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Controlling Weeds

Most gardens and landscapes require a weed management program to be followed to control weeds and prevent them from competing with cultivated plants. The eradication, or complete elimination, of weeds is virtually impossible, so the goal of such programs is typically to control or minimize the occurrence of weeds.

Most weeds can be controlled with integrated weed management. The four key components of such a program are prevention, cultural control, mechanical control, and chemical control.

Identifying Weeds

The first step in any weed managements system should be to identify weed species that are or might become problematic. Pictures, vegetative characteristics, and line drawings in books and online resources are typically used to identify weed species. However, care must be taken when doing so because weeds may look very different during each of their stages of development, during different seasons, under different conditions, and in different settings in which they grow.

To identify weeds, you should first look for unique characteristics to help narrow the list of potential weeds. This could include a variety of things like the conditions under which they are growing (wet vs. dry), whether its growth is erect or prostrate, leaf arrangements (alternate, opposite, whorled, simple vs. compound, etc.), broad vs. narrow leaves, smooth vs. dentate margins, and more. There are many books and online resources available to assist you with identifying weeds.

Weed Prevention

There are many things that can be done to prevent the spread of weeds. Not only do you want to prevent them from spreading within your own landscape, but if possible, between your site and others.

Ensure that all shovels, mowers, and other equipment used are cleaned before using them at another site to prevent the spread of weed seeds. Lawncare companies should clean their equipment between clients.

To the extent possible, you should maintain not only your landscape, but the areas around it. If your lawn, garden, or other landscape area is bordered by weed infested empty lots, ditches, or adjacent lawns, they can continue to re-infest your landscape. Try to keep these as free of weeds as possible by using nonselective herbicides. Convince neighbors that all should have an integrated weed management program to eliminate cross infestation.

Mulching is a great annual weed deterrent for gardening beds. A 2- to 3-inch layer of mulch will prevent weed seeds from receiving the sunlight needed for germination. Mulch has the added benefits of keeping the soil beneath it moist, cool in summer, and warm in winter as well as beautifying your landscape. Be sure the mulch you use has been treated so that it is free of weeds.

If you use compost or manure, be sure proper composting procedures are followed. The compost piles that reach 160° F will be free of most weeds. If you plant seeds, be sure that the seed packaging label states they are weed-free (or mostly weed-free). If transplanting trees or shrubs with burlap wrapped balls or container plants, make sure they are weed-free.

Cultural Methods of Controlling Weeds

When selecting and planting in your garden and landscape, it is important to ensure that your plants have the best chance to grow and thrive. Many things go into this such as picking the right type of plants for the conditions and growing zone, selecting disease resistant cultivars, planting during the correct time of year, and ensuring the soil is properly maintained with the correct pH, proper aeration, fertilization to name a few. When desirable plants grow and thrive in your garden and landscape, weeds have less chance of spreading and taking hold.

Spacing your plants so that roughly 80% of the soil is shaded can drastically limit most weed growth. Plant shrubs and other flowers densely so little soil is exposed to the sunlight needed for weeds to germinate. Care should be taken not to plant too densely, as this could lead to poor air circulation, diseases, and insect infestations. Crops should be planted in wide rows side by side to shade the soil between.

Moisture can cause desirable plants to fail while providing an environment for weeds to thrive. So, it is important that landscapes and crops are not overwatered. Allow the gardens, landscapes, and lawns to dry thoroughly between each watering. Plants should be grouped based on their watering requirements so that when giving one the required amount of water, you are not overwatering others. The proper used of irrigation heads and driplines can ensure different zones of a garden or landscape each get the correct amount of water.

Mechanical Methods of Controlling Weeds

Properly mowing your lawn can promote grass heath and control weeds. Never cut more than 1/3 of the grass height in a single mowing. Ensure that mower blades are sharp so that the cuts to the grass are clean and do not injure the grass blades. Injuries to the grass make it more susceptible to be overtaken by weeds, insects, and disease.

Hand pulling weeds can be one effective method for controlling annual weeds in smaller areas. It is less effective for perennial weeds because reproductive parts of the weed could break off and t behind in the soil. When pulling weeds, care should be taken to minimize soil disturbance which can lead to the germination of more seeds. Pulling weeds should be done when the soil is moist such as after a light watering.

