Weeds



Text adapted from Turfgrass Pest Management: A Guide to Major Turfgrass Pests in Oklahoma
Oklahoma Cooperative Extension Service


Introduction


Weeds are a primary turfgrass problem. This chapter provides a guide to the basic concepts of weed control, the use of safe and effective weed control methods, and how to identify weeds. It does not contain information about specific herbicide products. However, publications, circulars and Fact Sheets which give specific herbicide recommendations and more detailed information are listed at the end of this manual.

The best defense against any turfgrass weed is a healthy turf. Every weed control program starts with good cultural practices that encourage a dense, vigorous turf. Weed control alone will not produce satisfactory, long-term results.

Weeds often are the result of poor turfgrass management. Weeds usually take advantage of thin or weak stands of grass. The most common causes of poor turf are improper mowing, watering, and fertilizing. Other factors can also affect the condition of the turf, including insects, disease, and soil compaction.

Always determine why weeds invaded the turfgrass area and correct the problem. If the basic cause is not corrected, weeds will continue to be a problem even though the existing weeds are eradicated. A weed control program is more than just "spraying herbicides." A weed control program should be a planned approach to do the right procedure, at the right time, and the right way.

Sources of Turfgrass Weeds

Most turfgrass weeds result from seeds found in the soil. Some seeds may remain inactive for several years, and then emerge under proper temperature, light, and moisture conditions. These weeds generally germinate and mature when bare spots develop or if the soil is cultivated. Topsoils, manures and composts usually contain an abundance of weed seed. A new crop of weeds can be expected whenever these admendments are used.

Weed seeds may be transported from place to place by a variety of methods. Dandelion seeds are carried many miles by the wind. The tacky seeds of plantain are transported by clothing, equipment, and animals. Ripe seed pods of yellow wood sorrel can explode and throw their seeds many feet.

Management Practices

Successful weed control programs concentrate on an integrated approach using turfgrass competition, mechanical control, and chemical control methods. These three methods combined will provide a better weed control program than any single one used. The following briefly describes each of these weed control methods.

Turfgrass competition: A strong, vigorously growing turf will help prevent the invasion of new weeds and help to crowd out existing weeds. Proper irrigation, fertilization and core cultivation make a turfgrass compete favorably with weeds.
Mechanical control: Tillage before turfgrass establishment, proper mowing, and hand weeding are mechanical weed control methods. A few weeds can be pulled by hand, but this may not be effective on deep-rooted weeds or is too labor intensive for large areas. Regular mowing at the proper height and frequency controls several kinds of weeds.
Herbicides: The use of chemicals provides a convenient way to control weeds in turfgrass and is the only practical or effective method of control for many turf weeds. Herbicides are especially useful during lawn establishment or renovation of an old turf area. Once a dense turf is established and maintained properly, weed problems should be minimal. Do not depend on herbicides to take the place of good turfgrass management. Herbicides are an aid to good management, not a substitute.

Herbicides

Herbicides should only be used when necessary and always in conjunction with a good turfgrass management program. Always refer to the label directions before using a herbicide. Improper use can result in poor weed control, damage to the turf and damage to other landscape plants. The user is responsible for any damage that results from not following label directions. Also, it is a violation of state and federal law to use any product in a manner that is inconsistent with its label.

Herbicides are manufactured as liquid (emulsion), wettable powder, and granular forms under a variety of brand names and concentrations. The actual weed control chemical(s) and amount in a product will be listed on the label as the active ingredient(s). The rest of the herbicide product is the carrier or inert portion of the product.

Liquids, dispersable granules and wettable powders are concentrated formulations that are diluted with water and applied as a spray. Follow the label carefully because too concentrated a mixture may burn the turf, while inadequate coverage or too weak a mixture may not control the weeds.

Granular herbicides and fertilizers containing herbicides are ready to use and are applied with a rotary or drop spreader. Fertilizer-herbicide combinations are convenient to use, but a major limitation is that the best times for fertilizing and controlling weeds are not always the same. Granular herbicides and fertilizer-herbicide combinations are generally more expensive, based on the amount of active ingredient when compared to concentrated dispersable granules, wettable powders and liquids.

A selective herbicide controls weeds, but is also safe for your particular kind of turfgrass. However, some weeds are extremely difficult to control and can only be eradicated with a nonselective herbicide that kills all vegetation, including the turfgrass. The treated area must be replanted after the effect of the herbicide has worn off.

Contact herbicides kill primarily by contact with plant tissue. Uniform coverge and proper application rate are required for success with contact materials. Refer to the label for the proper temperature and spray volume when using contact herbicides.

Translocated herbicides are taken into the plant and accumulate at the active growth centers of the weed. In general, these products are selective for particular weeds which appear on the label.

Soil applied herbicides remain in the soil for a period of time and have some residual affects. These compounds are taken up through the plant roots and translocated throughout the plant, or simply inhibit normal root growth. Many preemergence herbicides are soil applied products.

Developing a Weed Control Program

There are several important considerations when developing a weed control program.
1. Know what kinds of turfgrass you have and the total area of each different type.
2. Properly identify the problem weeds and note what time of the year they occur.
3. Determine why the weeds invaded the turf area and correct the conditions or cultural practices that caused the problem.
4. When a herbicide is needed:

5. Follow a good turf management program along with the weed control program. An integrated approach which includes enhancing turfgrass competition, mechanical control, and chemical control methods will be the most successful weed control program.


