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when can i seed after using ortho weed b gon

Colorado State University

Contact your local county Extension office through our County Office List.

Control of Annual Grassy Weeds in Lawns – 3.101

by Anthony J. Koski * (12/14)

Quick Facts…

  • Crabgrass, foxtail, barnyardgrass and goosegrass can be problem lawn weeds below 6,000 to 6,500 feet in Colorado.
  • In summer, mow grasses as high as practical for the grass species in your lawn.
  • Keep bluegrass, tall fescue, fine fescue and ryegrass at 2 1/2 to 3 inches during the summer.
  • Irrigate properly to help reduce annual weed infestation. Light, frequent irrigation encourages weed seed germination, even if a pre-emergent herbicide has been applied.

Crabgrass, foxtail, barnyardgrass and goosegrass can be important lawn weed problems at lower elevations (below 6,000 to 6,500 feet) in Colorado. These weeds belong to a group called summer annual grasses. It’s important to understand their growth cycle if you want to control them.


The seeds of summer annual grasses fall to the ground the previous autumn and germinate the following year, from midspring through midsummer. Germination depends on soil temperature, not air temperature, and generally begins when surface soil temperatures reach 55 to 60 degrees F. Soil temperatures optimal for the germination of crabgrass will closely coincide with the
blooming of forsythia shrubs in the local area. Once germinated, these grasses grow quickly during the summer months. Their growth is favored by warm temperatures and a good supply of water. Each annual grass plant produces thousands of seeds from midsummer through the early fall, when the first frost kills them.

There are several approaches to managing annual grasses in the home lawn. They include both cultural (non-herbicidal) and chemical (herbicidal) techniques.

Non-Herbicidal Control

  • Mow as high as practical during the summer months for the particular grass species present in your lawn. Mow bluegrass, buffalograss, tall fescue, fine fescue and ryegrass at 2 1/2 to 3 inches during the summer. The seeds of some weeds require high light intensity to germinate. The shaded environment near the soil surface in a high-mown lawn helps deter weed seed germination. In addition, the higher mowing height produces a healthier grass plant. Crabgrass and other annual grassy weeds are much more common and aggressive in lawns that are mowed less than 2 inches.
  • Mow often enough so that no more than one-third of the grass blade is removed in a single mowing. Letting grass grow tall and then cutting it back to a low height reduces turf density, allowing weed seeds to germinate and grow more easily. It is especially important to mow a lawn more frequently in the spring, when the grass is growing faster. A lawn may require mowing every three to five days during the spring and early summer.
  • Irrigate properly to help reduce annual weed infestation (see fact sheet 7.202, Lawn Care). Light, frequent irrigation encourages
    weed seed germination, even if a preemergence herbicide has been applied.
  • Fertilize according to the needs of your lawn species. See 7.202 for information on proper fertilization of the common lawn grasses.
  • Core cultivate (aerate) the lawn at least once a year to reduce compaction and to control thatch.

Preemergence Herbicides

Preemergence herbicides control crabgrass by preventing seedling crabgrass from becoming established. To be effective, they must be applied before the crabgrass seed germinates. In southern and western Colorado, crabgrass seed can germinate from late March to early April. Along the northern Front Range, it can germinate from mid-April to mid-May.

Apply preemergence herbicides two to four weeks before the above dates. The actual germination of crabgrass varies from year to year, depending on the weather. Warm, moist springs cause earlier germination and cool, dry springs delay germination. A preemergence herbicide application will not control annual weedy grasses after the seed germinates and the weeds begin to form leaves. Preemergence herbicide applications made just before or at the time of forsythia blooming will provide effective annual grassy weed control.

Apply the herbicide uniformly across the lawn to establish a chemical barrier on the soil surface. Avoid skips and streaks, which may allow weeds to appear in the lawn later in the year.

