Oakleaf hydrangea shrubs grow relatively slowly. While flowers are borne on old wood and will age to a soft pink.

We tend to think of hydrangeas as one big group, but in actuality there is quite a bit of diversity among hydrangeas.

Hydrangea is both the common and the genus name, and there are 70 to 75 different individual species in the genus Hydrangea. Hydrangeas are native to southern and eastern Asia as well as North and South America.

In discussing hydrangeas, most sources divide this genus into five major groups: Climbing Hydrangea, Smooth Hydrangea, Panicle Hydrangea, Oakleaf Hydrangea and Bigleaf Hydrangea.

Let's take a look at each variety:

Climbing Hydrangea: Hydrangea anomala is a deciduous vine that can reach heights of 60 to 80 feet at maturity. However, it is a relatively slow grower and will take several years to attain this height.

As Climbing Hydrangea ages, it develops an attractive exfoliating cinnamon colored bark. It bears white flower clusters that are 6- to 10-inches in diameter in a pattern described as lacecap (large, sterile flowers surround the edge of the flower cluster, with small fertile flowers in the center).

Smooth (Snowhill Hydrangea): Hydrangea arborescens is a deciduous shrub that grows to be about 5 feet tall. Large white clusters of flowers are borne on new wood (branches produced in the current year). Flower clusters up to a foot in diameter are typical of this hydrangea. I remember my mom calling these “snowball” bushes growing up.

These shrubs are native to eastern North American forests. A popular cultivar, ‘Annabelle’ was developed by the late J.C. McDaniel from the University of Illinois.

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Panicle Hydrangea: Hydrangea paniculata is the largest of the major hydrangea groups, ranging in height from 10 to 25 feet. Some sources describe them as trees rather than shrubs. Their flowers are borne on new wood and start out white, but age to pink. These are the most hardy of the hydrangeas, surviving all the way to Zone 3. I have the variety ‘Quick Fire’ and it is loaded with blooms.

Oakleaf Hydrangea: Hydrangea quercifolia, is a deciduous shrub that grows to be about 5 to 6 feet tall. It grows relatively slowly and mature branches have exfoliating cinnamon colored bark much like the Climbing Hydrangea.

They get their name from their deeply lobed leaves that resemble that of the oak. They also have gorgeous fall color ranging from red to bronze and purple. White flowers are borne on old wood (branches produced the previous year), and they age to a soft pink.

Bigleaf Hydrangea: Hydrangea macrophylla are divided into two groups: hortensias, that have big “mopheads” of flowers and lacecaps, clusters of flowers with tiny fertile flowers in the center surrounded by a border of large sterile flowers. Depending on the cultivar, some Bigleaf Hydrangeas will flower only on old wood. Some will flower on both new and old wood. Flowers are generally blue or pink. The flower color of some cultivars is affected by the pH of the soil-- in acid soil the flowers are blue, in alkaline soil they are pink.

Scientists have determined that aluminum is the key element in determining flower color in hydrangeas. In acidic soil (pH less than 7) aluminum naturally present in the soil is available for uptake by the plant. The aluminum forms complexes in the flower cells, creating a blue appearance. In alkaline soil (pH greater than 7) the aluminum is still present in the soil, but it is not in a form that can be taken up by the plant. Without aluminum in the cells, the flowers appear pink.

For guidance in adjusting the pH of your soil, consult your local Extension office. They can help you with soil testing and amendments to change the pH. It can be tempting to forego the soil testing and start trying to adjust pH blindly, just adding what looks like “enough” of a given amendment. Adjusting pH can be difficult, and may take a lot more soil amending than you think. It is also easy to “overshoot” your desired pH, and it may be tough to recover from an overdose of amending. It is possible to have a soil too acidic or too alkaline to support plant growth.

Next week: How to overcome the frustration of a nonblooming hydrangea.

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Jennifer Schultz Nelson is a unit educator in horticulture for the University of Illinois Extension


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