Question: Why do I have
trouble keeping my St. Augustine grass growing under my
trees? Also, what is this clover-like plant that is killing
Answer: The attached images show that
your St. Augustine grass has a virus called St. Augustine
Decline (SAD)--you can see the mottling on the leaf blades.
This is why Floratam St. Augustine should be planted or
sprigged into the area--it is SAD resistant, resistant to
chinch bugs and is drought tolerant. See: http://www.plantanswers.com/grass.htm
The "clover" you have is oxalis which is a shade-tolerant,
weed/groundcover which invades as your turfgrass thins.
You can spray it with a herbicide which is safe for St.
Augustine such as Greenlight Wipe-Out but it is difficult
to control and the application may further weaken the St.
Augustine. No grass grows well in a shaded area---St. Augustine
will endure it longer than most.
Another "weak grass invader" image is attached.
It is the wide-leafed, tiny yellow bloomed ground cover
named Horse Herb or Sprawling Horse Weed (Calyptocarpus
vialis Less.) which is even harder to kill than oxalis.
It too takes advantage of thinning turf.