How to Grow Tree Hydrangeas (Panicle Hydrangeas)

Tree hydrangeas (Hydrangea paniculata), also known as panicle hydrangeas, are fast-growing flowering shrubs with an upright growth habit. These plants provide late summer blossoms when few other bushes are in bloom. The shrub has oval, toothed, dark green leaves, and it produces cone-shaped flower panicles that stretch around 7 inches long with clusters of small, creamy white blooms. The flower heads take on a pinkish hue over time before fading to tan or brown for the winter. Tree hydrangeas can be planted in the spring after the threat of frost has passed, as well as in the early fall. Note that they are toxic to both humans and pets, so take care where you plant them.

Tree Hydrangea Care

Tree hydrangeas are fairly easy to care for and can grow in a variety of conditions. They are tolerant of pollution and urban conditions, as well as of salt in the soil. They can handle being planted near roadways. Just make sure your planting site has good soil drainage and is sheltered from strong winds, which can damage the stems. 

These shrubs don’t typically have many pest or disease problems, especially when they’re grown in an environment they like. You might occasionally see aphids or mites on the foliage, which often can be mitigated with a strong blast of water from the hose. Expect to water and feed your shrub regularly, and prune to maintain its shape.

Light

Tree hydrangeas grow well in full sun to partial shade, meaning at least four hours of direct sunlight on most days. In hot climates, they will benefit from some afternoon shade.

Soil

These shrubs can handle a variety of soil types, including sandy, loamy, and even clay provided that there is good drainage. They prefer organically rich soil with a slightly acidic to neutral soil pH, though they also can tolerate slightly alkaline soil.

Water

Tree hydrangeas thrive in lightly moist but not soggy soil. Be careful not to overwater, which can cause root rot and other diseases. However, allowing the soil to dry out too much can cause the foliage to wilt and eventually damage or kill the plant.

Temperature and Humidity

These plants have good cold tolerance within their growing zones, especially compared to many other hydrangea species. In hot weather, it’s important to make sure they are well-watered to prevent stress on the plant. Humidity typically isn’t an issue as long as their water requirements are met.

Fertilizer

Feed tree hydrangeas twice a year in the early spring and in the fall immediately after the flowers have faded, using a fertilizer formulated for shrubs and trees. Over the summer, the shrubs will benefit from an application of compost.

Types of Tree Hydrangea

There are several varieties of tree hydrangeas:

• Hydrangea paniculata 'Grandiflora' grows to 25 feet with a 10-foot spread and has pure white flowers.
• Hydrangea paniculata 'Limelight' grows 6 to 8 feet tall and 5 to 7 feet wide and has a greenish color in its flowers.
• Hydrangea paniculata 'Big Ben' grows 6 to 8 feet tall and 5 to 7 feet wide and is valued for having flowers of a deeper pink color.
• Hydrangea paniculata 'Bobo' is a dwarf variety, growing only about 3 feet tall and wide.
• Hydrangea paniculata 'Pinky Winky' grows to a maximum height and width of 8 feet and is known for its bicolor flower heads (pink on the bottom, white on the top).

Pruning

This plant can be trained to grow as a small tree by judicious pruning. But it achieves its best form when grown as a large shrub with multiple stems. Blooms occur on the current season’s growth (new wood), so prune as needed in the late winter to early spring. Untimely pruning can sacrifice some of the flowers for that growing season. When kept in its shrub form, the shrub will bear larger flower clusters if you thin it to five to 10 primary stems.

To train the plant to grow as a tree, choose one main stem to secure to a sturdy stake. Prune away competing ground stems. Remove any shoots that emerge from your main stem from the ground to about 3/4 of the way up that stem. Continuously check for shoots around the base of the plant, and remove them as they pop up. Your main stem will continue to grow with foliage at the top, taking on the look of a trunk. It can require two or more years before a true tree shape is accomplished.

Propagating Tree Hydrangea

You can propagate tree hydrangeas from cuttings. The best time to do this is in the spring or early summer before the plant begins to flower. Cuttings not only are an inexpensive way to produce more plants, but they also allow you to make more of a particular tree hydrangea variety that you like. Here's how:

1. Choose a healthy stem that has not flowered yet, and cut off a piece that's about 6 inches long with sharp, sterile pruners.
2. Remove the lower leaves on the cutting, and cut the other leaves to half their length.
3. Dip the cutting in rooting hormone, and immediately plant it in a small container filled with vermiculite, coarse sand, or a combination of the two.
4. Water the cutting well, making sure the soil is moist but not overly wet. Place a plastic bag or dome over the container to retain moisture, taking care to make sure the plastic doesn't touch the cutting.
5. Place the container in an area that gets bright but indirect light. Keep the soil lightly moist but not soggy. Expect the cutting to take about a month to develop a root system, after which it's ready to be transplanted.

