Oakleaf hydrangea

Identification and origin

Hydrangea quercifolia belongs to the family Hydrangeaceae. It is native to the southeastern United States, from South Carolina, Georgia, and Florida west to Alabama, Mississippi, Louisiana, and Tennessee. In the wild it grows in moist woods, along stream banks, in ravines, and on bluffs. The epithet quercifolia means “oak-leaved,” and the genus name comes from Greek words for “water” and “vessel,” a reference to its cup-shaped fruits.

It is an upright, multi-stemmed, suckering deciduous shrub that forms a rounded clump. It usually reaches 4–8 ft tall and spreads 4–10 ft. Its large opposite leaves, leathery and dark green, have three to seven toothed lobes and are often more than 6 in long; fuzzy when young, they keep a whitish to brownish felt on the underside. In fall they turn wine red, orange, purple, and mahogany.

Flowers appear from late spring into summer in upright, pyramid-shaped panicles 4–12 in long. The large, showy sterile flowers sit mostly toward the base of the cluster; the smaller, creamy, fragrant fertile flowers fill the tip. White at first, they age to pink and then tan, and they hold on the shrub for a long time. The fruits are small brown capsules that persist into winter; songbirds eat the seeds, and butterflies visit the flowers. On older branches the brown bark peels in papery strips to reveal cinnamon-colored bark beneath, a fine winter feature.

Common cultivars include ‘Snow Queen’, with large white panicles that stay upright; ‘Snowflake’, with double flowers in panicles heavy enough to bend the stems; ‘Harmony’, with heavy clusters of sterile flowers; and compact forms such as ‘Pee Wee’, ‘Sikes Dwarf’, ‘Ruby Slippers’, and ‘Munchkin’. ‘Little Honey’ has golden foliage.

Light

It grows in full sun or part shade, but it needs some direct sun to bloom well. The sunnier the spot, the more reliably moist the soil must stay: in full sun, plant it only where the ground does not dry out. Avoid hot, dry, exposed sites. Shade that is too dense reduces flowering.

Watering

It prefers soil that stays moderately and evenly moist. Its big leaves lose water quickly, so watch them in hot weather: foliage that droops or browns at the edges is a sign of drought. Then check the soil a couple of inches down and water deeply at the base as soon as it is dry, rather than giving a little water often.

Water at the base when you can, with drip irrigation or a soaker hose, to keep the foliage dry; if you use a sprinkler, run it early in the morning so the leaves dry quickly. Once well established, the shrub tolerates moderate dry spells.

Potting mix

The ideal soil is fertile, rich in organic matter, moist, and well drained, on the acidic to neutral side. The shrub adapts to clay, loam, or sand, and even to slightly alkaline soil or ground that is occasionally wet. Chalky soil benefits from added organic matter. It has poor salt tolerance. A mulch layer 2–3 in deep keeps the soil moist in summer and moderates its temperature.

Temperature

It is hardy and grown in USDA zones 5a to 9b. In the northern part of that range, plant it out of strong winds and protect young plants in winter with a thick mulch or a burlap wrap. In harsh winters, below about -9 °F, it can lose many of its flower buds or even die back to the ground, so the next bloom is reduced or lost entirely. At the northern edge of its growing range, the top often dies back over winter.

Repotting

It transplants easily. Spring and fall are the best seasons to plant it; avoid midsummer, when it establishes less readily. Dig a hole two to three times as wide as the root ball but no deeper, backfill with the native soil, water thoroughly, then mulch. Allow 3–5 ft between plants for an informal hedge.

Pruning

It blooms on the previous year’s wood, and that is the rule to remember. If pruning is needed, do it right after flowering, so the shrub has time to set next year’s buds. Pruning in late winter or spring removes the coming flowers. In general it needs very little pruning.

In early spring, simply remove stems damaged by winter cold. For a fuller shrub, pinch the new growth or shorten a few old stems after bloom. Use clean, sharp bypass pruners, disinfected between cuts.

Propagation

The simplest method is to separate the suckers that come up at the base of the shrub, with their roots. Softwood cuttings also work.

Layering is very effective: dig a shallow trench near the plant, bend a flexible branch into it, remove a ring of bark about 1 in wide on the buried section, then cover it with soil, leaving 6–12 in of the tip exposed. Old clumps can be divided with a spade in early spring. Seed is another option.

Common problems

• No flowers: buds killed by winter cold (the most common cause), pruning at the wrong time, too much shade, poor soil, or excess nitrogen fertilizer.
• Drooping leaves, brown edges: lack of water, heat, or a site that is too hot and dry.
• Leaf spots, white powdery coating: leaf spot disease or powdery mildew; rake up fallen leaves and keep the foliage dry.
• Flowers or shoots covered in gray fuzz: gray mold (botrytis); improve air flow and keep the foliage dry.
• Pests: aphids on young shoots, spider mites, and less often scale insects, plant bugs, or black vine weevils.

The same symptom can have several causes: consider the past winter’s weather, when you last pruned, the exposure, and the soil moisture before drawing conclusions.

Toxicity

The bark, leaves, and flowers are mildly toxic if eaten, both to people and to dogs, cats, and horses. Hydrangeas contain cyanogenic glycosides; in people, ingestion can cause nausea, stomach pain, vomiting, and sweating, and in animals vomiting, depression, and diarrhea. Serious cyanide poisoning is rare. The plant does not cause contact dermatitis. If someone eats part of the plant, call a doctor, a poison control center, or a veterinarian.