Pain in the grass

Grassy weed control in turf can be challenging. You must understand the enemy, what weapons to use and when to time the attack.

Now is a good time to develop a program for management of summer annual weeds in turf. Review any notes you took on last year's results - they will assist in this effort. Also, consider environmental factors that impact herbicide effectiveness - they will help to maximize your weed-control efforts. Many factors contribute to a successful annual grass control program, and you must consider all of them.

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Weed identification and life cycles q Know the weeds. Accurate weed identification is necessary when choosing a weed-control product. Most pre-emergence crabgrass herbicides will control some other annual grasses like yellow foxtail, giant foxtail and barnyardgrass. However, if goosegrass is an important weed at some sites, you need to take this into account. Some of the crabgrass preventors do not provide acceptable control of this weed. For example, oxadiazon, prodiamine, dithiopyr and oryzalin generally provide greater goosegrass control than bensulide or siduron.

The leaves of goosegrass are folded in the bud, while crabgrass leaves are rolled in the bud. Goosegrass has a whitish base, which is why some people refer to this plant as silver crabgrass. It does not root at the nodes, but large and smooth crabgrass do root at the nodes.

Weed identification is also important for post-emergence herbicide application. For example, quinclorac will control crabgrass, but not goosegrass. Also, post-emergence crabgrass herbicides generally will not control annual bluegrass. Dithiopyr is primarily effective on crabgrass as a post-emergence treatment, but as a pre-emergence herbicide, it provides a broader spectrum of control.

You need to be able to separate grasses from sedges. Fenoxaprop, for example, will not control sedges, but MSMA and DSMA will. In the seedling stage, sedges will resemble an annual grass. Sedges have triangular stems ("sedges have edges"), which cause the leaves (blades) to be three-ranked (the leaves are borne in rows on each of the sides of the triangular stems). Grasses have round or flattened stems. Therefore, the leaves (blades) of grasses are two-ranked. Look down the stems from above. If the leaves are emerging from two sides of the stem (opposite each other), then you are looking at a two-ranked grass. If the leaves are emerging from three sides, then you are looking at a sedge.

q Understand weed life cycles. You may mistakenly believe that dallisgrass is a summer annual, perhaps because it goes off-color during the winter and resembles crabgrass during the growing season. However, it has rhizomes (underground stems), while crabgrass does not. Because it is a perennial, you cannot control dallisgrass with pre-emergence herbicides used for crabgrass control.

Another example is orchardgrass, a common cool-season, perennial weed in turf. It resembles goosegrass, but has a larger ligule, and tends to have a bluish-green color. Look at plant size early in the season. If you see a large plant in late winter or early spring, it must be orchardgrass. Goosegrass does not germinate until late spring and will not reach the tillering stage for a few more weeks after germination. As a general rule of thumb, use pre-emergence herbicides to control annual weeds and post-emergence herbicides to control perennial weeds.

q Know when different weeds species germinate. You must know this to optimize herbicide application timing. You should apply pre-emergence herbicides prior to the onset of germination so that the chemical can perform. The onset of germination should be the focus of your weed-management program.

q Know the length of the germination period. You may need to repeat applications. I asked my students in a quiz one time, "When does crabgrass germinate?" Mostly they answered, "March." While it is true that crabgrass can germinate in some areas in March, it also germinates in April, May, June, July and August! Depending on your location, the crabgrass germination period could last four or five months or more, especially if you live in the South. If you live in a region with a long growing season, you are more likely to need a repeat application for season-long control.

Pre-emergence application timing q You can apply pre-emergence herbicides any month of the year. I am frequently asked when is the best time to apply a pre-emergence herbicide. A related question is, "Is it too late to apply a pre-emergence herbicide?" You do not want to treat frozen ground or ground covered by snow, but otherwise you can apply a pre-emergence product almost any month. Your application timing, though, must ensure that the herbicide is applied prior to the onset of germination. Also, you need to maintain adequate herbicide levels in the soil throughout the germination period of the weed you are trying to control.

q Understand the residual nature of different herbicides. You are not given advance warning on when weed germination will begin each spring, so you must rely on information from previous years or use soil temperatures to predict germination. Apply the product a week or two before the expected onset of germination so that the herbicide can be activated by rainfall or irrigation. All pre-emergence herbicides require activation into the zone of seed germination. If no rain occurs for several weeks after application, effectiveness can be lost due to volatilization and photodegradation (breakdown by sunlight).

q Use split-applications when needed. Split applications refer to several applications of the same herbicide at reduced rates. For example, you could apply 3 pounds of active ingredient per acre of pendimethalin in one application. In a split application, you would apply 1.5 pounds of active ingredient followed 6 to 8 weeks later by another application of 1.5 pounds of active ingredient per acre. Split applications are generally targeted towards weeds that have germination times that differ by 1 to 2 months or a single weed that germinates over many weeks. Multiple or subsequent applications generally would target separate weeds that have completely different germination times. You would make an application of a herbicide at full rate to control spring and summer crabgrass germination. In the fall you would also make a full-rate application of the same herbicide to control a different weed.

It would be great if one pre-emergence application could give you season-long control. Because pre-emergence crabgrass herbicides may last for only three months in soil, depending on product and application rate, late-season germination of crabgrass is expected. In a study I conducted using single applications, none of the pre-emergence crabgrass herbicides tested provided season-long (5 months) control of crabgrass. In that study, prodiamine and oxadiazon provided longer-term crabgrass control than pendimethalin, dithiopyr liquid, oryzalin and bensulide followed by siduron and benefin. Make two applications of a pre-emergence herbicide, with the second one about six or eight weeks after the first. You will need two applications to achieve better control of late-germinating crabgrass.

