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Hydrangea serrata

Mountain hydrangea

Hydrangeaceae

Mountain woodlands of Japan on Honshu, Shikoku, and Kyushu, and the southern Korean peninsula

Attracts Pollinators Attracts Butterflies Container Friendly

SunPart Shade – Full Shade

WaterMedium

HardinessZones 6–9

Overview

Hydrangea serrata is a deciduous shrub reaching 2-4 feet (0.6-1.2 m) tall and wide, with slender stems and a mounded outline. Leaves are opposite, narrowly ovate to lanceolate, 2-5 inches (5-12 cm) long, thinner in texture and more sharply pointed than those of H. macrophylla, and turn red to purple before leaf drop. Lacecap heads 2-4 inches (5-10 cm) across open from June to August, with a flat centre of fertile flowers about 0.15 inch (4 mm) wide ringed by four to eight sterile ray florets 0.6-1.2 inches (1.5-3 cm) across. Sepal colour shifts with soil chemistry: blue below pH 5.5 where aluminium is available to the roots, rose to red above pH 6.5, and purple between the two. Flower buds form on the previous season wood, so a winter low near -10 F (-23 C) or a hard April frost removes the season bloom while the plant itself survives. Leaves of H. serrata var. thunbergii spp. contain phyllodulcin and are dried in Japan for amacha, a sweet tea. Foliage and buds contain cyanogenic glycosides and cause vomiting in dogs, cats, and people. Hardy in USDA zones 6-9.

Native Range

Hydrangea serrata is native to the mountain woodlands of Japan on Honshu, Shikoku, and Kyushu, and to the southern Korean peninsula. Populations occupy stream banks, ravine slopes, and forest understorey between 650 and 5,000 feet (200-1,500 m). Overlapping morphology has led some botanists to treat the plant as H. macrophylla subsp. serrata spp..

Suggested Uses

Planted in shaded borders, along woodland paths, and at 30-36 inch (75-90 cm) spacing as a low informal hedge. Container culture works in 7 gallons (26 L) or more, with the pot moved to an unheated shed or garage over winter in zone 6. Cut stems are used fresh in summer arrangements and hold their sepals when air-dried from the green stage.

How to Identify

Leaf blades are 2-5 inches (5-12 cm) long, matte, and taper to a drawn-out point, against the thick, glossy, abruptly pointed leaves of H. macrophylla. Stems are 0.1-0.2 inch (3-5 mm) thick and flexible, roughly half the diameter of H. macrophylla stems at the same age. Inflorescences are flat lacecaps 2-4 inches (5-10 cm) across in all forms of the wild species, with sterile florets held on stalks at the rim. Winter buds are narrow and pointed, seated on the previous season growth.

Appearance

Size & Dimensions

Height2' - 4'

Width/Spread2' - 4'

Reaches mature size in approximately 5 years

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

red

purple

Bloom Information

Bloom Period

~8 weeks

Summer

Heads open from mid-June to early July and hold colour into August, running six to nine weeks. Sterile florets keep their sepals after the fertile flowers finish, then turn green and dry on the plant. Buds set from August of the previous year, so bloom is lost where winter minima drop below -10 F (-23 C) or where late frosts strike swelling buds in April. Repeat-flowering selections carry a second, lighter flush on current-season shoots from late August.

Detailed Descriptions

Flower Description

blue below pH 5.5, rose to red above pH 6.5, purple at intermediate pH

Foliage Description

matte medium green, turning red to purple before leaf drop

Growing Conditions

Sun Requirements

SunPartial Shade SunFull Shade

Requires 3-6 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.0 - 7.0(Neutral)

357912

Soil Types

loam silt clay

Drainagemoist

Water & Climate

Water Needs

Medium

Frost Tolerance

hardy

Time to Maturity

3-5 years

Care & Maintenance

Care Guide

Three to six hours of morning sun with shade after midday suits the species; leaves scorch at the margins where afternoon sun combines with temperatures above 85 F (29 C). Soil is moisture-retentive loam high in organic matter at pH 5.0-7.0, and 2-3 inches (5-8 cm) of mulch holds root-zone moisture through summer. Water use runs to about 1 inch (2.5 cm) per week, with visible wilting inside a day of the surface drying in hot weather. Sepal colour is shifted toward blue with aluminium sulphate at 1 tablespoon per gallon (15 ml per 3.8 L) monthly, or toward pink with garden lime. Straw or hessian laid over the crown through winter holds flower buds alive in zone 6. Powdery mildew, Cercospora spp. leaf spot, and two-spotted spider mite occur in still air and dry heat.

Pruning

Cutting is done within four weeks of flowering, since buds for the next season form on the same wood from August. Dead stems are taken back to live tissue in April, once bud break shows which wood came through winter. Removal of one quarter of the oldest stems at ground level each year holds the framework young. Cutting in late winter removes the coming bloom on all but the repeat-flowering selections.

Pruning Schedule

summer