This is a very different Hydrangea from the H. macrophylla sorts that are commonly seen in gardens. It comes from China and is one of several similar (and confusing) Hydrangea, which are totally deciduous and almost tree-like in form (ht 2.5-3m), and which flower from late summer into autumn. It has slim, matt leaves and flattened lacecap blooms, which consist of tiny purplish-blue fertile flowers surrounded by large white or lilac-tinged sterile flowers. Bees love the flowers, the purplish-blue colour of which can be seen in the bees' pollen sacs as they buzz around the plant.
It has a more open form than other Hydrangea and seems more robust, coping far better with hot, dry spells than the H. macrophylla cultivars, with their large, lush leaves. It enjoys Sydney's climate but is also quite cold hardy. It can grow in sun or part shade, and needs no pruning beyond the removal of spent flowers at the end of its season. Companion plantings can include any of the many attractive for Plectranthus cultivars that are come into bloom around the same time, particularly those with purple flowers to echo the tints of this Hydrangea: such as P. 'Mona Lavender', P. 'Purple Dazzler' or purple P. ecklonii. Silvery P. argentatus is also pretty growing below the shrub.
The graceful Japanese windflowers (Anemone x hybrida) also come into bloom when the Hydrangea is still flowering and look delightful growing around it. The purple or white flower spikes of Liriope muscari can form an attractive groundcover at its base, as could a carpet of purple-leaved for Tradescantia pallida 'Purpurea'.
The plant sometimes sends out suckers and I have successfully dug these up to propagate it. My original plant came from the erstwhile Viburnum Gardens nursery in Arcadia more than 20 years ago, and I haven't ever seen it for sale anywhere else. I think it should be grown more often!