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When is the best time to put down weed control?

Nick Asked:
When should I put down weed killer?

We Found:


The how and when of weed control depends on what type of weed you are trying to control. Grassy weeds and broadleaf weeds are the problems in home lawns. Grassy weeds are usually more difficult to identify than broadleaf weeds. Grassy weeds are true grasses or monocots. A grass seed germinates and emerges as one single leaf. Crabgrass, foxtail, barnyardgrass and goosegrass are examples of grassy weeds. Broadleaf weeds are broad and flat, as with dandelions. However, the narrow leaves of wild garlic are the exception. In addition to dandelion and wild garlic, other common broadleaf weeds include wild violet, morning glory, thistle, goldenrod, ragweed, spurge, plantain, and chickweed.

Many grassy-type weeds can be controlled by a pre-emergent herbicide if applied at the right time of the year. We get more questions from viewers about crabgrass than any other grassy weed. When should I apply pre-emergent herbicides for crabgrass prevention, is the most often asked question.
Pre-emergence herbicides control crabgrass by preventing seedling crabgrass from becoming established. To be effective, they must be applied before the crabgrass seed germinates. In central Ohio, for example, it is recommended to apply an herbicide by the application deadline of April 15. The deadline in southern Ohio is about April 1, and in northern Ohio about April 30.

Apply pre-emergence 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 pre-emergence herbicide application will not control annual weedy grasses after the seed germinates and the weeds begin to form leaves. Pre-emergence herbicide applications made just before or at the time of forsythia blooming will provide effective annual grassy weed control.

There are several pre-emergent products on the market, including an organic product that contains corn gluten meal. This product is available and is more expensive than the synthetic products. Though a few crabgrass seeds germinate by the April 15 deadline date, the vast majority of seeds germinate in May. Germination begins when the temperature in the top inch of soil reaches 52-54 degrees or more for at least 5-7 consecutive days; soil moisture must also be present. Seeds also need light to germinate, so those thin areas in the lawn are likely places for crabgrass to become established.

If you have seeded the lawn this spring and still want to prevent crabgrass, there is only one pre-emergent herbicide to use that won’t kill the lawn grass seed. Look for Siduron or Tupersan and to be aware that this product is more difficult to find and more costly than other products. As with other pests, it is recommended that treatment be targeted. If crabgrass was a problem in the lawn last season, and it was allowed to go to seed, then you will probably want to apply a pre-emergent herbicide. If you are cutting new ornamental beds in the lawn, if areas along the driveway or sidewalks died out last season, or if the lawn is thin from grubs or disease injury, you might want to treat those areas with a crabgrass preventer. If you maintain a dense lawn and have not seen any crabgrass in the past two or three years, you probably don’t need to apply a herbicide.

What is the best way to control broadleaf weeds in my lawn? What’s the best time to apply herbicides?

The best way to control broadleaf weeds is to grow a dense stand of grass that can out compete weeds; fertilizing, mowing properly, and irrigation will all contribute to lawn density. If many weeds have invaded the lawn, you may want to review your fertility and mowing program. If you need to use an herbicide for control, September usually provides a good “window of opportunity,“ as temperatures cool and moisture returns. The other acceptable period is mid-May into mid-June.

If there are several species of broadleaf weeds present, use a “trimec” product, which contains 2,4-D, MCPP and dicamba in combination. Several manufacturers produce trimec products; check your hardware or garden center for labeled products. If weeds have invaded the entire lawn, then you may wish to make a broadcast application using a dry, granular product or use a liquid product in a hose end sprayer. If there are few weeds in limited areas, then make spot applications using a liquid product. Keep in mind that weeds must be actively growing in order for herbicides to work well.

Remember, broadleaf herbicides DO NOT eliminate grassy weeds, such as clumps of tall fescue or nimblewill. These grassy weeds can only be controlled with non-selective herbicide. These will also kill the lawn grass, and you will need to reseed or sod after applying one of them.

Applications of herbicides intended for postemergent broadleaf weed control will only kill those weeds present at the time that the herbicide is applied. They DO NOT prevent weed seeds from germinating and developing in the lawn at a later date. It is also important to remember weeds must be actively growing when the herbicides are applied so effective control can be achieved. This means that spring applications should be made from mid-April through early June, and fall applications should be made during the months of September and October. Herbicide applications during July and August are strongly discouraged because not only will weed control be more difficult to achieve but also an increased risk of causing damage/ discoloration to the lawn.

For more information on this topic click on the following OSU Extension Publications:

Posted by on 03/27 at 10:05 AM
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Posted by Denise Yost  on  04/02  at  12:59 PM

I am a Master Gardener.  The short answer to the best time to put down weed control is “never.“  Any chemicals applied to the lawn only serve to disrupt the balance of microbes which are essential to a healthy lawn.  Once the homeowner begins to apply chemicals, it’s a vicious cycle.  Various unwanted inhabitants - grubs and moles among them - are found mostly in lawns which receive supplemental fertilization and other chemicals.  By applying chemicals and by watering supplementally, the homeowner creates a weak and feeble root system and a lawn which cannot survive on its own through drought.  Conversely, redefining one’s definition of “lawn” to include anything green, mowing high and leaving clippings on the lawn, providing no additional chemicals and no supplemental watering creates a lawn which is tough as nails and can survive anything natures throws at it.  This approach also means less time spent in lawn care and a healthier environment for everyone.

Posted by Sprockets  on  04/02  at  03:56 PM
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