Answer: If the frost killed the early foliage, then they will have to regenerate buds and this will take time. Hopefully they were growing vigorously and healthy up until this happened.
You can spread a thin layer of good quality compost (maybe a quarter inch) over the soil surface, mulch with a thin layer of organic mulch (two inches thick -- no more, spread in a flat layer over the root area, do not allow it to touch the trunk of the tree), and if there is a dry spell water so the soil stays evenly moist like a wrung out sponge, not sopping wet and not dried out.
Then it's just a wait and see situation. When other dogwoods in the neighborhood show signs of growth then yours should, too.
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