The most common method today is to apply a pre-emergent.  Some of the most common pre-emergents today require two applications per year per label instructions.  Don’t let yourselves be fooled by the one treatment per year programs that are available, it rarely provides adequate control.
To achieve and maintain the proper barrier you need to apply twice per year.

General Exterminating’s application program provides two applications per year at six-month intervals.  Each application will be inspected after 90 days to insure proper control.  If at any time during the treatment period you experience an overabundance of weeds General Exterminating will treat the affected area at no charge.  No service can eliminate weeds completely but we believe we can reduce the growth by 95%. 
Best time to apply, September through February & again every six months.  Treatment must have 1/2 inch rain or artificial irrigation within 60 days of application to insure proper effects.

For more information about this affordable service give us a call or email.

PRE-EMERGENT HERBICIDES
Pre-emergent implies controlling the weed, or weed seed, before it "emerges" or germinates.   In actuality, some pre-emergents control the weed seed right after it germinates, by killing the young, delicate, emerging roots.

POST-EMERGENT HERBICIDES
Post-emergent implies controlling the weed after it has "emerged" or germinated. 

TIP: Post-emergent weed controls are most effective when applied to a weed that is "actively growing" (not to a weed that is under drought stress or in a semi-dormant state).  Many herbicides have "systemic action", moving from the plant leaf down to the root, and throughout the plant.  This movement occurs best during periods of rapid plant growth.
 

Links:
Pest Management & Identification - Weeds listed by common name