The ones with the highest Leaf Area Index. Buckeye is a good one, but your clue comes from driving around - the large trees with no grass underneath are a good place to start.

Thanks everyone, I think trees with little to no grass underneath could also be that way if they have very fibrous and shallow root systems that suck up lots of water (maples have that). So no grass underneath I bet is not necessarily always a Tell-tale sign of the coolest shade perhaps. But dense shade I bet is a large factor as well to no grass growing under the Norway maple.

So maybe the question might should be: What is a deciduous tree species with the most dense shade that also has large leaves or leaflets, and has relatively thick and glossy leaves as well?

-American basswood

-American beech

-Red oak

-Tulip tree

I posted this pic before of a small pine, but look at the background Sugar maple & its shade -- solid from a relatively high sun angle (the ball of light upper left isn't the sun -- it's a reflection). Grass grew scantily near the trunk, so I just dedicated that area to spreading out yard waste & creating a mulch zone. Irises grow fine right up against the trunk, tho. Go figure...

A decent plan for shade 20 years from now is an anticipation of the projected climate at that time - likely a bit warmer and more of what you have in precip. You can likely stretch a half-zone if you are looking to expand your choices. Surely in that state there are maps with projected changes.

.02

And lcadem, You made some good points about shade tolerant species. Never really thought a whole lot about that, and the shade they produce. Beech does grow on south facing shady slopes. Hemlock seedlings can take deep shade, and the adults make very deep shade so that makes sense.

The other consideration here would be the rate of evapo-transpiration--natures swamp cooler--in reducing the temperature under the tree. So, a tree transpiring a lot of water would generate cooler air than one that transpires little. Having plenty of growth--grass, ferns, moist soil, etc. under the tree would also increase evaporation and lower the temperature. That said, probably the thicker the canopy the higher the transpiration rate and thus better cooling.

The more leaves, the more evapotranspiration. Thus more cooling.

As beng and others have said, beech, sugar maples and norway maples are notorious for their dense shade AND thirsty roots.

John

Unless you're in an area devoid of ill-raised, unsupervised youth, might be worth a thought. That, and the slow growth.

How tall & how wide a plant do you want? Density of the shade will impact temp., but so will the surface area shaded. A larger patch of shade will tend to be cooler.

Richard.

One of the main drivers of cooling is ET. Nevertheless, one single tree of species X with dense shade is about the same temp reduction as species Y with dense shade, as species Z with dense shade. Might as well choose for form, fall color, bark texture, whatever.

I would think crown height would also improve a tree's ability to cool the air.

I've driven past big trees in warm weather -not under them -and felt a noticeable difference in the coolness of the air.

As to the beeches being slow-growing, I know some around here that would beg to differ. They can grow a foot or two a year, in good soil with normal rainfall. They do prefer slightly cooler climates, though. To me, they are one of the nicest trees (besides sugar maple!).