One of my favorite landscaping plants is the Lilac Chaste Tree. We have one in our courtyard and it is in full bloom right now, covered with bright bluish-lavender "spiky" flowers.
It is a perennial and needs to be cut back during the winter months, or early spring. Here is the way ours looked around the first of April ...
The flowers grow in 6"-12" spikes, and our tree blooms two or three times during the summer months.
A word of caution: Bumblebees love this plant, and will even spend the night on the flowers. However, when I took these pictures, some of which were up close, they were too busy gathering pollen to bother me.
I took this last picture in 2008, before we put the stone bridge across the pond.
To learn more about the Lilac Chaste Tree, you can click on the link.