Answer: There are several possible explanations for what is happening. Dogwoods in particular resent a dry location since they are shallow rooted and require an evenly moist yet well drained, humusy, acid soil. They also suffer in a full sun location without adequate moisture, remember they are originally an understory or edge of the woods tree from dappled or partial shade. Hydrangeas also prefer a location with an evenly moist soil although they are a little less fussy than the dogwoods; in full sun they too will appreciate adequate moisture.
Often a location at the top of a hill is relatively dry due to run-off. A rocky soil in and of itself is not necessarily a bad thing, but it may be that the soil in that location is truly poor (it may for instance have lost its topsoil through erosion or the road building process) and thus not very suitable for many of the more common plants. If it is at the crest of hill it may also be exposed and windy, another factor in increasing stress.
You might want to run some basic soil tests and find out what type of soil you are in fact dealing with and through consultation with your county extension determine which trees might have a better chance of surviving there.(They can also help you with the testing.)
Road salt could be a factor as well, if it is regualarly thrown there by passing traffic or carried there by runoff. It is possible to make a bit of a barrier using snow fence, burlap or even a decorative picket fence and/or creating a swale to divert the runoff, but the planting should still have a relatively good level of salt tolerance. The following list includes some of the more commonly used landscape plants in view of salt tolerance.
Salt Tolerance of Trees and Shrubs
In modern tree planting recommendations, it is suggested that you find a tree that is well suited to the native in the car, so maybe you can find another location thatis actually better suited for such a tree. Good luck with your new tree!
soil and localized growing conditions, then loosen the soil in the planting area going to the depth of the root ball and outward for a fairly large area, then planting without amending the soil. This is done to encourage the tree to root outward into the surrounding native soil as it would in nature.
Sometimes a tree planted in a carefully amended soil as you described will root only within the original planting hole, as though it was still in a planting pot, and this causes stress in subsequent years. The difference in soil structure from the hole to the surrounding area can also cause it to absorb water at a different rate, potentially causing it to dry out or possibly retain excess water creating an imbalance that way.
Additional care for a new tree includes watering it as needed to keep the soil evenly moist down deep in the root zone. This is done by watering deeply less often rather than a daily light sprinkling (dig down a bit and see if your watering has penetrated down six inches or so, test the soil an inch down to see if it is dry and you need to water) and using several inches of natural mulch over the entire root zone to both help retain soil moisture and keep down competing weeds. Watering should be done during the first growing season and into the fall until the ground freezes and then again in times of drought for the first three to five years the tree is in place.
It may be that this particular location is not really all that well suited to the smaller ornamental spring flowering tree you are hoping for. You may find that a juniper for instance would be more likely to thrive in that location; sometimes we need to revise our plans based on the realities of the site. It is also, in my experience, usually just as if not more enjoyable to see such a tree close-up near the front walk or rear patio than to pass by it quickly when coming and going
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