What is the differance between crabgrass and centipede grass?

Nothing is different about them. Both crabgrass and centipede grass are grasses but, only one is considered to be of the "desirable" variety for a formal lawn. Crabgrass is considered a "weed" because it is an undesirable plant. The definition of a weed is anything growing in an area where it is not wanted. Weed is a generalized word used to describe mostly "broadleaf" weeds, but as stated above, it can be any living or growing such as a tree, shrub, plant, or flower. If you don't want it there, it is.... a weed!

Now for the second part of your question. You can use weed killers on centipede grass. Use "roundup" (glyphosate) if you wish to totally get rid of the centipede. But, if you wish to control broadleaf weeds "in" a centipede turf, you will need a special herbicide that will not burn it or kill it. Firstly, you will need to use a "selective" herbicide, one that will not kill grasses, only broadleaf weeds. Centipede grass is very sensative to selective herbicides and there are only a handful that you can use safely without killing a centipede lawn. Most selective herbicides contain a percentage of 2,4D (chlorophenoxy herbicide) which centipede does not like. 2,4D has been around for more than 60 yrs and is still the third most widely used herbicide in the U.S. and Canada and the most widely used worldwide. In order to control weeds in a centipede and a st. augustine lawn as well, you will need to apply a selective herbicide such as Ortho Weed B Gone "for southern lawns". This special formula was developed for warm season southern lawns such as centipede and st. augustine. It must have and say on the label that it is for southern lawns or you may experience some browning and a possible kill of the turf. With crabgrass being a grass, it can only be controlled with a non selective herbicide such as "roundup" (if it isn't in the formal lawn area) or with a pre-emergent herbicide. Pre-emergents work by "coating" unwanted weed seeds with a hard shell, preventing them from ever germinating. The timing of applying a pre-emergent is very critical in its performance, especially with grassy type weeds such as crabgrass. Hope this answers your question...

...$Billy Ray$

Crab Grass: The lawn weed, crabgrass (Digitaria spp) is a warm-season annual weed, which means it reproduces by seed. Preemergent herbicides (also spelled "pre-emergent") come in either granular or liquid form and kill crabgrass seedlings as they germinate. Think of preemergent herbicides as forming an invisible shield across the soil surface that stops emerging crabgrass dead in its tracks.

Centipede Grass: Centipede is a tough, low-growing lawngrass that thrives with little care in the lower South, particularly in places where the soil is naturally acidic. Centipede's rough texture and slow rate of growth are offset by its ability to grow under conditions that cause more refined lawngrasses to fail. Centipede will grow in light dappled shade, but it grows most vigorously in almost-full or full sun.

A strong centipede lawn will survive periodic droughts, but it will be more resistant to pests and other problems if you water it during dry spells. Centipede is naturally a medium green color and should be fertilized less often than other lawngrasses. Over fertilizing will weaken this grass. A fertilizer that contains iron will insure a maximum green. Healthy centipede will "knit" itself into a tight turf that naturally resists weeds.

Weed killers are formulated to kill weeds.... not grass, and are formulated based on the weeds dna, thus, killing not only the weeds, but also their seeds.

Want to kill your Centipede Grass: Do not over-fertilize or attempt to make it grow as fast as other turfgrasses. If you do, you will kill it. So, the best way to kill centipede is to apply too much fertilizer or lime.