This is Gomphocarpus fruticosus, also known as Swan Plant Milkweed, and Tennis Ball Bush. A common synonym for this plant is Asclepias fruiticosa. This plant grows up to 6 feet high and is very popular with the birds, bees, and butterflies. It likes to live in sun to partial shade where it blooms from late Spring thru the middle of the Summer. This is a larval host plant for the Monarch butterfly. This plant can be started early, inside the house, and grown as an annual, but it is a tender perennial in USDA Hardiness Zones 10 to 11.