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This article deals with lawn and garden weeds commonly found in Middle Tennessee (area around Nashville) and how to control them. Some strategies for controlling weeds in Middle Tennessee include the use of pesticides. The Agricultural Extension at the University of Tennessee has issued this precautionary statement about using pesticides: “To protect people and the environment, pesticides should be used safely. This is everyone's responsibility, especially the user. Read and follow label directions carefully before you buy, mix, apply, store or dispose of a pesticide. According to laws regulating pesticides, they must be used only as directed by the label. Persons who do not obey the law will be subject to penalties.” Users should also always check the labels of pesticides for the most up-to-date information about using them.
e continual problems requiring preventive strategies. Other weeds may pop up during stress periods, requiring control at the time. The first step to control weeds is to assess the extent of the problem, including the identification of existing and anticipated problem weeds.
Chickweed
Even within one Middle Tennessee lawn, there can be a range of soil types, conditions (fertility and traffic or compaction) and sun exposure. As a result, weed problems may not be uniform. Begin the evaluation by drawing a map of the property. Section the map into easily identified zones (e.g. front, back, sides or areas bordering shrubs, walks and driveways). With this map as a reference, walk the property and record what you see. Note the following; a. Weed species present b. Weed stage of growth c. Weed population d. Previous control measures e. Health of lawn grass f. High traffic or low traffic areas g. Degree of sun exposure h. Dry areas or water-logging areas Separate weed species according to plant type (e.g. broadleaf, grass and sedges) and stage of growth. In assessing the size of the weed population or the health of your lawn, use a percentage rating. For example, in a particular zone, winter annual broadleaf weeds may account for 30 percent of the ground cover and lawn grass 70 percent. Using this technique, you could formally assess the health of your Middle Tennessee lawn twice each year, in the spring and again in late summer or early fall. The early fall assessment will be the most extensive, as summer annuals, winter annuals, biennials and perennials should be present, in different stages of growth, but identifiable. Summer annuals will be near the end of their life cycle, but will provide an idea of what needs to be done for the following spring. Winter annuals will be young and easier to control. Perennial broadleaf weeds are also easier to control in the fall. Use the spring assessment to evaluate the success of fall herbicide treatments and gauge the health of your lawn prior to summer stresses.
You may need to change your lawn care practices and/or application of appropriate herbicide(s). Your strategy will depend on the types of weeds present and population density or distribution. Certain weeds that can be anticipated in Middle Tennessee (like crabgrass and goosegrass) are best controlled with preventive or pre-emergence herbicides. For established weeds, two options are physical removal (which is essentially ineffective towards perennials, especially if only the top growth is removed) or treatment with curative or post-emergence herbicides. If annual weeds are few and in a localized area, physical removal may be your easiest choice. If weeds are annuals that are abundant and spread over a large area, or perennials, a herbicide treatment may be required. The herbicide may spread over the problem area or the entire yard (dependent on assessment results). Choose a herbicide based on how well it works on the weeds involved and how it will affect the grass. The timing of herbicide depends on the growth stage of the weeds and weather conditions. Generally, weeds are easiest to control early in their life cycle.
Broadleaf weeds are generally easiest to identify. Broadleaf weeds (like dandelion and clover) are distinctive from and are not botanically closely related to grasses and sedges. Broadleaf weeds have leaves that are broad, and are generally produced in pairs or multiples. Leaves are detached from the main stem by a sub-stem or petiole. Leaves may be simple (having one leaflet, like dandelion) or compound (having more than one leaflet, like clover). Veins within the leaf give a netted appearance in most cases. Selective herbicides for controlling broadleaf weeds generally are not effective for controlling grass weeds and sedges.
Grass weeds (like crabgrass and goosegrass) are botanically related to lawn grasses. They have a similar appearance and growth habits. Leaves of grass weeds are not detached from the main stem. Leaves of grasses are narrow, with a blade-like appearance.Leaves are produced one at a time in two vertical rows. Veins within leaves run parallel. Stems are usually round or flat. Grass weeds are often very difficult to control once established in the lawn. Thus, grass weeds are generally best controlled with preventive or re-emergence herbicides. Pre-emergence herbicides need to be applied prior to germination, as they act by preventing establishment.
