Question
Hi Floyd,

We live in Littleton MA, we recently had our lawn installed via hydroseed.  Along with the new grass we have lots of crabgrass.  I did some research and found a Scotts product Pro Step 1 for seeding and grabgrass preventative.  I applied that a week ago hoping to prevent the spread of the crabgrass.  Is there anything I can put on the lawn now to kill the already growing crabgrass?  Any help you could give us would be most appreciated, thank you!

Dave

Answer
Good evening Dave:
Both the smooth and hairy types of Crabgrass are probably the most undesirable summer annual weeds in the lawn. Annual grass weeds are grass weeds that germinate from seed, grow to maturity, produce seed and die within a 12 month period. A number of annual grass weeds routinely invade turfgrass areas including crabgrass, goosegrass, foxtail, barnyardgrass, fall pancium and annual bluegrass.
Crabgrass is usually lighter green in color, has wider leaf blades and a more spreading growth habit than  cool season cultured lawn grasses. Both preventive and control programs must be implemented to ensure crabgrass does not establish itself in the lawn. Crabgrass can be prevented to a large degree by maintaining a dense, healthy stand of grass. A high quality lawn will  shade the soil surface and discourage crabgrass germination along with the seedlings of other annual grass weeds. Therefore, it is important to select the proper species and cultivars of turfgrass for the site to be established. Proper mowing (height of two and a half to three inches and removing no more than one third of the grass blade at each mowing), fertilizer and water usage are the most important factors in maintaining turfgrass density.
Crabgrass seeds in the soil begin to germinate in the spring once the soil temperatures warm to nighttime minimum temperatures of 52 to 54 degrees F for at least 5 consecutive nights under conditions of moist soils (Although not 100% reliable, it is about the time the Forsythia begin to bloom).
The best way to stop crabgrass and other annual grass weeds from establishing themselves in the lawn is through the use of preemergent herbicides. These are products that stop the development of unwanted weed plants before they emerge from the soil. These herbicides control annual grass weeds by inhibiting cell division in the young root system. Lawns with thin stands of grass may require yearly applications of a preemergent herbicide. Scotts Halts is a pre-emergent herbicide. Apply Scotts Halts or Scotts Turf Builder with Halts Crebgrass Preventer before your daytime temperatures are consistently above 65?F.  A pre-emergent like Scotts Halts will create a barrier that will prevent crabgrass seeds from germinating.

If preventive annual grass weed control is desired in newly seeded lawns or where young, desirable seedling grasses are developing, a preemergent herbicide containing Siduron is the only herbicide that will provide control but not injure the development of  seedling turfgrasses. In fact Siduron may be applied at the time of seeding.
Preemergent herbicides are generally only effective if applied before the annual crabgrass weeds germinate. Therefore, early spring applications are essential if satisfactory crabgrass control is to be achieved. Herbicide applications should be completed and the herbicide watered-in at least 7 days prior to the initial germination date to allow time for the herbicide barrier to be established in the soil. Next spring consult with your local Cooperative Extension Office for the application date in your area.
If preventive or preemergent control strategies failed to satisfactorily limit 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 "Postemergent Herbicides." Postemergent 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 postemergent 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."
As with any herbicide, always read and follow the package directions.
I would suggest, that if you know for sure it is crabgrass that has invaded your lawn, you wait until next spring and apply a preemergent herbicide. At this time of the year it would be very difficult , if not impossible, to rid your lawn of crabgrass. Fortunately crabgrass is an annual and will die out over winter. It's the seed for next years crop you will need to destroy.
Have a good lawn:
Floyd McMahon