Watering recently planted Kousa dogwood
I planted this white kousa dogwood in May this year and as recommended by Sun Nursery was watering every other day for a couple of weeks, then twice per week. Each watering is 2-3 gallons. The tree is on South side of house, so getting lots of heat. Leaves are curling, especially at the extremities, so I reverted to watering every other day. Should I be watering more? Is there risk of overwatering? A dry stake driven in at 3 pm, 20 hours after watering, indicates moisture in region 16-26 inches down, less wet closer to the surface. It is a 92 degree sunny day. Think I’ll add more mulch. Is leaf curling normal in a new tree in high heat with full sun exposure? I planted a Cherokee Brave in the back yard at the same time, somewhat larger. In partial shade, and looking pretty good.
Howard County Maryland
Some degree of leaf drooping and curl on Kousa dogwoods (particularly some variegated cultivars like 'Wolf Eyes') is normal, especially in hot weather, regardless of moisture levels. It is best to check moisture levels a bit shallower (though your monitoring is good) - around a trowel's depth (~6" or so) is common, as deeper roots do stay moister for longer when waterings are thorough. The majority of a tree's roots will stay in the top foot or so of soil; this is especially the case on newly-planted trees. Roots grow primarily outwards away from the trunk rather than down into deeper soil. (In plant pots they have no choice but to go down at some point.) Dogwood roots do not fare well with constant soil wetness, so to avoid over-watering, you can give it slightly longer intervals between soaking to see how the soil moisture changes. If dry at around 6" deep or so, you can soak; otherwise, wait longer to let it dry a bit more. Two to three gallons may not be enough to soak the whole root zone well, but it is difficult to judge from the photo alone. If your moisture levels are consistent deeper than 6" than this may be sufficient.
If this is a full-sun exposure, that may be making the difference between this tree's apparent stress and the less-stressed 'Cherokee Brave'. All dogwoods prosper with some afternoon shade; Kousas are more tolerant of full sun, but they still can be stressed by heat (and reflected heat as you mention). Similar to Hydrangeas and Impatiens that wilt on hot days while being adequately moist, sometimes a plant "flags" (wilts) a bit during hot periods simply because it cannot rehydrate quickly enough to keep up with evaporation. If it perks up in the evening as the temperatures cool, this may be at least partly what is going on.
If your mulch layer is thin (under 2") you can add more, but around 3" deep should be the upper limit for shredded bark mulches. Mulch does have the advantage of helping to cool the soil surface, but at some point too much can be harmful in and of itself. Do keep mulch off of the trunk base, however, regardless of how deep the layer is. Ideally the root flare (where the roots begin to branch off of the trunk) should be visible at the soil surface and not buried.
We see that the tree is staked, though that may not be needed. Typically only trees in very wind-whipped areas or prone to tugging (by children, for instance) benefit from stakes. Either way, be sure to remove them after no longer than 6-12 months, because if they haven't done their job by then, they aren't going to work going forward. More importantly, make sure the ties are not abrading the bark and that they are loose enough that the tree can sway a few inches in any direction. This swaying action is what stimulates better root establishment and proper taper to the trunk as the tree matures.