As mentioned previously, mulching is an effective way of preventing the emergence of annual weeds. It is easy to implement as a mechanical control and relatively inexpensive. Mulch prevents weeds from germinating and growing by excluding sunlight.

Tilling or turning the soil is often used to prepare the soil for planting and to mix in organic material. It is also effective for controlling weeds. A hand operated tiller or tractor pulled plow can be used to till or turn the soil, depending on the size of the job. Doing so damages the vegetative parts of the soil and its roots. The roots become exposed and dry out. This is very effective with younger plants, but repeated tilling may be required for more established perennial weeds. By repeated tilling the soil, even mature perennial weeds will eventually deplete their food store. Even though turning the soil may lead to the germination of additional weeds whose seeds were previously dormant, repeated tilling will eventually kill those plants and deplete the seed bank as well.

Soil solarization is accomplished by covering the soil with clear or black plastic. While clear plastic typically produces higher soil temperatures, black plastic excludes light from reaching weed seeds and seedlings, preventing germination and photosynthesis. It should be implemented when the soil is most and during the summer months. Soil solarization traps heat under the plastic which kills weed seeds as well as nematodes and other disease-causing organisms. It is most effective against cool-season weeds.

Fire can be used as a method of controlling weeds. Not only can it kill the dry mature plant matter and new growth, but it can often kill buried weed seeds as well. Burning may be less effective if the buried weed seeds are dry. It is often used to control weeds in ditches and along the sides of roads. Flaming is a method that uses a fan shaped blow torch to burn weeds in smaller areas like along fences. It is most effective on young weeds, but repeated flaming can also be effective against tough perennials.

Biological Methods of Weed Control

Biological weed control is the utilization of living organisms to manage weeds. These living organisms can include arthropods like insects and mites, pathogens such as viruses, bacteria, fungi, and nematodes, and predators like birds, fish, and other animals. However, these methods are not typically used by amateur home gardeners and master gardeners.

Chemical Methods of Weed Control

Chemical weed control is the application of chemicals known as herbicides to the soil or weeds to control the germination and growth of weeds. Most herbicides work by interacting and interfering with the metabolic functions and biochemical pathways of weeds causing irreversible damage to the weeds’ tissue which eventually leads to its death.

Many factors should be considered when selecting the best herbicide for a specific situation including the desirable plants or crops being grown, the weeds being targeted, how close the weeds are to desirable plants or crops, the growth stage of both the targeted weeds and desirable plants and crops grown nearby, the type of soil where the herbicide is being applied, the season when it is to be applied, the cost, how it will be applied, and how it might affect the surrounding environment.

How Herbicides are Classified

Herbicides can be classified a variety of ways. Some common ways that they are classified include their mode of action, the site of action and the time of application.

Herbicide Classifications by Mode of Action

There are two major categories of herbicides when classified based on their mode of action: contact herbicides and systemic herbicides. Contact herbicides affect the portion of the plant with which they make contact. They are not translocated to other parts of the plant, so it is important that they be applied in sufficient quantity to thoroughly cover the foliage. Systemic herbicides are translocated to other growing parts of the plant via its vascular system after entering the plant through its foliage or roots. Most systemic herbicides are applied to foliage rather than soil.

Herbicides are also classified based on their selectivity. Selective herbicides control specific species of weeds or category of weeds as specified on the label, without harming or affecting other surrounding plants significantly. For example, a broadleaf weed killer might target the broadleaf category of weeds while not damaging the surrounding turf. Nonselective herbicides generally kill any plant with which they come into contact. Glyphosate or Roundup is an example of a nonselective herbicide.

Herbicide Classifications by Application Timing

Herbicides can be classified based on when they are applied. There are four such classifications: preplant incorporated or farrow application, preplant application, preemergence, and postemergence. Some of these terms normally refer to crop herbicides but may also apply to landscaping and gardening use.

