Post emergent weed control is a herbicide used to kill weeds after they have germinated. To do this the weed must be actively growing and not just green. Soil temperature is a good indicator of growth. For the most effective weed control in early spring is to only apply post emergent type weed controls when the soil temperature have been above 55 degrees for 10 days or so.
Remember with liquid herbicides you don't have to soak the weed for effective control. Just a light misting is all that is necessary. More than this and you may damage nearby desirable plants.
Weeds are a part of almost every landscape and you'll never get rid of them all, all the time. Spot treat them when you see them and they're actively growing.
If preventive or pre emergent control strategies fail to satisfactorily control the establishment of annual grass weeds, herbicides are available that will kill crabgrass after it germinates and begins to grow.
These products are referred to as "Post emergent Herbicides." Post emergent herbicide applications should be made as soon as the crabgrass is seen in the lawn. Occasionally one, but generally two applications are needed to control crabgrass. Make applications 1 to 2 weeks apart until the crabgrass is killed.
The success of the these treatments will be greatly improved if the crabgrass is in the 3 to 5 leaf stage of development. Once the crabgrass begins developing tillers (numerous new stems), control is much more difficult to achieve. Repeat applications may only provide partial control.
If additional crabgrass germinates after the applications, another series of treatments will be necessary. The most common post emergent crabgrass control products available to homeowners are products which contain methanearsonate. This material may slightly discolor Kentucky bluegrass, but no serious injury should result if used at the rate and under the conditions described under "Directions For Use."
On newly seeded turf, the stand must be well established before you can treat it with a post-emergence broadleaf product. You usually can treat if the site has been mowed three times.
Before using any chemical, identify the weed, then read the label on the package. If the weed is not listed on the label, don't use it because no one herbicide will control all weed species. You may need to use a combination of two or more herbicides to control specific weeds.