How to Grow, Care, and Prune Smooth Hydrangea?
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Smooth hydrangea, or Hydrangea arborescens L., is a native hydrangea species in the United States, originally from Florida. Whenever I discuss hydrangea with my neighbors, I can't help but tell my neighbors about its many merits.
Smooth hydrangea is full of vitality; its stems and leaves can grow as long as there is space. Its flowers bloom on new wood and old wood. This plant can grow well in the southern gardens but is also tolerant of cold in the northern.
There are many kinds of smooth hydrangea, some with white flowers and some with red or pink flowers. Flower heads are also different shapes; some are like lacecap, and some are like mophead.
1. Features
- Habit
Smooth hydrangea is a deciduous shrub. It can grow in USDA hardiness zone 3-9. Therefore, even without the warmth of the south, smooth hydrangea can still grow and bloom in the cold USDA zone 3.
Unlike some hydrangea varieties, smooth hydrangea requires at least 6 hours of full sun. The stem will lack the toughness to support the flower head without enough sunlight. In USDA zone 9, we can give partial shade to protect it from the blazing afternoon sun during hot summers.
This plant grows 3 to 5 feet tall and wide. According to the research of Michael A. Dirr, it can grow up to 10 feet in the native range. Moreover, growth over 100 years of smooth hydrangea in the Midwest and New England is more common.
- Leaves
The colors of the leaves range from medium to dark green. The leaf length is 2-8 inches, and the width is 2-6 inches. The shape of the leaf is oval, ovate, or heart-shaped, with a serrated edge.
In fall, the leaves will change from green to yellow. The frost resistance temperature of both young and mature leaves can reach 20-25 ℉ (-4 ℃). The leaves will begin to fall off in November if it is in zones 7-8.
- Flowers
There are two kinds of flower heads in smooth hydrangea: lacecap and mophead.
The lacecap flower head is 3-6 inches in diameter and has a mild fragrance. 'Hass Halo' and 'Mary Nell' are all lacecap flower heads. These two types of hydrangea attract bees and butterflies.
On the other hand, flower heads of 'Incrediball,' 'Wee White,' and 'Annabelle' hydrangeas are ball-shaped mopheads with white petals. While they look beautiful, they do not attract pollinators.
Smooth hydrangea allows us to see the color of the flower change from immature to mature. Some petals begin to appear green, then turn white, return to green, and finally parchment-white. From late summer to early fall, the color of flowers changes from grayish brown to brown.
Moreover, the soil pH does not change the flower color. 'Invincibelle Wee White' flowers are always white, and 'Invincibelle Spirit II' is always pink.
Smooth hydrangea can bloom not only on the old wood grown last year but also on the new wood. Some varieties like to bloom on old wood, while others, like 'Mary Nell' and 'Wee White,' prefer to bloom on new stems.
2. Care
- Soil
The soil type in Florida is sandy, and although it is well-drained, it lacks nutrients. Before I prepare to grow smooth hydrangeas in my backyard, I will improve the soil first.
- Dig down the area where you prepare to plant hydrangea by 5 inches (13cm).
- Put in 3 inches of manure.
- Use a rotating tiller to mix manure and soil thoroughly.
If you grow hydrangeas in pots, you can buy potting mix or mix peat moss and perlite.
No matter where, smooth hydrangea enjoys soil that is well-drained, moist, and rich in organic matter.
- Mulch
I will spread 2 inches of mulch evenly on the soil surface. Even in the hot summer, mulch can help keep the soil cool, prevent excessive water loss, and minimize weed growth. In winter, mulch can keep the soil warm and prevent plant roots from frostbite. I've been using organic mulch in the garden instead of rubber mulch.
- Water
Hydrangeas prefers moist soil to excessively wet soil. Even if the smooth hydrangea can withstand drought, excessive dry soil can cause dehydration and death.
Bigleaf hydrangea leaves can start to droop if the temperature goes above 80℉ (27℃), even if the soil is moist. Dr. Michael A. Dirr discovered this. So, if the hydrangea leaves look as soft as cooked noodles, it's a sign that the plant needs more water.
Especially in the hot season, I will water when the temperature is not so high. At the same time, I also adopted Dr. Dirr's active cooling method. In addition to the shade, I use sprinklers to spay fog to cool down every few minutes on a hot summer afternoon. This eases up the stress on plants brought about by the harsh external conditions and helps the overall growth of the plant.
