch, and soil remain weed-free while in storage or piles. Buy weed-free rootstock and maintain new stock in a separate area to watch for contamination. Keep non-crop areas near production sites as weed free as possible, and manage these areas to reduce possible contamination. Identify any potential sources of weed infestations and manage or eliminate these areas from concern.

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