Lilacs like to spread and have space because good air circulation helps to prevent leaf blight and other fungal infections. Planting shrubs at least six feet apart will give them room to grow but there are also lilacs that survive planted much closer so that they grow into a hedge or thicket. A closely planted lilac hedge works best if the area is very windy and in full sun. Enjoy that wonderful lilac fragrance. Keep growing, Marianne Binetti