Sedges (like yellow nutsedge) are not grasses , but have leaves that are similar in appearance and are thus often mistaken for grasses. Since herbicides used to control grass weeds are generally not effective on sedges, it is important to distinguish between the two types. Sedges have two key identifying characteristics: leaves arranged in three vertical rows and a triangular stem. Stems of grasses are commonly round or flat with leaves in two vertical rows.
Annuals complete their life cycle within 12 months. Summer annuals generally germinate in the spring, grow or develop during the summer, produce seed and die by the fall or after the first hard frost.
Winter annuals complete their life cycle in 12 months but generally overlap two calendar years. Winter annuals germinate in late summer to early fall and begin to develop. Winter annuals are dormant or semi-dormant through the winter, and flower the following spring. Winter annuals mature and die in late spring or early summer. Summer and winter annuals reproduce and spread by prolific seed production, serving as a ready source of infestation and establishment when conditions are favorable.
Perennials live for more than two years and may regenerate indefinitely. A simple perennial, like dandelion, may germinate from seed, but produces a tap root that, when severed, can produce a new plant. A complex perennial can spread by seed in addition to creeping above- or below-ground vegetative structures (such as stolons, rhizomes or nutlets) capable of initiating a new plant. Perennial weeds are often the most difficult to control. You are usually trying to control an established plant that has already produced considerable vegetative reproductive structures which may require repeated control measures. Removal of the above-ground shoot growth does little towards long-term control. Long-term control usually requires herbicide treatments that act on the above- and below-ground structures.
For controlling lawn weeds, there are generally two herbicide application types: pre-emergence and post-emergence.Pre-emergence herbicides prevent weeds from becoming established. Post-emergence herbicides are applied after weeds are established. Once established, certain weeds may be difficult to control. For example, trying to control an established grass weed in an established lawn grass can be very difficult. Herbicides that can select between the weeds and the grass are limited. Selectivity with a pre-emergence grass herbicide is achieved by application timing (applied to established lawn grasses, preventing grass seedlings from establishing). Post-emergence herbicides are generally used to control emerged, broadleaf weeds in established lawns.
Pre-emergence herbicides are applied to the soil and act as seeds germinate. These herbicides can act on summer and winter annuals, and some perennial weeds starting from seed. Pre-emergence herbicides need to be applied prior to germination. The time of germination for each weed species is life-cycle dependent. Pre-emergence herbicides generally provide some residual activity (meaning the weed control may last for several weeks after initial application). Pre-emergence herbicides may provide excellent grass weed control and have limited effectiveness on broadleaf weeds. Except where specifically noted, do not apply pre-emergence herbicides to newly established or reseeded lawns for at least three months. To ensure best performance with pre-emergence herbicides: 1. Remove trash, leaves and thatch to allow the herbicide to directly contact the soil. 2. Apply the preemergence herbicide uniformly over the treatment area (as directed on the product label). 3. After application, apply one-half inch or more of water. This activates the herbicide by moving it into the soil. Pre-emergence herbicides will not be as effective if not immediately activated by water (unless otherwise noted on the product label).
Postemergence herbicides act on weeds after they have germinated and are applied to the above ground plant tissue. Weeds are generally easiest to control when young (early in their life cycle). Postemergence herbicides may be contact or systemic. Contact herbicides only kill the top growth contacted by the herbicide. Systemic herbicides applied to the top growth, move within the plant to control below ground reproductive structures not contacted in the initial application. Systemic herbicides are preferred when treating established perennial weeds.
Herbicides for use in home lawns are available in several formulations. Generally there are two main types: granular, to be applied in a dry form using fertilizer-type spreaders, and liquids, powders or wettable granules formulated to be mixed with water and applied as a spray.
Granular herbicides are the most convenient and easiest to use. Many pre-emergence herbicides are marketed as granular formulations. These products may also be available in combination with granular fertilizers. Fertilizer/ herbicide combinations have several advantages: 1. Convenient, saving one additional trip over the lawn. 2. Can be conveniently applied with a common fertilizer spreader (no specialized equipment needed). 3. Spray drift is avoided that may occur with foliar spray. 4. The fertilizer may stimulate lawn grass growth and may reduce any ‘stunting’ effect of the herbicide. Fertilizer/herbicide combinations have several common misuses and disadvantages: 1. Fertility rate, especially with nitrogen, may be too high, depending on lawn grass type and time of year. 2. Use around or underneath trees and shrubs that are not tolerant of the herbicide. 3. An additional pass around or underneath trees and shrubs to give them extra fertilizer, resulting in herbicide overdose that can injure or kill trees and shrubs. 4. More expensive than products used separately. 5. Fertilizer/herbicide combinations containing post-emergence broadleaf herbicides (like 2,4-D and dicamba) applied preemergence are less effective than foliar sprays. Also, these products may injure desirable plants by root uptake.