Application Timing

There are three types of herbicide applications based on when they are applied:
Preemergence—a herbicide is applied to an existing turfgrass area before the weed seeds germinate to kill emerging weed seedlings.
Postemergence—a herbicide is applied to growing weeds after they have emerged from the soil.
Preplant—a herbicide is applied to eliminate certain problem weeds before the grass is planted.

Preemergence products are generally soil applied herbicides and are used primarily to prevent seed of annual grasses and some annual broadleaf weeds from emerging. These products should be applied at least two weeks before weed seed germinate because they are not effective after the weeds emerge. A second application may be required to extend the period of control. Most preemergence products should be watered according to label directions.

The time to apply preemergence herbicides depends on the weeds to be controlled and when they germinate. To control crabrass, foxtail and other warm-season annual grasses, the product should be applied in early spring (Figure 6). March 15 (2 weeks earlier for southern Oklahoma and 2 weeks later for northern Oklahoma) is an approximate application date, but weather conditions vary considerably from year to year.

Cool-season grasses, such as annual bluegrass and annual bromes, germinate in early fall after spring-applied preemergence chemicals are no longer effective. If these weeds are a special problem, additional fall applications may have to be used around early to mid-September.

Annual broadleaf weeds, such as chickweed, henbit, and shepherd's purse, are winter annuals that germinate in the fall. The appropriate preemergence herbicide should be applied about mid-September for these weeds. Specific products and application rates should be used for fallgerminating annual broadleaf weeds.

Postemergence herbicides may be selective or nonselective. They can control weeds by means of tissue contact, or uptake and translocation to a growth center. The important point is that postemergence herbicides cover an extremely diverse group of products. Carefully read the label for the correct application timing, method, and target weeds.

Preplant herbicides are only recommended when a turf area is to be renovated or established. These herbicides are generally non-selective. Therefore, all vegetation is usually controlled when they are used. Never use a preemergence product prior to seeding a new turfgrass area unless it is specifically labeled for this use. Most preemergence herbicides are not recommended for preplan" applications because they will prevent the desired turf species, as well as weed species from germinating.

Weed Identification

A weed is any plant growing out of place. Tall fescue in a bermudagrass turf could be considered a weed. Yet each plant is excellent for its own particular purpose. With few exceptions, broadleaf plants, such as dandelion, chickweed, and spurge, are weeds. Most weeds are very competitive and survive under adverse conditions.

Types of Turfgrass Weeds

Grassy or monocot weeds emerge from the seed with a single seedling leaf. All grasses fall into this classification. The most distinguishing characteristic of a grass are leaves with veins that run parallel to each other (Figure 7). Examples of grassy weeds include crabgrass, goosegrass, and annual bluegrass.

Broadleaf or dicot weeds emerge from the seed with two seedling leaves. The most distinguishing characteristic of a broadleaf weed are veins that form a net-like pattern on the leaf (Figure 8). Examples of broadleaf weeds are dandelion, shepherd's purse, henbit, and clover.

Both monocots and dicots may be annuals, biennials, or perennials. Annuals live for a single year. In other words, they germinate from seed, grow into a mature plant, produce seed, and die in one year. Biennials germinate from seed the first year and stay in a vegetative growth stage for the entire year. During the second year, they flower, produce seed, and eventually die. Perennials germinate from seed the first year, but can live indefinitely because they can regenerate from underground crowns or stems. They continue to grow and flower year after year, and some can spread by stolons or rhizomes.

Figure 7. (a) Monocots emerge from the seed with a singie seedling leaf.
(b) Monocots are characterized by parallel venation.

Figure 8. (a) Dicots (Di meaning two) emerge from the seed with two seedling leaves or cotyledons.
(b) The most distinguishing characteristics of a dicot are veins that form a net-like pattern.


Turfgrass weeds can be divided into four general types for determining the appropriate method of control: 1) annual grasses, 2) annual broadleaves, 3) perennial grasses, and 4) perennial broadleaves. They are further classified as warmseason (summer) or cool-season (winter) to determine when they should be controlled. Winter annuals, for example, germinate in the fall and die the following summer. Summer annuals germinate in the spring and die in the fall. Winter and summer perennial weeds follow a similar pattern; however, they become dormant rather than dying at the end of their respective growing seasons (Figure 9).

Learning to identify weeds requires a certain amount of study and observation. Each growing plant has its own characteristics, peculiarities, and growth habits. A certain amount of basic knowledge can be invaluable when prescribing herbicides to control problem weeds.

Grassy Weed Identification


There are several characters that can be used to distinguish grassy turfgrass weeds (Figure 10). Vernation is the arrangement of a leaf or leaves in the bud shoot. They may be classified as rolled or folded. The tubular portion of the leaf which wraps around or encloses the stem is the sheath.
Edges of the sheath may join, overlap, or be closed. The sheath may also be rough or smooth, cylindrical, flattened, or compressed. Ligules clasp the stem at the junction of the leaf blade and sheath. The type (membranous or hairy) and shape (tall, short, or jagged) remain fairly uniform and constant within each given species. The collar is the area on the outer side of the leaf where the blade and the sheath join. It is generally much lighter in color and varies in size and shape from species to species. Auricles are found on either side of the collar and may be claw-like, long, or short. Rhizomes are underground stems that can produce new plants, while stolons are above ground stems capable of producing new plants. The seedhead or inflorescence is the collection of flowering parts which are arranged in various ways (i.e., spike, panicle, etc.). Spikelets are a small unit of the seedhead which contains the seed parts.