Split-applications are also beneficial when dealing with goosegrass, which tends to germinate a few weeks later than crabgrass. Time your first application to control early-germinating crabgrass. Your second application will target peak germination of goosegrass.

q Use your knowledge of weeds and herbicides to extend your application window. Pre-emergence herbicides break down slower under cold soil conditions, so these chemicals persist longer during winter applications than summer applications. Use this information when scheduling applications to properties you maintain. You may have difficulty treating all of your properties in a short time (during spring). You can start applications several months prior to the onset of germination, especially if using a longer residual product. You will still obtain good early-season control. However, you may need to make a second application (you may need to in any event).

Another way to extend the application window for a pre-emergence herbicide would be to make applications after, rather than prior to, the onset of crabgrass germination. Add a post-emergence herbicide, such as fenoxaprop or quinclorac to a pre-emergence product for control of existing weeds. For example, we have successfully combined quinclorac and fenoxaprop with pendimethalin. By making this application in May, you can control pre-tillered crabgrass, while relying on the residual pre-emergence herbicide to control weedy grasses that germinate over the next few months. Alternatively, you could use dithiopyr, which also controls small crabgrass plants and provides residual control. With this strategy, you may be able to achieve season-long control with one application.

Weather conditions that impact pre-emergents q Rainfall. The greater the rainfall, the less time pre-emergence herbicides last. You will experience a loss of herbicide due to leaching and surface run off when excessive rainfall occurs. Ideal rainfall conditions for weed control would be a half inch of rain immediately following application, with no rain occurring after that. This is a highly unlikely scenario, so you must be aware that your applications may not be achieving optimum control. Certain parts of the country, including my area, experienced above-normal rainfall last summer. If you were in a similar situation, you may have seen more late-season crabgrass. You may be inclined to blame the herbicide, but excessive rainfall will adversely affect all pre-emergence herbicides.

Too little rainfall may also affect your applications. If it does not rain for three or four weeks after application, considerable herbicide losses may occur due to volatilization or photodegradation. When it does rain, there may not be sufficient herbicide remaining for acceptable weed control. Irrigate if it has not rained within a week after a pre-emergence herbicide application. Irrigation will incorporate the herbicide into the soil and reduce losses.

q Temperature. Certain products are volatile, and if you apply them during warm conditions, they may leave the soil surface as a vapor. For example, if you apply dithiopyr during cool weather, you will lessen the potential for volatility losses.

The herbicide formulation you use can also impact volatility. Using granular formulations of dithiopyr will lessen volatility and increase activity compared to sprayable emulsifiable-concentrate formulations. The new sprayable formulation of dithiopyr (Dimension Ultra WSP), in water-soluble packages, may also improve activity by reducing volatility losses compared to the emulsifiable concentrate.

Post-emergence application timing In general, make post-emergence herbicide applications to small, actively growing weeds. For crabgrass, this means the pre-tillering stage. Once a crabgrass plant has several tillers, it becomes difficult to control and higher application rates are needed. For example, if you postpone applications, the label for fenoxaprop recommends rates that are about 50 percent higher when treating 1- to 2-tiller crabgrass, compared to untillered plants.

Weather conditions that impact post-emergents q Temperature. Post-emergence herbicides work best when applied during warm (but not hot) weather. If you make applications when air temperatures are less than 50F, the result may be erratic or poor. Ideally, you should make applications when air temperatures are between 60 and 85F. When you make applications during high temperature and humidity conditions, the chances of turf burn increase. Check labels for specific precautions.

q Wind. Apply post-emergence herbicides when there is little to no wind. If you apply during windy conditions the chance for drift and damage to desirable plants becomes greater. Also, drift will decrease effectiveness by decreasing application uniformity.

q Rainfall. Only apply post-emergence herbicides when there is no potential for rain for at least 1 day. Because they are taken up through foliage, post-emergence controls must remain on the leaf surface long enough to enter the plant. Rainfall following applications will remove the herbicide and may significantly affect activity.

q Pre-application environmental conditions. You will achieve the best results when you make applications to actively growing weeds with good soil moisture. Post-emergence herbicides can fail when applied to drought-stressed weeds. The weeds do not absorb and translocate the herbicides.

Adjuvants and tank-mix combinations Some post-emergence herbicides require adjuvants for maximum effectiveness. The quinclorac label, for example, specifies the addition of methylated seed oil or crop oil concentrate to improve absorption. Other products may already contain a surfactant, so no further adjuvant is required. The label should indicate whether an adjuvant is needed.

Use caution when making herbicide combinations. Certain products can be antagonistic. For example, we observed lower crabgrass control when fenoxaprop was mixed with 2,4-D combination products. Other post-emergence crabgrass herbicides that we tested do not show antagonism when combined with post-emergence broadleaf herbicides. Also, you may achieve good control when mixing fenoxaprop with pre-emergence herbicides. There does not appear to be any antagonism, so some tank mixes can be beneficial when using fenoxaprop. Although broad-spectrum weed control through combinations of herbicides is desired, check for any incompatibility problems when mixing products. See "How to: Check for tank mix compatibility".

Take notes on each of the properties you maintain this year, list the dates and rates that herbicides were applied. Monitor rainfall patterns to gauge the need for repeat applications. Also, identify the major weed species present at each site. With this information, you will be able to fine-tune your grassy-weed management program and achieve the best results.

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© 2014 Penton Media Inc.

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