Apply the herbicide uniformly across the lawn to establish a chemical barrier on the soil surface. Avoid skips and streaks, which may allow weeds to appear in the lawn later in the year. Preemergence herbicides break down during the summer months, most quickly when summers are warm and precipitation or irrigation is plentiful. Therefore, weather or watering that favors a faster than normal breakdown can lead to a lawn infested with a late germinating annual grass. Thus grassy weeds can become a problem in lawns that are not mowed, fertilized or irrigated properly, even when a pre-emergent herbicide is used.

With normal weather patterns, most preemergence herbicides give good to excellent control of crabgrass, foxtail and barnyardgrass. Control of goosegrass and field sandbur often is less satisfactory, depending on the herbicide used. For best weed control, use the following guidelines. In all cases, read the pesticide label for more detailed information before using the

  • Do not use preemergence herbicides at the time of seeding except for a product containing siduron. Wait until the new grass is mowed three times before applying a preemergence herbicide.
  • After using a preemergence herbicide, wait two to four months before seeding, depending on the product used. Refer to the label for the specific time that must elapse before it is safe to seed.
  • Do not apply preemergence herbicides to the soil before laying sod or to new sod. Rooting may be restricted by some preemergence herbicides.
  • Apply sufficient water (1/2 inch) to wash the herbicide off the grass onto the soil surface within one to two days of application.
  • Do not thatch the lawn after the preemergence herbicide application, as the herbicide barrier can be disturbed.
  • Conventional core cultivation (aeration) does not reduce the effectiveness of preemergence herbicides that have already been applied.

Postemergence Herbicides

There are postemergence herbicides (applied AFTER the weeds have begun growing in the lawn) that can be used to control existing annual grasses. One type that is easily obtained by homeowners from any garden center is called MSMA (monosodium methanearsonate). This product is often sold under the simple trades name “Crabgrass Killer.” This material
is most effective against young seedling weeds and can be applied only as a spray. Once the weeds become larger and more mature, MSMA is largely ineffective.

Other herbicides used for postemergence control of crabgrass and other annual grasses include quinclorac (sold under the trade name “Drive”) and fenoxaprop ethyl (trade name “Acclaim Extra”). Both of these herbicides can provide excellent control of seedling annual grasses, and fair to good control of more mature (larger) weeds. Due to their higher
product cost they have traditionally been used only by professional lawn care operators. However, quinclorac/Drive is now available to homeowners in a product sold by Ortho under the trade name Weed B Gon MAX® Plus Crabgrass Control Ready-to-Use. This ready-to-use (no mixing required) spray herbicide also contains 2,4-D, MCPP and dicamba, which provide control of broadleaf weeds like dandelion and clover.

The fenoxaprop-p-ethyl product is sold by Bayer Advanced under the name Bermudagrass Control for Lawns. This product will also control crabgrass, foxtail, barnyardgrass and field sandbur in cool-season (bluegrass, fescue, ryegrass) lawns. It should NOT be used on buffalograss, bermudagrass or zoysiagrass lawns.

Some points to consider when using MSMA (sometimes called MAMA or DSMA on the herbicide label) and other postemergence herbicides for annual grassy weed control include:

  • These products may cause a temporary discoloration of the turf. If the soil is dry, apply enough water the day before the treatment to moisten the soil to a 3-inch depth. During hot, dry weather apply another 1/2 inch of water two days after the lawn has been sprayed.
  • Do not apply postemergence crabgrass herbicides to a new seeding until it has been mowed at least three times.
  • Follow ALL label instructions carefully in order to obtain greatest effectiveness and to avoid unintentional injury to the lawn and surrounding landscape plants.
Table 1: Preemergence herbicides for annual
grass control and expected level of control.
Herbicide Name Trade Name CrabgrassFoxtail Goosegrass Barnyardgrass Field Sandbur
benefin Balan G-E F G NA
benefin/trifluralin Team E G G-E NA
bensulide Betasan G-E P-F G-E NA
corn gluten meal Many G NA NA NA
dithiopyr Dimension E G-E G-E NA
oxadiazon Ronstar G E NA G
pendimethalin Pre-MPendulum E E G-E G
prodiamine Barricade E G G NA
siduron Tupersan F-G P-F F-G NA
E=excellent, G=good, F=fair, P=poor, NA=information not availableExclusion of chemicals or product trade names does not imply criticism, nor does inclusion imply any endorsement, by Colorado State University or the author. Read all label directions before using any pesticide.

