The pictures are a little fuzzy when I try to blow them up to see detail in the leaves of the weed, but it looks like Virginia Buttonweed. Buttonweed makes a little 4-petal white flower about 1/2 or less from tip to tip. After the petal fall a seedpod developes with 2 seed about 3/16 of an inch long - side by side in the pod. See the picture attached.
Va. Buttonweed is just about impossible to get rid of without doing serious damage to you St. Augustine. Virginia buttonweed spreads both by seed and vegetative reproduction via spreading rhizomes. The weed is a vigorous grower and easily withstands very close mowing. Even in healthy, established St. Augustine grass lawns, Virginia buttonweed can invade and form a dense mat of weeds. You might try pulling it out by hand, but be sure to get all of the roots, undergrond stems and above ground stems, or you will just be pruning it. In severe cases you might want to try applying a selective broad-leaf herbicide like one containing 2,4-D, dicamba, MCPP or MCPA to your St. Augustine grass , but read the label of what ever product you apply and do so exactly like the label says, or you will KILL you St. Augustine graveyard dead.
If I can find it should i consider Celcius from Bayer Scientific?
The picture shows only limited foliage, but it looks like Virginia buttonweed which is fast becoming the number 1 weed problem in St. Augustine in Texas. You may want to Google "Virginia Buttonweed" images to compare.
There are no easy ways to get rid of this sneaky pest. If you are talking about isolated patches, try pulling it up and complete out. That means all the above-ground stems and below ground rhizomes. Bag and remove all weed fragments. Also, it is best if you would bag and not mulch you turf in the vicinity of buttonweed. Small stem fragments, followed by irrigation or rainfall will simply re-root.
Va. buttonweed is perennial. The plant produces a little 4-petaled white flower about 1/2 inch across that will yield a seed pod 1/4 inch long with two plump healthy seed. The pod is very buoyant an and will float with high rainfall into lower elevations of the landscape.
Pre-emergent herbicides applied in April and again in June with help control seed germinination but will do nothing for the existing plants.
Most of the herbicides on the market will make life miserable for buttonweed, but will not kill it out. In many cases, more damage is done to St. Augustine than buttonweed.
Here is a plan that keeps LSU recommends to keep it in check but requires you to buy an expensive herbicide called Celsius: In April, apply Ortho Weed B Gon in spot treatments at a rate of 2 oz of the herbicide/gallon of water/1000 square feet. 4 weeks later, apply a professional product called Celsius (expensive/available at places like Ewing Irrigation and John Deere Landscape locations). Usually, a 3rd herbicide application (Celsius again) is needed 1 month to 6 weeks later. It has taken me as many as 4 herbicide applications with severe infestations.
The herbicides mentioned above can be safely applied in St. Aug, centipedegrass, bermudagrass and zoysiagrass when used as directed.
I personnally have used Trimec, which is a mixture of 3 different herbicides. I would apply it verbatum with label instructions, wait about 3 weeks & reapply and again 3 weeks later.