It's also possible that even though the Northern Red Oak is growing right next to the last and most yellow Willow Oak, that it's roots aren't growing into the same kind of soil in it's immediate vicinity. It's quite common to see rows of Pin Oak(Q. palustris) where some are dark green, while the odd one here and there are quite chlorotic and yellow. The soil conditions are just different enough from tree to tree to have different results along with possible genetic variations between each individual tree influencing alkalinity tolerances. Although in the case of the Northern Red Oak, generally Northern Red Oak is more tolerant of higher pH soils than Willow Oak and a soil pH which might make a Willow Oak look chlorotic wouldn't necessarily be high enough to make a Northern Red Oak look chlorotic.

Symptoms of overwatering very much mimic underwatering. Akthough if you can overwater a willow, you may have other problems. But at the same time... What leaves are dropping? The whole branch? Or are the tips staying on? Cuz a willow can take a powerful large amount of water and would drop leaves in an effort to reduce water loss in a dry summer spell.

We don't worry about roots in my part of the country unless things have been extremely dry. So check on things if no rain at least every week, especially if it has been very sunny, hot or windy. Then, if it is getting too dry, just water normally, just enough so the soil gets visibly moist down through the root zone, not sopping wet. You don't want to drown things. And do this only if it's getting dry again from the last time you watered or got rain. We have sandy soil. As mentioned, if the tree is zone hardy for your area, cold will not be a problem. Unless it is in a notoriously exposed or windy spot. The biggest problem I had in the past and this is mostly with fruit trees, is the mice would chew the bottom of the trunk bark, during the winter, under the snow and girdle the trunk. I learned to wrap the trunk with heavy duty aluminum foil from the ground to about a foot up. Because mice will get through the rabbit fencing unless it is very fine hardware cloth. So if your H cloth is 1/4" squares or smaller, you won't have to worry about the mice. And rabbits like any kind of tree so put up some HC, chicken wire or rabbit fence around at about a 2 ft. diameter hoop and as high as needed to protect even if the snow gets a few feet deep. If no snow, 2 ft. high is usually enough. Some of mine are 4 ft high and I might have to deal with higher if a lot of snow blows in. And if deer are a problem, you may have to extend that hoop of rabbit fence up to 6ft. high and add a couple of stakes for sturdiness, so they deer can't reach over the top and snack on the branches.

how large are the trees ... how did you plant them.. did you amend the soil ... bare root??? i am of the opinion.. that trees .. in mother earth.. NEVER need fert ... do you fert your lawn??? take a hand trowel ... or tablespoon.. and dig 3 or 4 holes in the gob planted.. and in native soil.. 3 to 6 inches deep.. AND FIND OUT ABOUT MOISTURE.. at depth.. in your soil.. in your yard.. once you find out.. you can adjust watering accordingly ... one thing for sure.. watering is an art.. rather than a science.. and doing it by the calendar may not work ... have you ever done a perk test??? : https://duckduckgo.com/?q=garden+perk+test&t=ffcm&ia=web this will help you understand how water flows thru your soil ... and finally.. how about a pic or 3 ... so we can see whats going on ... ken