* Colorado State University Extension turfgrass specialist and professor, horticulture and landscape architecture. 10/97. Revised 12/14.

Fall is the time to control perennial broadleaf weeds

Spring is usually the time of year we see multiple advertisements for lawn care product, especially weed n’ feed products. The labeling on these products always contains a photo of a weed-free lawn with photos of weeds the product will control. What is the weed most commonly shown? Dandelions!

Dandelion is a perennial broadleaf similar to other perennial broadleaf weeds (broadleaf plantain, buckhorn plantain, ground ivy, violets, clover, etc.) that have a central taproot. In the spring, weeds such as these will flower and produce seed. When this happens, plant foods and energy move upward in the plant for reproduction of seed. In early fall, perennial broadleaf weeds prepare for winter by building up carbohydrate reserves. The storage vessel is the large, central taproots characteristic of perennial broadleaf weeds. Therefore, most internal plant activity is carrying these reserves downward.

Broadleaf weed numbers are best reduced by maintaining a taller mowing height (3.5 to 4"). While we know that taller mowing heights reduce weed populations up to 80%, you will still notice some weeds like dandelions and others. Increased mowing heights may provide you some savings in weed control by resorting to some hand pulling or spot spray applications of broadleaf herbicides. Blanket applications of broadleaf herbicides are usually not necessary for just a few weeds and are costly. A ready-to-use spray product for broadleaf weeds takes just a few minutes to treat a few weeds for the desired result.

So, why is fall the best time to control perennial broadleaf weeds? All broadleaf herbicides sold over-the-counter for homeowners are considered systemic herbicides. This indicates that the plant absorbs the product then is translocated throughout the plant to disrupt normal plant functions. That large taproot we discussed in the engine house for that plant to survive. If the taproot lives, so does the plant. Therefore, that downward movement of food reserves into the taproot during fall allows the herbicide to translocate more easily into the root for a more efficient control. Broadleaf herbicides are also considered selective herbicides, which indicate they will control the broadleaf weeds without harming your desirable turfgrass.

One precaution to keep in mind when controlling perennial broadleaf weeds in fall is to read the label concerning re-seeding intervals. Fall (September) is also a great time to over-seed lawns. Many broadleaf herbicides require a wait time of 3 to 4 weeks following an application of product before seeding. In some situations it may be better to seed first for proper timing, and then follow up with broadleaf weed control.

Broadleaf weed control products will contain some combination of various active ingredients. They usually include some three-way or four-way mix of the following – 2,4-D, MCPA, MCPP, sulfentrazone, dicamba, or triclopyr. Most products are three-way mixes of 2,4-D, MCPP and dicamba. Some products have removed 2,4-D and replaced it with triclopyr. These products tend to do a better job on the tougher perennial broadleaf weeds like ground ivy and violets. Active ingredient of the products can always be found on the front panel/label of the container under the ACTIVE INGREDIENT section. Some of the more common trade names include: Ortho’s Weed b Gon and Weed b Gon MAX, Pbi/Gordon’s Trimec, and Spectracide’s Weed Stop for Lawns.

For best results, it is recommended not to mow two days prior to two days following applications. Do not irrigate the lawn following applications for at least 24 hours. Always check the forecast for rain prior to applications as well.

All product information is presented with the understanding that no endorsement of named products is intended, nor criticism implied of similar products that are not mentioned. Before using any weed control product please read the label carefully for directions on application procedures, appropriate rate, first aid, storage and disposal. Make sure that the product is properly registered for the intended use (specific weeds and site).