Preplant incorporated herbicides or farrow application herbicides must be plowed or tilled into the soil before planting to be effective. If left on the soil surface, these highly volatile herbicides will quickly evaporate into the air. This is usually applied in an agricultural setting well in advance of sowing where crops such as tomatoes, soybeans, corn, and strawberries are being grown commercially.

Preplant herbicides are applied prior to sowing but do not require incorporation into the soil. They are typically applied a few days before sowing.

Preemergent herbicides are applied after a crop, garden, or turf is planted or is established which targets weed seeds before they can germinate. These herbicides kill weeds either by preventing the weed seeds from germinating or by killing the seedling before it emerges from the soil. These are generally selective herbicides and must be applied at the correct time since they are typically only effect for 10-12 weeks. To be effective, the application area should be watered (typically with a half inch or so of water) to move the chemical into the soil. Otherwise, results will be poor.

Postemergent herbicides are applied after weeds have emerged from the soil. They are applied directly to the foliage of existing weeds and therefore do not provide residual protection against the emergence of new weeds. Apply to the point where the herbicide begins to run off the foliage and stop. Any liquid that drips off the plant’s foliage is wasted. The application of postemergent herbicides is most effective when the plant is actively growing rather than when it is seeding or undergoing stress from mowing, heat, drought, or cold weather. After application, you should generally let the herbicide dry on the foliage for at least 6 hours before exposing to rain or irrigation as early watering can wash the herbicide from the plant, though there are some postemergent herbiceds that are rainfast and dry within an hour or so. These herbicides tend to be most effective when applied during times when the temperature is between 60° and 85° F. Temperatures colder than 60° can slow the translocation of the herbicide within the plant while temperatures above 85° F can cause harm to desirable plants not being targeted by the herbicide.

Herbicide Formulations

Herbicides can be purchased in many forms. They are formulated to make handling, mixing, and applying them easier. A letter designation can usually be found on the label indicating the formulation. The major formulation categories and corresponding letter designation are: soluble concentrates (SC), soluble powders (SP), emulsifiable concentrates (E or EC), wettable powder (W or WP), dry flowables (F or DF) and water dispersible granules (DG or WDG), granules (G) and pellets (p), and ready to use (RTU).

Soluble concentrates mix readily with water requiring no agitation. They stay in solution and do not separate or settle in their containers once mixed. They are non-volatile, non-abrasive, do not clog screens or nozzles, and the equipment cleans easily. Soluble powders are the same as soluble concentrates only in powder form making them easier to store, transport, and handle.

Emulsifiable concentrates contain emulsifiers to form stable oil:water mixtures. These emulsifiers wrap around oil soluble chemicals to suspend them in a water-based or aqueous solution like the way small droplets of oil or fat are suspended in water to form milk. In fact, when emulsifiable concentrates are mixed with water, the resulting suspension is “milky white” in color.

Wettable powder is the name attributed to the original dry herbicide formulations. They can be applied as is directly to plants or mixed in water. These herbicides are problematic due to how fine they are (the consistency of flour or talcum powder) which makes them dusty and easy to inhale. The powder tends to stick together and come out the bag in clumps when attempting to weigh and mix them creating clouds of dust.

Dry flowables and water dispersible granules are terms used for a new and improved breed of wettable powder. There is really no difference between dry flowables and water dispersible granules formulations other than what their manufacturers call them. The powder is pre-clumped together into aggregates or granules which disperse when added to water.

Granules and pellets, like most dry products, are attached to inert particles used as the carrier. These formulations have large particles with a low concentration of herbicide. They can be applied directly to the soil from their packaging using a spreader without any preparation or need to dissolve them in water.

Ready to use herbicides are premixed in the carrier and can be applied directly from the purchased container. These containers often come with a spraying nozzle already attached.

Herbicide Safety

Once you have identified the desirable plant and problem weed, then you should select an appropriate labeled herbicide for controlling the weed without harming the plant. Read and understand the entire label on the herbicide container as they always provide information on the plants to which it can be applied, sites where it can be used, and which weeds it will control. The packaging label is the best source of information on how to use the product correctly and safely. Follow all instructions on the label, never applying more than is recommended which increases the risk of injury to desirable plants.