Answer: Fall is usually a good time to plant, particularly with deciduous trees and shrubs. Evergreens are usually best planted in spring but can also be successfully moved in the fall.
This year however many areas have had a very dry season and consequently the soil moisture is just inadequate overall. The hot and dry conditions can also negative impact containter grown or balled and burlapped plants being held at the nursery. So it is a difficult time to be planting.
The yellowing and drying could be caused by overwatering or uinderwatering and/or transplant stress. Watering is critical to transplanting success. The soil should be kept evenly moist but not sopping wet. Note that this must be maintained up until the soil freezes.
You must take care that the root ball is kept moist and that the surrounding native soil is kept moist because roots will not grow into dry soil. Sometimes the two soil types will hold moisture differently, so the only way to know for sure is to check the soils with your finger or a trowel, digging down a few inches. You want to avoid creating a waterlogged or overly dry situation in one soil or the other -- or both.
Watering is best done very slowly so the water can penetrate deeply, watering again when the soil proves to be dry. (The interval will depend on the soil type and the weather.) Drip irrigation or a slowly poured bucket can work best, especially on a slope. A daily light sprinkling is to be avoided because it will not reach the bottom of the root ball and it will encourage shallower rooting of the plant. You should water, wait half a day or so and then dig down to see how far your watering penetrated. It can be surprising.
A layer of several inches of mulch over the root zone but not touching the bark of the plant will help moderate the soil temperature and help preserve moisture.
There are also some other conditions that could cause yellowing, such as a soil imbalance or use of hot mulch or accidental herbicide drift and so on. Since the job was just completed I would strongly suggest you consult with the lanscape designer, installer and nursery supplier as to what is happening. You might also wish to consult with your local county extension. There is just no substitute for an on-site evaluation when there is a widespread problem.
I hope this gives you some ideas for troubleshooting, but I really do urge you to find some local expert professional advice.
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