Most post-emergence and some pre-emergence herbicides are sold in formulations meant to be mixed with water and applied as liquids. The actual product may be in a solid form, such as a powder or water-dispensible granule that mixes well with water. These formulations are meant to be applied using a sprayer or water hose-end attachments. Sprayers may be pressurized by air, using a hand pump or by water pressure during filling, as in the case of hose-end sprayers. Avoid spray drift to desired plants by using low pressure (to maximize droplet size) and by not applying when the wind is greater than 5 mph. Hose-end attachments force the herbicide into the water flow. The accuracy of herbicide application is low with this equipment. A more appropriate use of hose-end attachments is the application of fungicides, insecticides and liquid fertilizers. Many post-emergence herbicides are packaged as liquids ready to use for spot treatments in squirt bottles or aerosol cans. Be cautious with aerosol cans. They can be misdirected and the herbicide may contact your face and eyes. For spot treatments, many post-emergence herbicide labels may provide directions for use with a brush and can. The liquid herbicide mixture can be “painted” with a brush (or cloth or sponge) onto undesirable plants. This method is convenient for treating individual plants or a few plants in small problem areas.
Granular spreaders (gravity drop or spinner types) usually provide an easy gauge for selecting the approximate setting for the desired application rate. In addition, usually the package for granular herbicides or herbicide/fertilizer combinations provide specific directions on application rate and equipment calibration. To ensure uniform coverage, or to avoid misses, more even distribution will be achieved by applying one-half the total rate in each of two passes in opposite directions (total rate to be applied is generally given as pounds per 1000 square feet). CAUTION: If you are applying hormone-or phenoxy-type herbicides (such as 2,4-D, MCPP or dicamba), do not use the same spreader to later apply fertilizer to desirable plants other than lawngrasses. There may be enough herbicide residue remaining in the spreader to injure desirable plants. Thoroughly rinse the spreader with soapy water and then rinse with clean water and let dry.
Herbicide applications with pressurized sprayers require accurate calibration. When treating large areas, proper calibration and consistent application are essential to avoid overdosing and/or missing areas. To calibrate a liquid pressure sprayer, use the following steps: 1. Fill the tank full or to a marked level with water. 2. Spray an area 10 feet wide by 10 feet in length (or 100 square feet). Pay attention to your walking speed. A consistent walking pace is essential for consistent application. 3. Record the amount of water needed to refill the tank to the marked level. This is the amount of water required to treat 100 sq. ft. Multiply this amount by 10 to get the total amount of water required to treat 1000 sq. ft. 4. Empty out this water or add water until the tank is half full. 5. Then add the quantity of herbicide appropriate for the amount of water the tank holds. 6. Refill the tank with water (the action of the water during fill will aid in uniform mixing of the herbicide with the water). Shake the tank for 15 seconds. 7. During application, it may be necessary to stop and shake the tank to ensure continued uniform mixing. This is particularly true for wettable powder or water-dispersible granular formulations of herbicides.
Mechanical weed control involves removing weeds while they are small and preventing them from producing mature seed. It consists principally of mowing, plowing, rototilling, hoeing and hand pulling. These procedures give immediate results and require little, if any, specialized equipment. Their main disadvantage is that they work only on growing weeds and may need to be repeated frequently. Remove weeds while they are small because it is quicker, easier and does less damage to desired vegetable plants. Hoe or till shallowly (less than 2 inches deep) to avoid damage to desirable plants and to minimize moisture loss from the soil. The deeper the soil is disturbed, the more weed seed will be brought to the surface where it will grow. Most weed seed that germinate are in the upper 2 inches of soil. Weed seed may survive many, many years in the soil. Each time it is worked, more seed is pulled to the surface. Gardeners often quit weeding as a crop matures. Weeds do not affect crop yields as much at this stage. However, they will still produce seed. A single weed of some species can produce hundreds of thousands of seed. Weed removal should continue until the vegetable crop is completely harvested. While crops are not growing on the garden spot, the soil may then be kept tilled, mowed or heavily mulched to prevent weeds from propagating. Frequent tillage has the additional advantages of turning organic material under where it will add to soil organic matter and of mechanically destroying any insects that are present.
