Oak Leaf Hydrangea (Hydrangea quercifolia)

Oak Leaf Hydrangea, Oakleaf Hydrangea

One of the most commonly grown garden shrubs, hydrangeas are beloved for their large showy flowers and attractive foliage. Oakleaf hydrangea, named for its deeply lobed oak-like leaves, has the added benefit of four-season appeal. The large dramatic foliage unfurls in spring, followed by cone-shaped panicles that bloom for weeks beginning in early summer. The brilliantly colored fall leaves drop off the plant to reveal attractive cinnamon-colored bark, lending winter interest to the landscape.

I. Appearance and Characteristics 

Hydrangea quercifolia, commonly known as oakleaf hydrangea or oak-leaved hydrangea, is a species of flowering plant in the family Hydrangeaceae. It is native to the southeastern United States, in woodland habitats from North Carolina west to Tennessee, and south to Florida and Louisiana. A deciduous shrub with white showy flower heads, it is grown as a garden plant, with numerous cultivars available commercially.

Its specific epithet combines the Latin words quercus (“oak”), and folium (“leaf”). However, it is not closely related to oak species (Quercus).

Hydrangea quercifolia is a coarse-textured deciduous shrub growing to 3–12 feet (0.91–3.66 m) tall with an open crown. The plant sprouts shoots from underground stolons and often grows in colonies. Young stems are covered in a felt-like light brown bark, and the larger stems have attractive cinnamon-tan-orange bark that shreds and peels in thin flakes.

The leaves are yellowish green to dark green on top and silvery-white underneath. They have three, five or seven pointed lobes and are 4–12 in (10.2–30.5 cm) long and almost as wide. They vaguely resemble larger versions of oak leaves, similar to Quercus species with lobed foliage. Plants in the shade have larger leaves than those grown in sun. The leaves turn rich shades of red, bronze and purple in autumn that persist in winter accompanying the persistent dried flower-heads.

Flowers

Hydrangea quercifolia flowers are borne in erect panicles 6–12 in (15.2–30.5 cm) tall and 3–5 in (7.6 –12.7 cm) wide at branch tips. Flowers age in colour from creamy white, aging to pink and by autumn and winter are a dry, papery rusty-brown.

Unlike bigleaf hydrangea (Hydrangea macrophylla), flower color does not vary with soil pH. Flower color does, however, vary throughout each growing season. The flowers are a pale green as they emerge and open to a bright white, aging to either pink or brown (depending on the cultivar/seedling).

Hydrangea quercifolia and Hydrangea paniculata are the only hydrangeas with cone-shaped flower clusters (i.e. panicles); all the others have their flowers in ball-shaped or flat-topped clusters, called umbels.

Like many other Hydrangea species, oakleaf hydrangea inflorescences are composed of two flower types; showy flowers with enlarged petal-like sepals and inconspicuous flowers with non-showy petals and sepals. The showy flowers, whose function is to attract pollinators, typically obscure the inconspicuous flowers to some degree.

Although faint, Hydrangea quercifolia flowers have a pleasing scent. The floral scent in oakleaf hydrangea is stronger than that of the other commonly cultivated Hydrangea species. Presumably the function of the floral scent is to attract pollinators, although this has not yet been studied. The fragrance can be described as being most similar to Syringa reticulata (Japanese tree lilac), although not as strong.

Fruit

The fruit of Hydrangea quercifolia are dry dehiscent capsules which are green following pollination and ripen to brown as they mature. The stigmas remain on the fruit and dry to form horn-shaped appendages atop the capsule with an opening between them to allow the seeds to disperse.

Seeds

Hydrangea quercifolia seeds are quite small, approximately 0.6 mm long (and with a thousand seed weight of 0.0356 g) allowing several to develop within each capsule. Seed color varies from light tan to dark brown with longitudinal striations. Seed shape is ellipsoidal to ovate. Seed storage behavior is considered to be orthodox in nature, allowing them to store for relatively long periods of time at cold temperatures. The seeds typically germinate readily in 7-14 days as long as they are not allowed to dry out after sowing.

Distribution

Hydrangea quercifolia is native to the southeastern United States (Alabama, Florida, Georgia (w.), Louisiana (e.), Mississippi, North Carolina, South Carolina, Tennessee), but naturalized in other parts of the US and widely cultivated elsewhere.

In nature, Hydrangea quercifolia is found in a variety of habitats but almost always on well drained, shady slopes. These include riverbanks, bluffs and rocky outcroppings. The tendency of the species to grow on calcareous soils or cliffs as well as deep alluvial soil suggests it has a wide tolerance of soil substrates as long as they are well drained.