How to Grow Tree Hydrangea From Seed

Though it is possible to grow tree hydrangeas from seed, keep in mind that it is quite difficult. That's why most gardeners go with propagating through cuttings.

If you do choose to propagate through seed, let several flowers fade on the plant. Harvest these flowers in a paper bag after a few months. This gives them time to dry out. Store the flower heads in the bag for another week to ensure they are dry. At that point, give the bag a good shake. This will loosen the tiny seeds from the flower heads. Keep in mind these seeds are about the size of a grain of salt or pepper, so they will be tough to find.

You can sow the seeds directly into the ground in the fall, or you can choose to hold onto them through the winter months and start them indoors in the early spring. Fill a container with potting soil, and sow the seeds on the surface; do not cover them with soil. Keep the soil lightly moist. Place the container in a sunny spot, and expect germination of the seeds within a matter of weeks.

Potting and Repotting Tree Hydrangea

Container-grown tree hydrangeas will likely need to be repotted every year or two as they grow, depending on your pot size. The best time to repot hydrangeas is during the spring when the plant is no longer dormant but isn't yet stressed from the heat of summer. It's also OK to repot a hydrangea during the winter to allow it more time to adjust to its new surroundings.

Choose a container that is a few inches larger than the previous one. The best container material is unglazed clay to allow excess soil moisture to evaporate through its walls. If you are potting a tree hydrangea for the first time, go with a pot of 8 to 12 inches in diameter. The best growing medium is well-mixed compost with a fertilizer designed for hydrangeas. When planting, cover the roots and stem with the soil. Keep the container in the shade for the first few days, gradually introducing it to more sun to acclimate the plant to the light and humidity. Keep the soil moist but not waterlogged.

Overwintering

To help your plants through the winter, keep the soil moist right up until the ground freezes. Cover the roots with 3 to 4 inches of mulch, taking care to remove it as soon as the temperatures warm up.

Common Pests & Plant Diseases

Hydrangeas of all varieties can fall prey to aphids, rose chafers, and scale. Organic pesticides can help get rid of these pests. Slugs and snails will also be tempted to nibble on the plant. Remove the slugs and snails by hand, and deal with heavy infestations with natural methods, such as slug pellets.

Moreover, hydrangeas are prone to fungal diseases, such as powdery mildew, blight, and leaf spot, all of which can be treated with an appropriate fungicide. The plants might also fall victim to a variety of viruses, which can cause leaves to turn yellow and a plant to lose its flowers. The only real treatment is the removal of affected plants.

How to Get Tree Hydrangea to Bloom

Tree hydrangea flowers feature rays of four to five petals that stretch around 1 to 3 inches across. Deadheading, or removing the spent blooms, can help to encourage further flowering, though many gardeners opt to leave the dried flower panicles on the plants through winter for visual interest in the garden.

With any sort of hydrangea, there are a few things to look for if the plant is not blooming. Start by ensuring that the ground is moist but not overly wet. Hydrangeas also need some sunlight but not too much, so make sure they remain out of harsh light. This is especially relevant if you live in a hot climate. Furthermore, hydrangeas tend to bloom better when they have more acidic soil, so test your soil to see whether amendments are necessary. Also, don't fertilize too much, as overfertilization can lead to lush foliage but few blooms.

Common Problems With Tree Hydrangea

Tree hydrangeas are fairly easy to care for, as long as you get their growing conditions right. They're somewhat susceptible to pests and diseases, but they're also resistant to salt and pollution in their environment. The following are some common issues to watch out for.

Leaves Turning Black/Brown

Black or brown spots on foliage can often be a sign of a fungal disease. If the affected area is small, you might just want to prune it off. Or you can use a suitable fungicide.

Leaves Turning Yellow

Yellow foliage can often be a sign of unsuitable moisture conditions—both overwatering and underwatering. The soil should never be soggy or be allowed to dry out completely. Yellowing also can be a sign of too much fertilizer, as well as some diseases.