Plant Profile: Tree Form Hydrangea

This article by Debbie Lonnee appeared in the Sept/Oct 2019 issue of Northern Gardener

Plant to Pick

Sept/Oct 2019

Hydrangea paniculata

Panicle Hydrangea

Tree Form

Nothing adds color to the late summer landscape more than Hydrangea paniculata, also known as panicle hydrangea. It is one of the hardiest species of hydrangea we can grow in Minnesota.

Rather than highlight one of the many new cultivars on the market today, I’d like to showcase how it can be grown into a different shape or form.

When you think of a shrub, you probably envision a multistemmed plant that is round and bushy. When you think of a tree, you likely envision a single trunk with a branched top. How about combining the two and grow what my colleagues and I jokingly call a shrub on a stick!

This is now commonly done with Hydrangea paniculata, and a well-shaped tree form is a welcome addition to small-space gardens.

It takes the nursery grower more than five years to bring a good-sized panicle hydrangea tree to market. Without getting into too many technical details, the growers take a young rooted cutting, cut out all but the strongest, straightest stem to create a trunk, and later prune the trunk at a certain height (usually about 4 feet) to create a shrubby round head. Those bareroot trees are harvested and shipped to other growers or planted in larger containers, typically 7- or 10-gallon containers that will eventually be shipped to the garden centers after another year of rooting and growth. These plants have the look of a young ornamental tree with a very visible trunk.

Typically, the taller panicle types are used for tree production. Think about great cultivars such as ‘Limelight’ PP12,874, First Editions^® Vanilla Strawberry^™ and Quick Fire^®. These are all varieties that mature in the 6- to 8-foot height range and are best for trees. The newer dwarf cultivars do not lend themselves to tree forms. The conical flowers start to form in July and bloom through August and into September.

Growing Tips

So, how do you grow and maintain a tree-form hydrangea? They have the same cultural requirements that the shrubs do: full sun and well-drained, average soil. Most cultivars are hardy in USDA Zone 4, with many hardy to zone 3. Pruning is the main difference. Panicle hydrangeas bloom on new wood, so they can be pruned as dormant plants in March, April or early May. You will need to be diligent, especially with a newly planted tree, in the first years to maintain that round shape. You can cut the stems back 25 to 50 percent (don’t be shy!), and stand back as you prune to watch that you are maintaining a round shape. If any shoots appear on the trunk, they can be easily pruned to maintain the trunk look.

A light fertilization in spring is helpful. In the landscape, these smaller shrubs on a stick offer a plant that looks like a tree but that fits into a very small space. Small city lots and townhouse gardens will all benefit from the verticality of a small tree. I see many designers using them in large display pots, but they will have to be stored indoors during our cold Minnesota winters.

Just a few thoughts about the three cultivars I have suggested above:

‘Limelight’ PP12,874 From the Proven Winners^® Color Choice program, this is an excellent variety as a tree. The flowers open in midsummer, first lime green, then a nice white. As the flowers mature, they will take on some light shades of pink, burgundy and green.

First Editions^® Vanilla Strawberry^™ Bred in France, this is one of the first panicle hydrangeas to display incredible shades of pink, then strawberry, as the panicle matures. It starts out white, and then the color begins at the bottom of the panicle. This color is best expressed when we get cool nighttime temperatures in late July into August. The last two seasons we have had spectacular color on this variety. The red stems are a bonus.

 

Quick Fire^® (‘Bulk’ PP16,812) Also part of the Proven Winners^® Color Choice program, this is one of the earliest panicle types to bloom in summer, sometimes a month earlier than other varieties. The flowers are more open than other types but are beautiful in their own way. Quick Fire will take on some pink colors in August as the flowers mature.

Debbie Lonnee works in the horticulture industry and gardens in South St. Paul.

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