II. How to Grow and Care

Sunlight

In their native habitat, oakleaf hydrangeas are understory plants, so they appreciate afternoon shade, especially in southern climates where nearly full shade may be necessary. In the North, oakleaf hydrangeas can get by with full sun. Too much shade may reduce the intensity of the fall color.

Temperature and Humidity

Oakleaf hydrangea generally does well in climate conditions throughout its hardiness range (zones 5 to 9), but winter damage to flower buds may occur in the colder part of the range, especially with young shrubs.

Watering

These plants appreciate moist soil, and the more sunlight they receive, the more water they need. Blanketing the ground beneath the shrub with a thick layer of mulch will help maintain soil moisture.

Soil

Grow oakleaf hydrangea in well-drained soil with a slightly acidic soil pH (5.0-6.5), amended with plenty of compost.

Fertilizing

This plant generally requires no feeding, especially if you are mulching over the root zone. When growing in alkaline soils, occasional feeding with an acid fertilizer may be beneficial.

Planting Instructions

Loosen soil in the planting area and amend with compost or other rich organic matter. Dig a hole 2 to 3 times wider and slightly shallower than the root ball. Remove the plant from the nursery pot and loosen the roots if potbound. Place in the hole so the top of the root ball is at or slightly above soil level. Fill in the hole with soil, tamp down gently to remove air pockets, and water thoroughly. Provide regular water until plants are established.

Pruning

Oakleaf hydrangea shrubs tend to spread out and often do so unevenly with branching in all directions. Annual pruning or trimming back unruly branching every two years will keep your hydrangea shapely and healthy. In ideal conditions, this plant may shoot up to 10 feet, which may require that you trim it down. Since it blooms on old wood, wait to prune until after flowering when blooms start to fade, usually late summer into September or even October. Damaged or diseased branches can be cut away whenever they appear.

This plant spreads through suckering, so you may want to remove spreading ground shoots to control the spread of the shrub.

Propagation

Oakleaf hydrangea shrubs can be successfully propagated through stem cuttings. Follow these propagating steps:

  • With clean, sharp pruners, snip two to four cuttings from green stem tips that have no flowers. Make sure the cuttings have at least one growth node (a knobby line across the stem).
  • Remove the leaves from the lower half of the cutting.
  • Dip the end of the cutting into rooting hormone.
  • Plant the cuttings in moist, sterile potting mix.
  • Place the planted cuttings in a plastic bag and keep them out of direct sunlight.
  • Make sure the soil remains moist but not soggy.
  • Within about 12 weeks, root growth should be sufficient to transplant the plant into the garden or a larger pot for continued growth. 

Overwintering

At the northern end of its hardiness range (zone 5), young plants should be given some winter protection, such as a burlap wrap.

Mulching

Each spring, apply compost, shredded bark or other organic matter around the base of plants. For winter protection in colder climates, mound 6-8 inches of mulch around the base after the ground freezes in fall. Pull back the mulch in early spring to allow the soil to warm up.

Pests and Diseases

There are several notable diseases that infect Hydrangea quercifolia. Root rot diseases can be fatal while foliar diseases tend to be only cosmetic. Root rot diseases include Phytophthora, Pythium and Fusarium, which are all fungal diseases. Management techniques to decrease incidence of root rot include proper watering (allowing the soil to dry between waterings), removal of diseased plant parts, and fungicides if needed.

III. How to Get Oakleaf Hydrangea to Bloom

Failure to bloom can have several reasons. The shrub might not get sufficient sunlight. While hydrangeas prefer dappled shade, they do need sun to bloom. Lack of water might be another reason. Keep the soil moist and add a generous layer of mulch to prevent the soil from drying out.

Avoid fertilizers high in nitrogen, which results in lush foliage but no flowers. Switch to a bloom-boosting fertilizer that is high in phosphorus (P).

Be careful when pruning to avoid removing flower buds which can also be killed by a late freeze in Spring.

If you tried all of those options and your hydrangea still does not bloom, it might be simply too young. Continue to give it the proper care and keep your fingers crossed that you will see it bloom next year.

IV. Uses and Benefits 

  • Effective as a specimen or accent for foundations or other locations near homes or patios. 
  • Group or mass in shrub borders or in open woodland areas. Good informal hedge.
  • Exfoliating mature branches provide interesting color and texture in winter.

Oak Leaf Hydrangea (Hydrangea quercifolia) Details

Common name Oak Leaf Hydrangea, Oakleaf Hydrangea
Botanical name Hydrangea quercifolia
Plant type Perennial
Hardiness zone 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b, 9a, 9b
Growth rate Slow
Harvest time Fall
Height 4 ft. 0 in. - 8 ft. 0 in.
Width 4 ft. 0 in. - 8 ft. 0 in.
Sunlight Full sun (6 or more hours of direct sunlight a day)
Soil condition High Organic Matter
Flower color Cream/Tan
Leaf color Green