Your viburnum bushes are deciduous, which means they will drop their leaves each fall. Their growth rate should increase after the first year. Most shrubs and perennials will not grow significantly the first year they are planted, at least there won’t be much visible growth. Instead the growth will be concentrated underground, establishing and expanding the root ball. If you are looking for screening plants that will remain green all year you might consider using a few evergreens like arborvitae and juniper trees. Another possibility would be an ornamental grass screen. Many ornamental grasses will get 6-8 feet tall, especially in your zone 5 area. Be careful choosing your grasses, some are vigorous growers and may become invasive. Either grass or evergreens will provide you with the screening you want and help accent the viburnums already planted.