Best grown in rich, medium moisture, slightly acidic to neutral, well-drained soils in part shade. Tolerates full sun only if grown with consistently moist soils. Soil pH affects the flower color (blue in highly acidic soils and lilac to pink in slightly acidic to alkaline soils) of many big leaf hydrangea varieties. Add aluminum sulfate to the soil to make the flowers bluer or add lime to the soil to make the flowers pinker. Begin soil treatments well in advance of flowering, as in late autumn or early spring. Bloom occurs on old wood. Prune after flowering by cutting back flowering stems to a pair of healthy buds. Prune out weak or winter-damaged stems in early spring. Winter hardy to USDA Zone 6. Can be grown in USDA Zone 5 with protection (e.g., mulch and burlap wrap), but 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.
Big leaf hydrangea (H. microphylla) is a deciduous shrub with a rounded habit that, in the St. Louis area, typically grows 3-6’ tall and as wide unless damaged by harsh winters or pruned smaller. It generally features dark green, serrate, obovate to elliptic leaves (4-8” long) and large clusters of long-blooming summer flowers. Flowers appear in two types or groups: (1) LACECAP GROUP which features flattened flower clusters (corymbs) of small fertile florets with scattered showy sterile florets often forming a marginal ring, and (2) MOPHEAD OR HORTENSIA GROUP which features rounded, globose, mophead-like clusters (corymbs) of mostly showy sterile florets. ‘Merritt’s Supreme’ is a mophead-like cultivar that typically grow 3-4’ tall and as wide. It features rose to red sterile florets in huge spherical clusters. Flowers blooms in early July for up to two months.
No serious insect or disease problems. Some susceptibility to bud blight, bacterial wilt, leaf spot and mildew. Aphids are occasional visitors. Notwithstanding its unique flowering buds, winter hardiness can still be a problem in the St. Louis area.
Group or mass in a sheltered location in the shrub border. Also a good specimen or accent for protected locations near homes or patios.