Based on the shape of the flowers, I'm betting on Kyllinga vs. Nutsedge. Either way, it's not something you want in your turf. I've never Kyllinga myself, but I'm sure someone can give you advice on how to eradicate it.

Hi! I'm so sorry, Jean001a, I haven't replied sooner but i have been away (got the neighbor to look after the garden :). Thank you for you response. I had used miracle grow organic all purpose plant food NPK: 7-6-7 when i transplanted the plants to their grow bags. (i don't know what it is made up of though). Luckily it seems as though they have recovered. the leaves are mostly green. However, i think that the fertilizer had too much nitrogen as the plants are now 1m tall and are only just starting to set their first trusses of flowers.

It looks to me like it was predominantly fescue. I believe the light green grass is either annual bluegrass (Poa annua) or roughstalk bluegrass (Poa trivialis). Hopefully someone can confirm which one of the 2 you may have. Both of these grassy weeds like moist conditions. Your daily watering is providing the perfect environment for Poa to germinate. You should be watering 1" a week, all at once. This forces the fescue to grow deep roots to reach the moisture. Poa (both kinds) have very shallow root systems (that's why they are easy to pull), are drought sensitive and will die when the top 2" of soil dries out. Both Poa's usually die out in the summer heat without irrigation. Poa annua is an annual, and drops seeds in the summer that will germinate in the fall, overwinter, and the cycle repeats in the fall. You can combat Poa annua by putting down a pre-emergent (i.e. Prodiamine or Dimension) in the late summer/early fall to prevent next year's crop. Poa triavialis is the real nightmare. It is a perennial and spreads through rhyzomes. The roots can stay viable for years even after the leaves have completed browned. The only sure (and cheap) way to kill Poa trivialis is to use RoundUp and reseed the areas. If you do indeed have fescue and DEEP pockets, you can use Velocity selective herbicide to kill the Poa (both types; it'll cause some stress on the fescue); but it's VERY expensive ($600 for a 1 pound bottle (only size available); primarily geared towards golf courses and sod farms).

I see you have a hose and oscillator sprinkler. That's an excellent start. What is your watering schedule? How often and for how long do you water? Did you fertilize yet this spring? Whether you did or not, now is the time to apply for the first time (or second if you did it in the early spring...which you should not have). Have you applied any herbicides or insecticides? If so, when, and what were they? As for your picture, we cannot see a thing at this altitude. You need to be 3 inches away, not 10 feet. And 3 inches needs to be in the shade or have someone cast a shadow where you are photographing. Get a picture with both green and gray grass. Rather than tease you, though, if this is a disease, regardless of which one, I would recommend applying corn meal at 20 pounds per 1,000 square feet. You can get corn meal at a feed store in 50 pound bags for roughly $12 or less this summer. Corn meal is a biological control that seems to work on St Augustine but not on other grasses. Try the corn meal before you try any chemical approach, because the chemicals ruin the biology rendering the corn meal useless as a bio control. Apply to the entire lawn, because you might not realize that the disease has infected the entire lawn. Also do the entire lawn, because corn meal is an organic fertilizer and it will leave deep green spots where you apply, so do the entire lawn. Once you use corn meal to cure St Augustine fungal diseases, you will completely stop worrying about it in the future. It is a complete solution and permanent...until you do something to let it happen again, which I seem to do annually :-( But at least I have a simple, safe, solution. Give the corn meal 3 full weeks to do the job. You will get very anxious about it for 2 weeks thinking nothing is happening. It simply takes 3 weeks, so relax. Between 2 and 3 weeks you will have forgotten about it. Then after about a month you will notice that "suddenly," the yard is perfect and deep green. Also if you are not already doing it, always mulch mow St Augustine at the mower's highest setting. You can let it go 2-3 weeks or longer if your mower can stand it. This will give you a deep rooted grass system that can withstand long periods of heat and go for days without watering. But do, please, tell us about your watering.