Tall fescue is probably better suited to transition zone areas. I doubt many will recommend for your zone; I don't. It won't do well in your winters. Your best bet would be a KBG and fine fescue mix like you have, but if you have KBG why overseed. Add organic material and water. All of the cool season varieties have similar water requirements. Build the soil with organic material so it holds the moisture better. When did you last core aerate?

Generally, when I hear someone say that "Otherwise, you might be disappointed if you plant course tall fescue in with your Kentucky bluegrass..." then I immediately think of the old standard of forage tall fescue called K31. When planted lightly or in a finer blade grass like KBG or rye it tends to develop a single tough crown that looks a lot like crab grass. That is why you'll be disappointed, but it is also the wrong grass and wrong cultural approach. K31 is a good, drought resistant, utility grass generally found in the transition zone and southern areas and needs to be heavily seeded (400-600 pounds per acre). Modern turf type tall fescues do well with a little KBG added, but not a little TTTF with a lot of KBG.

Thanks for helping, Soccer Dad. I was under the impression that Kentucky bluegrass required more water than other grasses. The manager of a local nursery also said that when people tell him they want KBG, he tells them "no you don't!", although I can't remember if he said why.

My lawn has a sprinkler system installed, yet some of my neighbors with no sprinkler, and less topsoil have lawns that are as green or greener then mine when it gets a little dry. When it gets very dry and I water a lot, mine stays greener. The only difference besides grass type is that my lawn is on a hill facing the sun and the other lawns are on a hill facing away from the sun. Maybe that makes more of a difference than I thought.

To answer your question, it's been at least five years since I core aerated my lawn. I rent a dethatcher every spring. I understand that dethatching shouldn't be necessary if I go organic. In your opinion, should I concentrate on aerating and adding compost, but skip the seeding?

IMHO, your best value is core aerating. There is no need to dethatch if you aerate. Add compost if you can (some of the places I looked this week had stuff that was more mulch than compost, so be forewarned). A good heavy (20#/1000) feeding of Soybean meal would be next (or whatever organic fertilizer you chose). KBG is a heavy feeder for optimum color and health. You could probably feed Sep, Oct, Nov at 20#/1000 and not worry. These things will make your existing KBG thrive and would probably limit germination of any new seed without a complete renovation.

Water. Cool season grasses need about 1" of water each week. Your sloping lot and soil type may require some more. Do the tuna can tests so you know how much your applying. Your goal is to keep the root zone irrigated, so often you hear to water all at one time. Let your grass and your particular situation guide you on the amount of water. I suspect you are correct that your southern exposure is a bit drier in the summer, but slope plays a part too.

I've read several university studies that compare the evapotranspiration rates and KBG was demonstrated to be less than turf type tall fescue. Old wives' tales are often hard to dispell.

Soccer dad--do you have any links to those studies? I've read that TTTF doesn't save a lot of water, but I've read that it only needs about 3/4 the water of KBG.

I've also seen some anecdotal stories that newer varieties of KBG don't need nearly as much water as older varieties.

I agree that KBG would be the best choice for MI in sunny areas. In shade, fine and/or hard fescues might be a good choice. Creeping red fescue will spread, although more slowly than KBG. Some fine fescues do well with much less water than KBG.

In my experience the creeping red fescue is more drought resistant than the KBG. Though not as bright green in the best conditions it will grow back well after a summer dormancy.
Not at issue in your case as you do have irrigation, however you may want to push a spade into the ground and take a visual of just how deep the water is getting into your soil.

