There is a short length of time every summer during which it seems that almost anything will start to grow. Take a cutting and stick it in a pot, and it forms roots.
Hydrangeas are easy. It's usually how most of mine are started. Cuttings from bedding plants seem to take during this time. All of the semi hardwood shrubs seem to love this time of year. Vines are great ones to try also. During other seasons, plants need more encouragement to grow, and some rooting hormone. A little luck, and some background knowledge.
Honestly I don't know why this works out so well, but almost anything that you stick into soil seems to grow during this short period of time, maybe the moon is in it 4th house, or Jupiter is rising, maybe its just the longer days. There probably is some technical gardening explanation, but it doesn't matter to me.
I do know that there is nothing more rewarding to a gardener than seeing new growth coming from a plant, and right now the possibilities are endless. So I am on my way to go out and buy some more pots, and make more space.