Bigleaf hydrangea

Identification and origin

Hydrangea macrophylla belongs to the family Hydrangeaceae. In the wild it grows in Japan, including the Volcano Islands south of the main archipelago. Its species name comes from Greek and means “large leaf.”

It is a fast-growing deciduous shrub that forms a rounded clump of many stems. It usually reaches 3–6 ft tall and as wide. Its broad leaves drop in fall.

Flowers appear from late spring into summer on stems formed the previous year. There are two main groups. Mopheads carry round heads made up almost entirely of large, showy, sterile flowers. Lacecaps have flat clusters: tiny fertile flowers in the center, ringed by large sterile ones.

Color depends on the soil. In acidic soil, at a pH of about 4.5 to 5.5, aluminum is available and the flowers turn blue; in slightly acidic to neutral soil they shift to mauve; in alkaline soil they are pink. White, red and green varieties do not change.

Common cultivars include ‘Endless Summer’ and ‘Penny Mac’, which also rebloom on new wood; ‘All Summer Beauty’, a rebloomer; ‘Nikko Blue’; ‘Blue Wave’, a blue-to-pink lacecap; ‘Ayesha’, with lightly fragrant, spoon-shaped petals; ‘Pia’, a dwarf with rose-pink flowers; and ‘Lemon Daddy’, with chartreuse foliage. Compact ‘Red Hot Violet’ and ‘Altona’ suit containers.

Light

In the garden it prefers part shade or dappled light: ideally morning sun and afternoon shade. Avoid hot, exposed sites. It tolerates a sunnier position if the soil stays moist all summer.

A blooming gift hydrangea indoors belongs in a bright spot out of direct sun, away from blasts of hot or cold air.

Watering

It needs soil that stays consistently moist: as soon as the surface begins to dry, water deeply. In hot weather, and even more in sandy soil, its needs rise sharply. Water a young plant whenever a week to ten days pass without rain.

In containers, check moisture often and move the pot into shade in summer. For a blooming indoor hydrangea, keep the root ball evenly moist, water as soon as the surface feels dry to the touch and never let water collect in the decorative foil or pot cover.

Water blue-flowered varieties with rainwater to keep their color.

Potting mix

It likes moist, well-drained soil rich in organic matter, and it adapts to clay, sandy or loamy ground. For containers, mix two parts peat-free soil-based compost with one part peat-free multipurpose compost; to get blue flowers where water and soil are alkaline, use an acidic mix made for azaleas and other acid-loving plants.

To turn flowers blue, add aluminum sulfate products or sulfur to the soil as new growth emerges; this does not work on very chalky soil. For clearer pinks, spread ground limestone in winter.

An established plant usually needs no fertilizer. Too much fertilizer favors leaves over flowers and makes shoots more prone to frost damage. On poor, sandy soil, a balanced spring fertilizer helps a struggling plant.

Temperature

In the ground it is grown in USDA zones 6a to 11b. Young flower buds are vulnerable to late spring frosts. Florist varieties are often less cold-hardy.

In containers, shelter it in winter in a cold greenhouse or against a sunny, protected wall; wrapping the pot insulates too little, so raise it on pot feet instead to avoid waterlogging. If you bring one indoors, give it a cool rest between 36–61 °F for about six weeks, without letting the root ball freeze or dry out: this chill is needed for flowering. Then move it to a sunny room with nights of 55–61 °F. In bloom indoors, it lasts longer somewhere cool, around 61–64 °F.

Repotting

Plant in spring or fall; a container-grown plant can go in the ground at any time as long as the soil is neither frozen nor very dry. For containers, choose a wide pot with drainage holes and the mix described above.

To get a gift hydrangea to bloom again, respect the chilling period described above.

Pruning

It blooms on the previous year’s wood: hard or late pruning removes the flower buds.

On mopheads, leave the faded flowers on all winter: they protect the buds below from frost. In mid-spring, cut each spent stem back to the first or second strong pair of buds. On lacecaps, which are hardier, remove faded flowers right after blooming, just above the second pair of leaves. Never prune after midsummer.

To renew the clump, cut one or two of the oldest stems to the base each year. Cutting everything to the ground rejuvenates a neglected plant, but there will be no flowers the following year. Dead stems, shortened to 12 in – 2 ft, can be left standing for the native bees that nest in them.

Propagation

Cuttings are easy. Softwood cuttings, taken from the soft, flexible young shoots of the current season from late spring to midsummer, root readily. Semi-ripe cuttings in midsummer and hardwood cuttings in winter also work. Expect two to three years before the first flowers.

To layer, bend a flexible stem into a shallow trench, remove a ring of bark on the buried section and cover it with soil. Old clumps can be divided in early spring.

Common problems

• No flowers: pruning too hard or too late, buds killed by frost, too much fertilizer, or a florist cultivar that is not hardy enough.
• Pale or yellowing leaves: drought stress (mulch the soil) or a nutrient deficiency on sandy soil.
• Spots or white coating on leaves: Cercospora leaf spot, anthracnose, powdery mildew; bacterial wilt also occurs.
• Flat, white, waxy blobs on stems: hydrangea scale, most visible in early summer.
• Other pests: aphids, hydrangea leaftiers, rose chafers; deer browse the foliage.
• Damaged shoots in spring: late frost.

A single symptom can have several causes: look at the soil, exposure, recent pruning and weather before drawing conclusions.

Toxicity

Bigleaf hydrangea is toxic to dogs, cats and horses. Its leaves, flowers and bark contain cyanogenic glycosides; in animals, eating it mostly causes vomiting, diarrhea and depression, while cyanide poisoning is rare. For people the toxicity is low: eating a large amount can cause nausea, stomach pain, vomiting and sweating. Contact with the foliage can aggravate some skin allergies, so wear gloves. If ingestion occurs, contact a veterinarian or a poison control center.