- Fertilizer
To keep healthy, the smooth hydrangea plant needs 16 different nutrients. In addition to nitrogen (N), phosphorus (P), and potassium (K), it needs 13 essential nutrients to thrive. Nitrogen (N) can help plant leaves, stems, and roots grow. If some essential nutrients are lacking, smooth hydrangea will not show its best growth.
For example, hydrangea veins show dark green, and the part between veins turns yellow in alkaline soil lacking iron. If you plant hydrangea in a pot, even if the soil pH is 5.5, this will happen if the soil is deficient in iron.
Dr. Dirr's research found that smooth hydrangea grows best in soil with a pH between 5.5 and 6.0. When the soil is slightly acidic, the plants can maximize using all 16 essential nutrients. But aluminum (AI) won't dissolve in water if there's too much phosphorus (P). This means plants won't be able to absorb aluminum (AI) from the soil.
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I will evenly spread 14-14-14 slow-release fertilizers around the plants in early March. In Florida, smooth hydrangea sprouts in late February, and new leaves will gradually unfold in March. This shows that the roots are beginning to be active. Entering the summer, I start to apply iron (Fe), sulfur (S), zinc (Zn), and other nutrients.
3. Pruning Tips
I will prune smooth hydrangea at three critical times: when new leaves grow, the blooming period ends, and entering winter.
In early March, smooth hydrangeas began to grow new leaves. At this time, I will cut off all the dead stems and inward-growing stems. For thicker branches, if I am unsure whether they are dead, I will check by scraping off a bit of bark. If it is brown inside, I will cut it off.
When the blooming period is over, I will cut off all the flower heads that have turned brown.
Smooth hydrangea can accept hard pruning. When the smooth hydrangea goes into dormancy, I prune the plants to a height of 10 inches from the ground. This can prevent it from overgrowing and encourage its bloom on new stems.
4. Diseases
We should pay attention to powdery mildew in early summer and early autumn.
Smooth hydrangea seedlings planted in containers are more likely to suffer from bacterial leaf scorch (BLS). The soil is too wet or dry, too hot, and root rot can lead to this disease. The disease leaves water-soaked spots on the leaves at an early stage. If we do not remove diseased leaves in time, leaf spots will expand, and mature leaves will die.
5. Pests
In addition to aphids, Japanese beetles, and spider mites, we should also pay attention to Hydrangea Leaftier Moth. Hydrangea Leaftier Moth will eat the new buds and stems and glue the leaves together, making the leaves deformed. Once plants have pests, we must remove these diseased parts in time. At the same time, we can also use neem oil or insecticides for further treatment.
6. Animals
Deer like to eat smooth hydrangea, panicle hydrangea, bigleaf hydrangea, and oakleaf hydrangea. If there are deer around your garden, these hydrangeas can hardly grow. Deer will constantly gnaw at their stems and leaves. Even if you mix smooth hydrangea with other plants, deer will find them.
Bibliography:
- Hoadley, S. (2022). The Wild Hydrangeas of Mt. Cuba Center. Arnoldia, 79(3), 20–31. https://www.jstor.org/stable/27234671
- Janse, J. D., & Obradovic, A. (2010). XYLELLA FASTIDIOSA: ITS BIOLOGY, DIAGNOSIS, CONTROL AND RISKS. Journal of Plant Pathology, 92, S35–S48. http://www.jstor.org/stable/41998754
- Schultz, Peter B., and David J. Shetlar. "Major insect pests of ornamental trees and shrubs." Handbook of Integrated Pest Management for Turf and Ornamentals. CRC Press, 2020. 237-247. https://doi.org/10.1201/9780138752798
My name is Arthur Mo. I am a gardener who loves gardening so much. I graduated from Ocean University of China. After I returned to Japan, I began to learn how to cultivate hydrangeas and daisies. Mr. Mitsuhiro (my master) taught me to sow, cut, fertilize, and manage diseases and pests. For nine years, I have been sharpening my planting skills. In 2022, my family and I moved to Florida. The warm climate of Florida gives me the impulse to continue cultivating hydrangeas and daisies. I began my gardening trip in my backyard and unswervingly contributed my efforts to these plants. I love these plants deeply, and at the same time, I also thank my family, my master, and friends for their understanding and support.