The only reason thatch builds up in a lawn dep enough to cause problems is because there is not enough of the Soil Food Web to digest what thatch there is, and that is most often because of lack of organic matter in the soil as well as using "stuff" that poisons the soil, synthetic fertilziers, "weed" killers, and many of the turf pest controls. Our sandy soils need lots of organic matter, stuff that will help hold both moisture and nutrients in the soil instead of having them flow out with the water. "Topsoil" does not replace organic matter and for many of us any "topsoil" we get will simply be sand with some organic matter added, unless the seller scrapes it off some new development.
Start making that lawn better by adding more organic matter which will get the Soil Food Web functioning which will then digest the thatch. Whether any Kentucky Blue Grass needs more water then any other grass depends on which cultivar, and there are about 100 now, you get, but most often you will find that a blend of Kentucky Blue Grass, Perennial Rye, and Fine Fescues is the predominant mix sold in Michigan. However, that mix does need a very good, healthy soil to grow, unless you want to spend a ton of money on fertilizers and pest and "weed" controls, because that lush green growth that is forced is simply unable to fight off those things and with little or no organic matter in the soil to hold both moisture and nutrients you will need to keep dumping them on, at great expense.
Start your lawn rejuvination program by applying compost, about 1/2 inch at most at any one application. There are expensive products you can buy that will do the same thing, kind of, and "Ringers Lawn Restore" is one. Made from the waste of poultry production it will do wonders on sandy soil, at great expense (As I Recall it is around $25.00 per 2,500 square feet). Mow high, the more grass blade exposed to the sun the more nutrients those grass blades will manufacture to feed the grass crowns and roots which will cause the plant to send out more stolens, tillers, and roots to grow more grass plants. Mulch mow, this will supply organic matter to the soil as well as 1/2 the needed Nitrogen annually. Turfgrasses need about 2 pounds of Nitrogen per 1,000 square feet per year to grow well. Water as needed, many "authorities" will tell you that lawns need an inch of water per week, but in sandy soils that can be, depending on the weather, twice that unless you live someplace with an extremely high water table with the water just below the soils surface.

BP, this is the one I can recall. Hope this link works. Well, I can't get the pdf to link. Search Colorado State U for presentation on Truth About Kentucky Bluegrass.

Here is the link.

I read it and it says that tall fescue can use about 10% less water than KBG if it can grow deep roots, but can also use more if conditions won't allow the deep roots.

One thing that it notes that many people forget is that tall fescue can stay green longer in drought than KBG, but once it goes dormant, it will die sooner than KBG. KBG can stay dormant for many months.

Fine fescues use quite a bit less water than tall fescue and KBG and can also stay dormant for many months like KBG. The more I read about fine fescues, the more I like them as a lawn choice. I've planted a mixture of sheep fescue and creeping red fescue in my back yard. I'm hoping I can leave it with little or no additional water.

For some reason, the link I posted didn't work. I'll try again.

Here is a link that might be useful: Truth about bluegrass

I turned off the sprinklers at the end of July and left them off for a month. We didn't get any rain during that time and daily highs were generally in the high 80s to mid 90s.

Nearly all the KBG at least went dormant, but nearly all the K31 tall fescue (which I hate) stayed green. I'm not sure about any other tall fescue that may be in the lawn since improved varieties are less easily distinguished from KBG.

I spread seed on 8/31 and have been watering 3x a day since then. The brown areas of KBG are still brown, so I think it died instead of just going dormant.

bp,
What I took from that brief from CSU was that the water requirement, based on ET, was less for KBG than TTTF, but maybe I should go back and re-study the whole thing. That is only one part of the equation in my mind. There is the resiliency of the grass to come back from draught as well as the length of time the grass can go without water and survive. In my area, soil structure, heat, and the amount of slope in my yard are all factors as well. I suspect that is true with most homeowners. Sometimes I wonder how much of the Universities' works are really applicable to homeowner lawns.

Hope your new seed sprouts soon!

I think they did say that tall fescue lost more water to evapotranspiration, but as you say, there's more to it than that. The reason tall fescue can last longer before going dormant is that it can develop deeper roots. If the soil won't allow that or there's no moisture deeper down, that won't help. Sheep fescue deals with drought very differently from either of these. Instead of deep roots, it has roots that expand laterally and develops tremendous rootmass, but not root depth. Any water that comes down is absorbed immediately. If it starts to dry, the leaves roll up to slow evaporation and it nearly stops transpiration without going dormant.

Once tall fescue goes into dormancy, the time between dormancy and death is much shorter than for KBG (at least usually). The tall fescue that stayed green for me isn't really a turf type tall fescue. It's the cheap unimproved K31. It stays green for a long time, but isn't an attractive turfgrass.

In my case, I think that since I was under watering before I quit, the grass was already stressed, so it went dormant very quickly and then died faster than normal due to preexisting stress.

The grama sprouted first 9after about 4 days). I hope it gets enough growth before it gets cold, because it's a warm season grass and is usually planted much sooner.

The wheatgrass sprouted a day or two after that, which really surprised me, since it's usually a real slow performer. I presoaked the seeds for 24 hours this year, so I guess that was the difference.

The fescue is finally starting to germinate (day 8 and 9). I expected it to germinate a week or two before the wheatgrass because what I've read says 7-14 days. I'm hoping to see more of it germinate over the next few days.