Answer: When newly planted shrubs suddenly wilt and then turn brown it almost always has to do with soil conditions. It might not drain well and the roots are sitting in a puddle, or they are not getting adequate water. Either way, the distress shows up on the foliage. Viburnums will grow in full sunshine, but if the site has refected heat (such as near a wall or sidewalk) it will affect the health of the plant. Make sure the site drains well and that you're giving them adequate water; one inch per week is usually all they need. Watering is more efficient if you have a basin or watering well beneath each plant and you flood it with water, allow to drain, then flood it a second time. This concentrates the water over the root mass and allows it to trickle down, wetting the entire root mass. Once a week is all it should need. In the meantime, prune away the dead parts of the plant and watch for new growth - new growth will indicate the roots are established and the plants are ready to make up for lost time.
Best wishes with your viburnums!
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