Most cultural methods of weed control emphasize prevention. These include mulching, solarization, and multiple or wide-row planting.
Any layer of material spread over the soil surface may be considered to be a mulch. There are two classes of mulch: organic and inorganic. Both reduce weed growth, retain soil moisture and influence soil temperature. Hay, straw, sawdust, ground bark, leaves, compost and even newspapers are among the many substances utilized as organic mulches. All reduce weed emergence and make it easier to pull those weeds that do emerge. Some weeds such as nutsedge will grow through mulch, even through plastic mulch. Inorganic mulches include black and other opaque plastic as well as landscape fabric. Landscape fabric is much more expensive than plastic mulch. However, it has the advantage of allowing water to pass through into the soil. Black plastic (polyethylene) is the most commonly used inorganic mulch. Clear or translucent plastic should not be used as mulch. These plastics allow light to penetrate, which permits germinated weed seed to grow under the plastic mulch. Black plastic eliminates growth of most, but not all, weeds. For example, yellow nutsedge will grow through plastic mulch. Also, weeds will grow in the holes cut in the plastic for the vegetables. Opaque plastic in colors other than black is sometimes used as mulch and will also control weeds. However, many of these plastics are quite expensive. Some data suggests that tomatoes grown on red plastic or that muskmelon grown on blue plastic will produce higher yields. Yield increases of up to 25 percent have been documented. However, the results are not consistent enough to justify the expense of the colored plastics. Due to high temperatures and intense solar radiation, the use of black plastic may cause damage to plants transplanted or seeded during the hottest part of the summer. White plastic can be utilized during the summer months to prevent root and stem damage to the plant. White plastic will allow light to penetrate. Therefore, white plastic with a black backing (known as white on black) is preferable. White on black plastic is more expensive and may be difficult for home gardeners to locate in affordable quantities. Another option is to whitewash or paint the black plastic with white latex paint. The paint will eventually wear off, but not until the plants have shaded the plastic around the base of the plant, reducing the chance of damage.
Solarization is the use of clear polyethylene plastic sheeting (2 to 6 mils thick) to capture the radiant energy of the sun, thereby raising the soil temperature to levels lethal to many weed seeds. The plastic sheeting is placed over bare, moist soil during a summer fallow period. In Middle Tennessee, best results are obtained by using two layers of plastic sheeting and separating the layers with boards, bricks or small squares (ca. 3” X 3”) of polystyrene insulation called spacers. Spacers are placed every 1.5 to 2 feet on top of the first layer of plastic. The second layer of plastic sheeting is then placed on top. This creates an air gap between the two layers, which provides a reservoir of warm air that minimizes nighttime cooling. The two layers of plastic sheeting are then sealed together by placing soil along the outer edges of the top layer. The top layer of plastic should be kept clean to maximize solarization efficiency. An occasional dusting with a dust mop or rinsing with a garden hose should be all that is necessary. The plastic is left in place for 4 – 5 weeks. During this period, soil temperatures are frequently raised to 120 – 125 degrees F. This practically eliminates viable weed seed in the top 2 to 3 inches of soil. After the plastic is removed, care must be taken not to mix the deeper layers of unsolarized soil with the nearly weed-free solarized soil. Rototilling no deeper than 1 – 2 inches is recommended. As long as the soil is not rototilled deeper than this, weed control by soil solarization can significantly reduce weed infestations for 12 months or more. Many soil-borne plant pathogens will also be reduced during the solarization process. This improves plant stands and vigor and may double yields. The best time to solarize soil in Middle Tennessee is from May through early August. Two or three periods, each consisting of several days in a row of warm weather and bright sunshine, are necessary. Late summer and fall vegetables may be planted the day after removing the plastic.
Closely spaced double or triple rows may also assist in weed control. Simply plant two or three rows of a vegetable close enough so that the leaves will cover the area between them rapidly as the plants grow. Very small vegetables such as radishes may be broadcast in a long row a foot or so wide. These techniques allow growing vegetables to shade the soil, which reduces weed growth.
