Culture
Best grown in rich, evenly moist, well-drained soils in part shade. Tolerates full sun only if grown with consistently moist soils. Soil pH generally affects the flower color for this shrub, namely, blue in highly acidic soils (pH below 5.5) and pink to purple pink in alkaline soils (pH above 7.0). Add aluminum sulfate to the soil to make the flowers bluer or add lime to the soil to make the flowers pinker. Soil treatments should be commenced well in advance of flowering. Flowers bloom on old wood. Prune after flowering by cutting back the flowering stems to a pair of healthy buds. Prune out weak or winter-damaged stems in early spring. Winter hardy to USDA Zone 6. If grown in the St. Louis area, shrubs should be sited in sheltered areas with protection from cold temperatures and brisk winter winds (e.g., mulch and burlap wrap). Even with protection, shrubs may lose significant numbers of flower buds or die to the ground in harsh winters, thus respectively impairing or totally destroying the bloom for the coming year.
Noteworthy Characteristics
Hydrangea sargentiana, knows as Sargent's hydrangea, is an upright, rounded, deciduous shrub that typically grows 3-6’ tall and as wide unless damaged by harsh winters or pruned smaller. Strigose brown branches are clad with opposite, heavily pubescent, ovate, dark green leaves (lower leaves to 10" long with upper leaves decreasing in size to 4" long). Branch bark peels with age. Flattened, lacecap-type flower clusters (to 8" diameter) bloom for two months in late summer (July to early September). Each cluster features small, blue to purple-violet, fertile flowers surrounded by white ray flowers. Fertile flower color becomes pinker as the soil pH becomes alkaline. This hydrangea is primarily native to dense forests (valleys and slopes) in central China (Hubei). It was introduced from China in 1908 by E. H. Wilson (1876-1930) from a plant collecting trip undertaken for the Arnold Arboretum.
The genus name Hydrangea comes from hydor meaning "water" and aggeion meaning "vessel", in reference to the cup-like capsular fruit.
Specific epithet honors Charles Sprague Sargent (1841-1927) who was a leading American dendrologist and first director of the Arnold Arboretum.
Problems
No serious insect or disease problems. Some susceptibility to bud blight, bacterial wilt, leaf spot and mildew. Aphids are occasional visitors.
Uses
In the northern edge of its growing range, it is best grouped or massed in sheltered locations in the shrub border. Also a good specimen or accent for protected locations near homes or patios. Hedge. Attractive foliage and summer flowers.