Tardiva because it flowers later in the season than the species; it's tardy. This is a rangy coarse plant that's nonetheless a great addition to the late season garden. The sterile florets on the acutely conical paniculate inflorescences (sorry about that, but it's accurate) begin white and are gradually suffused by rose pink. I remember growing a cultivar called 'Pink Diamonds', but the generic 'Tardiva' gets as much color as the plant really needs.
This would be a great plant for nurseries if more people shopped for plants in the late summer or fall. It grows like the wind, flowers on new growth, tolerates much abuse in its watering, and isn't especially susceptible to pests or diseases. Superplant. For a few years my role at Behnke's included technical oversight of the woody plant production facilities. They had a pretty good idea what they were doing, but I enjoyed walking around and looking at the plants. I always marveled at the quantity and quality of Hydrangea paniculata produced including 'Tardive'.
We had our own issues this year with this particular plant. (You can see it halfway down the large stairway across the road from the huge weeping Katsura) It had gotten a bit out of control and we cut it back hard and removed a large number of root suckers. This treatment normally induces an explosion of watersprouts that grow 3-4 feet, produce large panicles of weighty flowers, that cause the branches to sag giving the whole plant a strange appearance. We wanted to avoid this so Amanda nipped the most vigorous shoots several times inducing growth in many lateral buds, and so dispersing the repressed energy of the plant into so many shoots that none of them grew to more than a foot or so. It turned out to have been a good strategy rigorously enforced by Amanda and the plant is spectacular right now.