WEED IDENTIFICATION, MANAGEMENT, AND CONTROL IN THE HOME LAWN. Michelle Le Strange*

Transcription

1 Introduction WEED IDENTIFICATION, MANAGEMENT, AND CONTROL IN THE HOME LAWN Michelle Le Strange* Though weeds are a natural component of the landscape, by definition we don't usually like where they end up growing. In the front lawn weeds are easily seen and are usually considered objectionable. The great majority of the 500 species of weeds in California are flowering plants, those higher forms of plant life that produce seed. Fifty species are considered as major weeds in turf. Why identify weeds in the first place? In the days when weed control and management was mechanical or pulled by hand, specific weed identification was not so important. Today the management tools are more sophisticated. Timing of fertilizer and water applications influences the growth of specific weeds. The herbicides are developed to be selective to major plant groups. Some are selective to plant families, others to species. Also rising labor, fossil fuel, water, and chemical costs put an emphasis on controlling weeds the first time. It is important for anyone conducting weed control programs to learn bow to tell the difference between weeds. Weed Classification Botanists recognize two natural divisions in the flowering plants. The monocotyledons and the dicotyledons. Two characteristics distinguish monocots from dicots. One is the number of leaves that the young seedling has when it emerges through the soil surface. The monocots have one seedling leaf, the dicots two. The second characteristic is the venation within the true leaves. In the monocots the veins run parallel to each other. In dicots the veins branch in several directions in a netted fashion. All of the grasses plus bamboo, irises, nutsedge, and palm trees are examples of monocots. Roses, hardwood trees, poison oak, and clovers are members of the dicot group. In weed control dicots are referred to as broadleaf weeds and the monocots as narrowleaf weeds, but it is their venation and number of seed leaves that determine their category. Depending on the number of years that they live, flowering plants are also classified as annuals, biennials, or perennials. Annuals grow from seed, flower, produce seed and die in one year or less. Where there are distinct seasons, annuals are found to occur either as winter or summer season plants. Winter annual weeds germinate in the fall, grow throughout winter, and mature in spring before the heat of summer. Summer annual weeds germinate in the spring, grow during summer, and mature before frost. Biennial plants have a 2 year lifecycle. Seeds germinate the first year and grow vegetatively in a rosette shape. During the second year a flower stalk arises, seed is produced, and the plant dies. Perennial weeds can live indefinitely, although the tops may die during the winter. They can start from germinating seeds, but many perennial plants spread naturally by *Farm Advisor, University of California Cooperative Extension, Tulare County 2

2 vegetative means such as root pieces, rhizomes, or tubers. Perennial plants continue to spread in turf until all of their parts are killed or weakened enough so they no longer compete with the turf. Seedling plants are the easiest to destroy whether they are annuals, biennials, or perennials. generally harder to control. Once perennials become established, they are Some examples of annual weeds in turf include: Narrow leaf Annual bluegrass Large crabgrass Smooth crabgrass Goosegrass Italian ryegrass Broad leaf California bur clover Common chickweed Common groundsel Birdseye speedwell Spotted spurge Some examples of perennial weeds in turf include: Narrow leaf Bermudagrass German velvetgrass Kikuyugrass St. Augustinegrass Bentgrass Dallisgrass Yellow nutsedge Broadleaf Dichondra Dandelion White clover Creeping woodsorrel Plantains Mouseear chickweed English daisy Cultural Management of Weeds There are several management tools or cultural practices that best enable the turf to compete with weeds. A healthy vigorous turf stands a much better chance against weed invasion than a weak stand of water-stressed, unfertilized lawn. Mowing, watering, and fertilizing the lawn have a direct impact on weeds. The GOAL of MOWING is a smooth, uniform, dense turf. Large weeds that stick up over the turf are mowed away--that's why most problem weeds are prostrate. There are two KEYS in mowing--height and frequency. Each turf variety has its requirements. In general cool season grasses have a two inch optimum for stress tolerance and resistance against weeds. Warm season grasses are usually mowed 1/2-1 1/2" in height. At these heights the grasses are the most vigorous. With respect to frequency, the rule is to never remove more than 1/3 leaf area. In spring this is often more than one time/week for the cool season grasses as this is when they put on a tremendous flush of growth. For the homeowner it's just a matter of getting out there and doing it, but under commercial or municipal situations it gets expensive. For this reason proper selection of turf varieties is important. The proper irrigation scheme for vigorous turf and weed control is to water INFREQUENTLY and water DEEPLY. Just how infrequent and how deep depends on the turf, temperature, and soil type. Using a long interval between irrigations in the spring serves two purposes. It encourages deep 3

