Depending on what variety of dogwood, the lilacs may eventually swallow them up
as they grow bigger. I would highly reccommend you plant the lilacs in a row
and then continue the dogwoods after you run out of lilacs. Space the lilacs
at least 5 or 6 feet apart for better air circulation, and the dogwoods about 4
or 5 feet apart. Consider the idea mentioned above, alternating them in the
hedge will look obsurd.