From an article in the Telegraph on Annabelle hydrangea

The flat-headed mass of white flowers appears in June- July (about 2 months) but the green veining on the four "petals" (which are, in fact, long-lasting bracts) prevents 'Annabelle' from looking brash in summer sun.
Although there are no large sterile flowers to give that fragile lacecap look, 'Annabelle' has poise and balance, and makes a strong presence for more than six months....The cool white Hydrangea arborescens 'Annabelle' produces one of the most magical of all winter seed heads.
After the flowers have faded and fallen, a skeleton of fine tracery remains to catch the frost and low winter sunshine. So this is one hydrangea that deserves a prominent place in every garden, somewhere it can shine in three seasons - summer, autumn and winter.
If not using en masse...
Good companions
Mop-headed flowers such as hydrangeas and elders mix well with herbaceous plants. But all flat-topped flowers need spires to avoid the hummocky look. Rich blue aconitums go well, and the earlier flowering, prolific 'Spark's Variety' and the later 'Bressingham Spire' also make good partners.
The cool white flowers are an excellent foil for dark foliage and flowers. Set 'Annabelle' close to the rounded, wine-red leaves of Cotinus coggygria 'Royal Purple', accompanied by the powder blue Clematis 'Perle d'Azur' for summer dazzle.
Or use spiky dark dahlias, such as the brown-red 'Rip City', the rounder darker red 'Arabian Night', or the deeply purple 'Hillcrest Royal'. Colourful asters also look more vibrant against white 'Annabelle'.
'Annabelle' fades elegantly to shades of brown and green, and will blend well with late-season grasses, whether shorter forms of Miscanthus sinensis (such as 'Yakushima Dwarf' or 'Kleine Fontane') or fluffy pennisetums.
'Annabelle' also comes into leaf late, so an underplanting of early spring bulbs (miniature daffodils or small blue bulbs) will have room to shine before this hydrangea bursts into life and eclipses them.