Smooth hydrangea (*Hydrangea arborescens*) is a deciduous shrub, naturally found on streambanks, gorges, and wooden slopes in many parts of Northern America. It is commonly recognized by its showy inflorescences – big white corymbs which bloom during summer. Smooth hydrangea is often cultivated as an ornamental, and it is an excellent choice for a border plant.
I. Appearance and Characteristics
Hydrangea arborescens, commonly known as smooth hydrangea, wild hydrangea, sevenbark, or in some cases, sheep flower, is a species of flowering plant in the family Hydrangeaceae. It is a small- to medium-sized, multi-stemmed, deciduous shrub up to 2 m (7 ft) tall that is native to the eastern United States.
The inflorescence is a corymb up to 15 centimetres (5.9 in) wide. Showy, sterile flowers are usually absent or if present they are usually less than 1 cm in diameter on the edge of the panicles. Flowering occurs May to July. Fruit is a ribbed, brown capsule about 2 mm long. Many are produced in October and persist through the winter.
The leaves are large (8 to 18 cm long), opposite, serrated, ovate, and deciduous. The lower leaf surface is glabrous or with inconspicuous fine hairs, appearing green; trichomes of the lower surface are restricted to the midrib and major veins.
The stem bark has a peculiar tendency to peel off in several successive thin layers with different colors, hence the common name “sevenbark”.
Smooth hydrangea can spread rapidly by stolons to form colonies.
II. How to Grow and Care
Sunlight
The native environment of smooth hydrangea is under sparse forest, so it is adaptable to partial shade. Too little sunlight will reduce the flower number or even produce no flowers, and too much sunlight may cause sunburn to flowers. In gardens, the best place to plant smooth hydrangea is a place where it can fully receive the morning sunlight while being partially shaded in the afternoon. The colder the climate, the longer sunlight exposure is needed. In the coldest hardiness zone, it requires at least 6 hours of sunlight a day. In the warm areas where it is best adapted, 3 hours of sunlight a day is enough.
Temperature and Humidity
Smooth hydrangeas are one of the most cold-hardy hydrangea species and don’t generally need additional winter protection. However, they can die back when winters are particularly cold. The ideal daytime temperatures are around 70 degrees Fahrenheit during the growing season. They prefer humidity over dry areas and should be protected from winds.
Watering
Smooth hydrangea demands a very large amount of water. The recommended watering frequency is at least three times a week. Please note that the frequency should be appropriately increased in dry areas or during hot summers. The soil should always be kept moist, but free of accumulated water. The watering frequency may be adjusted in different seasons according to this criterion. Furthermore, water should be quickly supplemented if smooth hydrangea leaves begin to wither.
Soil
Smooth hydrangea requires soil that is rich in humus and well-drained. It likes moist soil but is not resistant to waterlogging. If the soil is heavy, coarse sand or organic fertilizer can be added to improve draining and permeability. This can also increase the organic matter content. If the soil easily dries out, cover it with organic mulch to keep it moist while maintaining ventilation.
The soil pH will affect flower colors of some varieties of bigleaf hydrangea. Smooth hydrangea growing in acidic and aluminum-rich soils will bloom in blue, while in alkaline soils they bloom in pink. An appropriate amount of aluminum sulfate or lime can be added to the soil to adjust the pH if you want to control the flower color.
Fertilizing
Smooth hydrangeas do well being fed a balanced, all-purpose fertilizer. If you want to increase the number of blooms and their size, select a feed higher in phosphorous, such as one labelled “bloom boost” or with a balance of 10-20-20. Spring feeding is particularly important. Too much fertilizer, however, can burn or yellow the foliage, and too much nitrogen encourages foliage growth rather than blooms.
Unlike Big Leaf hydrangeas, the bloom color of the smooth hydrangea cannot be manipulated by raising or lowering soil PH.
Planting Instructions
The ideal planting time for smooth hydrangea is early spring or late autumn. A very high survival rate can be obtained by transplanting smooth hydrangea after it has gone completely dormant. Additionally, it can also be planted in late spring and early summer, but make sure the soil is always moist and well-draining. During the initial stage of transplanting, pay attention to shading. Proper shading will reduce the evaporation rate and prevent sunburn.
For transplanting, dig a pit 61 cm wider than the diameter of, and as deep as, the plant’s root ball. Cover the pit with soil and pile up a small soil mound aboveground to help with water drainage. If planting in a flowerpot, make sure the diameter of the pot is at least twice as big as the root ball.
Pruning
Because they bloom on new wood, pruning smooth hydrangeas is easy. Plants can be pruned to the ground every winter or early spring to promote healthy new stem growth and uniform flower production. Alternatively, simply remove any weak or damaged stems.
Propagation
This fast-growing hydrangea suits propagation from cuttings—root or stem. To propagate softwood cuttings, follow these steps (as long as it’s not a trademarked cultivar):
- Take a 5-inch stem cutting and remove the lower leaves
- Dip the cutting in rooting hormone
- Use a sterile, well-drained potting medium such as damp sand or vermiculite and keep it moist
- Cover loosely with plastic, so it does not press against the leaves
- Keep in bright but indirect sunlight and water whenever the soil starts to dry
- Roots should develop within a few weeks
Overwintering
Smooth hydrangeas will go dormant in the winter. If grown outdoors, you’ll need to wrap and cover to protect the plant and root system. Hydrangeas grown in pots can be brought indoors or into a greenhouse during the cooler months. Water once a month to keep roots moist until the plant grows again in the spring.
Pests and Diseases
Common Pests & Plant Diseases
Smooth hydrangeas don’t have problems with any serious pests or diseases, but some common garden pests are attracted to the plants. Keep an eye out for things like aphids, Japanese beetles and spider mites.
The common fungal disease powdery mildew can also sometimes be an issue because the plant prefers higher humidity and moisture.
Common Growing Problems
There are a few common growing issues to be aware of when growing smooth hydrangea.
Yellowing Leaves
If you notice your smooth hydrangea has yellow leaves, it might be due to a nutrient deficiency (in particular a lack of chlorophyll production due to low iron). Fertilize with iron-enriched fertilizer and make sure the soil is kept acidic.
Drooping Leaves
A smooth hydrangea’s leaves can droop if the plant has too much moisture or too much sun exposure. Ensure optimum levels of each in order to keep your plant healthy and strong.
III. Uses and Benefits
- Ornamental uses
Smooth hydrangea is a flowering shrub that is sometimes grown ornamentally for its unusually shaped green-white flowers. This plant spreads rapidly and is, therefore, a good choice of ground cover plant. It also makes for a great flowering hedge and looks great in mixed borders. Azaleas grow well with hydrangeas since they bloom early, offering garden color before smooth hydrangea flowers.
It has additionally been marked as a pollinator plant, supporting and attracting bees and butterflies.
- Medicinal uses
Smooth hydrangea root was used medicinally by the Cherokee, and later, by early settlers for treatment of kidney and bladder stones.
IV. Harvesting and Storage
Smooth hydrangea inflorescences can be divided into two categories according to their shapes: Lacecap and Mophead. Both can be used as cut flowers. The inflorescences can be cut with sharp scissors as soon as they fully bloom. The best time to pick flowers is early morning, which helps avoid the effects of evaporation. The stem should be cut at a 45-degree angle and soaked in cool water as soon as possible to prevent moisture loss.
