Here is the deal. Its too HOT and DRY to try and keep your fescue lawn green if it gets full sun, period. We are losing water faster than you can apply it with irrigation. There is this term called “ET”, which basically measures how much moisture we lose to the atmosphere on a daily basis, through plant usage and just plain evaporation. As of 6-25-12, we are basically losing about 1/3 of an inch per day in the Nashville area. That's a hair over two inches a week. That is astronomical for June! It is more typical to lose about 2/10 of an inch a day. The hot air temps combined with lower than normal humidity (the hot breeze vs the stagnant steam we usually have) works as follows:
Think of your car engine. If you have antifreeze in your car and the water pump is working, it keeps the car engine cool enough. If the water pump quits working, the fluid will get hot and engine will overheat. If your engines water pump works fine, but theres not enough coolant in the system to circulate, engine overheats. With tall fescue, if you have enough water and the humidity is low, the water will go into the roots, through the plant, and exit out the leaves, keeping the plant cool. If you don’t apply enough water, there's no coolant to keep plant from (overheating)wilting and turning brown, AKA, drought stress. When the humidity climbs, and you try to compensate for the drought stress, the amount of moisture in the atmosphere is too great to allow rapid movement through the plant and into the atmosphere. Which, is like the water pump not working on your car. Even though the soil profile is wet, there is no cooling effect. Overwatering at that point will make hot, wet soil, which will most likely cause pithium root fungus and will grow lots of nutsedge, Virginia button weed, and other undesirable weeds.
So the choice becomes, do I want to spend a boat load of money on water to try and “green up” my burnt fescue, only to encourage pithium, nutsedge, and other weeds??? Trust me, “Brown Patch” disease control won’t do squat when pithium strikes! Game over at that point.
My recommendation is to water enough to keep the grass alive, but not necessarily “green”. Apply a max of 1.5 inches a week, until drought breaks, if possible. This should keep your lawn from becoming a sea of nutsedge and from pithium blight wiping you out.
This is the first time I have ever seen it this dry and hot, this early. Its like late July, August conditions out there. Fescue basically quits functioning once air temps hit 90%, so its on life support at this point anyway. Sustained high temps for more than a week will cause irreversible damage in many full sun areas. It doesn't matter what we apply to your turf if its to dry to live, especially when your trying to grow tall fescue in Sahara Desert! Fescue is a “cool season” plant, NOT A CACTUS!