Answer: Dogwoods as a rule grow better in morning only sun or in bright dappled light all day. Full sun all day is a hot location that does not suit it very well and can stress the tree, especially a new tree that is not yet fully rooted. They do best if transplanted in very early spring, so their roots can grow some before the weather turns very hot. They also need a soil that is well drained yet evenly moist like a wrung out sponge. It should be organic, acidic, and humusy. Dogwoods are very sensitive to over and underwatering.
Based on your description I think several things may have happened together: transplant stress, heat stress, overwatering.
Your goal in watering is to keep the soil damp like a wrung out sponge, not sopping wet/saturated and not dried out. Watering is not done according to a set schedule because the need for it depends on the soil type and the weather. You are watering -- if needed -- to supplement rain. To know if you need to water, dig down and check the soil with your finger. If it is still moist, do not water yet. When you water, water slowly and thoroughly so it soaks down to the deepest roots. To know how much to water, wait a few hours after watering and then dig down to see how far it went; sometimes it can be surprising. Usign this method you might need to water after five to seven days without rain as a rule of thumb, but check the soil and see tobe sure. Using an organic mulch in a flat layer over the root zone (about two to three inches thick, do not allow it to touch the trunk) also helps keep the soil cool and moist.
It sounds like your tree is severely stressed. The best you can do for a stressed tree like this is careful watering and using a mulch, then patience. Since it is a new tree, I would suggest you also consult with your retailer.
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