3 root development before high demands of summer stress the system, and it reduces to a minimum the surviving infestation of annual summer weeds such as crabgrass. You can help control weeds by mowing a little high and with infrequent, deep waterings. The high mowing reduces the light penetration to the bare soil, and the infrequent watering lets the top 2" of the soil dry out causing weed seedlings to die. The greatest influence of fertilization on weeds is the indirect effect. Fertility increases grass vigor, thus reducing the competitiveness of the weeds. Nitrogen is the limiting element in most areas. Not only is the amount applied critical, but so is the timing of the application. If fertilizers are applied at the optimum time for grass growth, weeds will be less likely to invade and compete. Fertilizer applications should be made to encourage turf before crabgrass germination in spring or before annual bluegrass germination in the fall. A well fertilized, adapted turf will reduce invasion by dandelion, large and smooth crabgrass, and clovers. Controlling Broadleaf Weeds in Lawns There are herbicides available that are selective to controlling broadleaved annuals in lawns. Virtually three chemicals or their combination will control just about all of the common annual broadleaf weeds in lawns. They are 2,4-D, mecoprop, and dicamba. These herbicides are applied directly to the foliage of the weeds as they are growing in the lawn. The chemical then moves within the weeds to the growing points and kills them. These chemicals are readily available, and have many trade names. Nearly every company specializing in lawn products has them in some way, shape, or form. Oftentimes they are in the so called "weed and feeders", that is, the herbicide or herbicides are impregnated onto a granule and mixed in with a nitrogen fertilizer. The same selective herbicides that control the annual broadleaves will control the perennial broadleaves. Each herbicide has its list of weeds upon which it is most effective. In some cases it is necessary to use a combination of two or all three of them. For example, 2,4-D works well on dandelions and plantains, but it is weak on clovers. Dicamba, on the other hand, is good on clovers, but it is weak on dandelions and plantains. Controlling Narrowleaf Weeds in Lawns Crabgrass control includes managing irrigation so that water is applied deeper and less frequently. Several pre-emergent herbicides offer good control of crabgrass, if their application is properly timed. A single application of a selective, pre-emergent, soil applied herbicide such as benefin, bensulide, or DCPA will control crabgrass in turf for an entire growing season. The best time for crabgrass control is January-February for the most part of the state. With these chemicals it is important to sprinkle irrigate them into the soil, and to avoid using them on bentgrass because of its sensitivity to them. To control crabgrass that has emerged the methane arsonates, like CMA, DSMA, and MSMA, are used. Repeated applications of these selective foliage applied herbicides are often needed to get rid of the crabgrass. Several companies offer these products in their home lawn products line. Pendimethalin is primarily used by the col1ullercial applicator, 4

4 but it is becoming more available to the homeowner. Oryzalin is registered for use on warm season turf, but care should be taken if you have the hybrid bermudas, because some phytotoxicity has been noticed in some of the varieties. Controlling annual bluegrass in established turf is very similar to controlling crabgrass, except the pre-emergent application date is August and September. Oxadiazon is another pre-emergent material that is registered for controlling annual bluegrass in established lawns. Available for commercial applicators, but not the homeowner, is a post-emergent herbicide, pronamide. It works well in controlling established bluegrass in dormant bermudagrass. However, because of its specificity it is rather expensive to use. Dallisgrass is a perennial problem in lawns. The best management tool is to prevent it from getting into the lawn. If it is in the backyard, but not in the front, keep it out by hosing off the mower after use to remove any seed heads caught in the blades. There are no pre-emergent materials registered specifically for dallisgrass, but it is rarely a problem where crabgrass pre-emergents are used. For post-emergent control again rely on MSMA and DSMA. When these materials are used, care should be taken with respect to the kind of lawn and the temperature. Lawns may be temporarily discolored. After application be sure to not water the lawn for at least 48 hours. Time is needed for the herbicide to move into the plant, and sprinklers will wash it off the leaves and into the soil. Dallisgrass will require several applications at approximately monthly intervals. Spot treatment is another alternative. Nutsedge appears similar to grass except that it has triangular stems. The leaves are shiny, dark-green, stiff and erect, and its rhizomes form underground tubers. This weed thrives in soggy conditions. When lawns are watered deeply and less frequently they are more competitive and can crowd out though not necessarily eliminate the sedge. Post-emergent chemical control is with MSMA and DSMA. Bentazon is available for use by commercial pest control operators/advisors. If a home lawn has a bad case of nutsedge, it might be worthwhile to hire a professional applicator. Spurge and Oxalis Control Spurge is an aggressive broadleaf annual which grows prostrate on the ground. The stems form circular mats from a single taproot, and it quickly becomes dense in lawns. Spurge is a prolific seeder and germinates throughout a long growing season. It may be green or reddish but it is easily identified by the red spot on the upper center of the leaflet. It is extremely difficult to get rid of a bad infestation of spurge by management practices alone; however, a healthy vigorous turf is essential to prevent the spurge from taking over. February-March is the time to make a pre-emergent application to prevent the germination of spurge seeds. DCPA, which is sold under a variety of trade names, oxadiazon, and oryzalin are registered pre-emergents though none offer complete control. Also available is pendimethalin as a pre-emergent. The new combinations of pre-emergents, benef in plus oxadiazon, oryzalin, or trifluralin, may help in achieving 5

5 better spurge seedling control. as effective as bromoxynil. Post-emergent applications of 2,4-D are not Creeping woodsorrel is the oxalis problem in lawns. This perennial has running rootstocks and prefers shade where it can become a very dense mat. The flowers are small and yellow. The leaves are green or reddish in color and look very much like a clover leaf, but it is not in the clover family. Many compounds on the market claim oxalis control, but control really has not been very successful in California until recently. Triclopyr and glyphosate have been registered for spot treatments. Now triclopyr has a new registration as a post-emergent over the top of grasses and offers very good oxalis and plantain control without harming the lawn. This is good news. Conclusion A healthy vigorous lawn is a key factor in controlling weeds. Weed identification is important for proper use of herbicides, but especially for understanding the weed's lifecycle and biology. Cultural practices can be altered to maximize turf vigor and minimize weed infestation. 6