Chemicals (herbicides) are only occasionally used by home gardeners to control weeds because suitable herbicides are seldom legally available in small, economical amounts. There are also several other difficulties with the use of herbicides by home gardeners. Many herbicides are nonselective and will kill the vegetables, as well as the weeds.Others are selective and can be used only with certain vegetables or control only certain weeds. Home gardens generally contain many vegetable and weed species, which makes using selective herbicides difficult. Some herbicides may also damage nearby vegetables or remain in the soil and damage future plantings. Even if available and effective, herbicides may not be legal for use on a specific vegetable or at the time when they are needed. The herbicide application rate may be very low and extreme accuracy in application may be absolutely essential. Overlapping applications may kill vegetable crops and, if areas are skipped, weeds will not be controlled. Required pre-harvest intervals (PHI's) or waiting periods between application and harvest can be lengthy and must be observed. Herbicides may also be effective only for a short period of time or produce results slowly. Despite all these problems, there are occasions when herbicides may be successfully used in home gardens. The following suggestions will assist home gardeners in effectively using herbicides. 1. Understand the difference between pre-emergence and post-emergence herbicides. Pre-emergence herbicides are effective only before weeds germinate. Post-emergence herbicides work on weeds that are actively growing. 2. Understand the different formulations of herbicides available and how the formulation affects use. Some of the more common formulations are emulsifiable concentrates (EC), flowables (FL), wettable powders (WP) and dry flowables (DF). All are designed to be mixed with water and sprayed on the area to be treated. Wettable powders and dry flowables may settle out unless the sprayer is shaken periodically. Herbicides may also be formulated as granules (G). These are to be spread evenly over the soil surface. 3. Plan the garden in detail. Plan to locate all the crops for which a specific herbicide may be used near each other.This allows treatment of larger areas with less effort. 4. Follow all instructions on the label. This is extremely important. Failure to follow the label instructions precisely may result in harm to the applicator, the environment or the crop. Pre-emergence herbicides require weed-free soils without lumps or clods. Most are best applied to moist soil and shallowly incorporated by tilling, irrigating or natural rainfall. They must be applied uniformly and at the proper rate to be safe and effective.Post-emergence herbicides may be applied over the top of the growing crop and weeds. Again, it is essential that all aspects of the label directions be understood and followed. If you have any questions, consult your county Agricultural Extension agent. 5. Rinse spray equipment. Residual herbicide in sprayers may damage crops. Many gardeners who use herbicides purchase spray equipment for herbicides only and keep it separate from equipment used for insecticides and fungicides. 6. Calibrate application equipment accurately. Inaccurately applied herbicides may be ineffective or dangerous. If there is a question on how to apply a specific herbicide, contact your local county Agricultural Extension office. The two most suitable herbicides for use on home gardens are Trifluralin (Treflan™) and Sethoxydin (Poast™). Trifluralin is sold in many formulations, each designed for specific uses. Trifluralin prevents the germination of most grasses and some broad-leaved weeds (for several weeks) and must be applied before these weeds germinate. Sethoxydin kills growing grasses. It requires only a short waiting period between application and harvest and may be applied to a wide range of vegetables. Proper use of herbicides may involve grouping vegetables according to the herbicide that may be applied to them, as well as paying attention to rate, timing, uniformity and method of application. Remember, label directions must always be read and followed.
The stale seedbed technique is a system that controls weeds prior to the planting of the crop. The theory is that most weed seeds that germinate are found in the top 2 to 3 inches of soil. With soil temperatures of 70 degrees F or higher and moist soil, most of the weed seed in the top 2 inches of the soil will germinate in 2 to 4 weeks after a tillage operation. Therefore, a generalized procedure follows. 1. Broadcast fertilizer and incorporate with a final tillage. 2. Allow the weed seed to germinate for 2 to 4 weeks. 3. Once most of the weeds have germinated, kill the emerged weeds. This can be done by using a nonselective herbicide such as glyphosate (Roundup) or by performing a shallow tillage (less than 2 inches). 4. Then seed or transplant the crops with minimal soil disturbance. Since warm soils are required for this technique to be effective, it generally is not used for spring-planted gardens. It can be very effective when utilized prior to the planting of a fall garden.
Much of the information for this article was taken from material on the UT Agricultural Extension Web site.
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