Answer: Without seeing the trees and where they are planted it is difficult to tell you exactly what has happened. Trees that are stressed are more likely to be attacked by a pest or disease, so often there is a combination of factors that combine and kill a tree.
These trees are both potentially subject to problems such as borers, so avoid pruning or damaging the trunk or bark in any way as this provides an easy entry to the pest. Start with a healthy specimen and plant it at the same depth, no deeper, than it grew in the container or when dug.
The cherry needs a location in full sun and soil that is well drained, meaning never soggy or saturated. To encourage proper rooting, when planting a new tree dig a wide hole and loosen the soil over a wide area. Do not dig deeply as this can cause excessive settling after planting. IF there are any encircling roots, loosen them and direct them outward. The most important thing for a new tree of any kind is to water correctly. Losing a tree at the end of the first summer is often a sign of drought stress and/or poor rooting.
The dogwood is potentially subject to a number of pests and diseases as well, including a severe wilt disease that has become somewhat prevalent in the mid-Atlantic. In some areas, it is not longer recommended to plant these for this reason. The dogwoods are native to the edge of the woods, so try to place it in a location with bright dappled light all day or in morning-only sun. The soil must be acidic, humusy, evenly moist and yet well drained. In a dry spot or in a wet spot this tree will fail. Again, avoid damaging the trunk. This tree is very sensitive to being planted too deeply, and it is very shallow rooted and especially sensitive to root disturbance and to drought as well.
Both trees should be mulched with two to three inches of organic mulch over the root area year round. Spread it over a wide area but do not allow it to touch the trunk. Water as needed to keep the soil slightly moist, meaning damp like a wrung out sponge, not sopping wet/saturated. You need to check the soil moisture on the dogwood any time the soil is not frozen. Dig into the soil with your finger. If it is still moist, do not water yet. When you do water, water slowly and thoroughly so it soaks down deep. After watering, wait a few hours and then dig down to see how far the water went; it can be surprising.
The dry winter and spring may have severely stressed the dogwood so although it is still alive the prognosis may not be that good. I would strongly suggest you consult with your local county extension to try to determine what is happening to the dogwood, in case it is something that could infect its replacement, and whether or not you have a suitable spot for one in your yard.
Although I do not know for sure what happened to the cherry, I would not replant the new cherry in the same spot, either. Again, your local county extension may have some suggestions as to where to plant. I'm sorry you have had trouble with your trees.
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