Culture
Best grown in rich, medium moisture, well-drained soils in part shade. Tolerates full sun only if grown in consistently moist soils. Soil pH affects the flower color of most cultivars except white (blue in highly acidic soils and lilac to pink in slightly acidic to alkaline soils). 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. Plants generally need little pruning. If needed, prune immediately after flowering by cutting back flowering stems to a pair of healthy buds. Prune out weak or winter-damaged stems in late winter/early spring. Best to mulch plants year-round with 3" of shredded bark, peat or compost. Winter hardy to USDA Zone 6. For added protection, however, plants grown in USDA Zone 5 should be sited in sheltered locations and given additional winter protection, as needed, for the purposes of minimizing the risk of loss of significant numbers of flower buds or possible die-back to the ground in an extremely harsh winter. A burlap wrap of stems or circle of chicken wire filled with leaves or straw to 8-12" are time-consuming and visually unattractive landscape options, but can be effective. Regardless of protective measures taken, most bigleaf hydrangeas simply will not bloom (or will bloom poorly) in some years because of a variety of winter occurrences beyond the control of the gardener (e.g. low temperatures, sudden wide temperature fluctuations, icy conditions, late frosts). Some newer cultivars have been selected that flower on both old and new wood. Their pruning and flowering can differ from that of the species.
Noteworthy Characteristics
Hydrangea macrophylla, commonly called big leaf hydrangea, 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 serrate, obovate to elliptic, dark green leaves (4-8” long) and large clusters of long-blooming summer flowers in either lacecap form (flattened flower clusters of small fertile florets with scattered showy sterile florets often forming a marginal ring) or mophead form (globose flower clusters of mostly showy sterile florets).
The genus name Hydrangea comes from hydor meaning "water" and aggeion meaning "vessel", in reference to the cup-like capsular fruit.
Specific epithet comes from the Greek words makros meaning large and phyllon meaning leaf in reference to plant leaves.
'Robert', commonly sold under the trade name of LET’S DANCE MOONLIGHT, is a mophead cultivar that is part of the Let's Dance Series of reblooming hydrangeas. It typically grows to a compact 2-3’ tall and as wide. It produces rich rose-pink or blue mopheads (depending on soil pH) on both old and new wood, resulting in a somewhat continuous bloom from July through September. Within its growing range, it will reliably bloom each year, regardless of the severity of winter temperatures. If buds on old wood (i.e., buds typically formed in August-October) do not survive winter, reblooming hydrangeas will still bloom in late summer, albeit less floriferously, on the new growth. LET'S DANCE MOONLIGHT has good winter hardiness. It may be grown in slightly acidic to alkaline soils (pH of 6 to 6.5 or more) for rose-pink flowers or in highly acidic soils (pH of 5.5 or less) for blue flowers. Foliage acquires good bronze-red color in fall. This plant is protected by patent number PP20020.
Problems
Some susceptibility to bud blight, bacterial wilt, leaf spot and mildew. Aphids are occasional visitors.
Uses
Group or mass in the shrub border. Also a good specimen or accent for foundations and other locations near homes or patios. Hedge. Containers.