Plant Palette

Plant Palette

Hydrangeas

Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture
jaschult@illinois.edu

Hydrangeas are one of those plants that both enamor and frustrate gardeners. As much as we love them, when they don't live up to either how they looked when we purchased them, or they don't look like the catalog picture, we wonder what we're doing wrong. I planted my first hydrangea as a new homeowner six years ago, and for the first two years hoped no one noticed that the neighbor with her Ph.D. in horticulture couldn't figure out how to get her hydrangea to bloom.

The name hydrangea comes from the Greek "hydra" meaning water, and "angeon" meaning vessel, in reference to the plants affinity for water and shape of the seed pods.

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.

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 if not many years to attain this height. I have also found in my own garden that the local rabbits find it tasty, which has further slowed its growth!

As Climbing Hydrangea ages, it develops a very 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, or Snowhill Hydrangea, Hydrangea arborescens is a deciduous shrub that grows to be about five 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 the understory of eastern North American forests. A popular cultivar, 'Annabelle' was developed by the late J.C. McDaniel from the University of Illinois.

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.

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, which 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 these varieties. 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.

If you feel you can't worry about soil pH just yet because your hydrangea stubbornly refuses to flower, you need to put your detective hat on. One source I found summed up the "non-flowering" issue as a problem of excesses-- too much cold, too much shade, or too much nitrogen.

Now for the million dollar question, why wouldn't my hydrangea flower? In figuring out this question, the type of hydrangea is important. My non-flowering hydrangea was a Bigleaf Hydrangea. It had beautiful blooms on it when I planted it, but produced no new flowers that summer or the following year.

Knowing the type of hydrangea has to do with the first excess: "too much cold". If you are growing a hydrangea that blooms on old wood, that plant forms next year's flower buds on this years branches in late summer and fall. If the winter is particularly cold, those flower buds may be killed. Added winter protection may help keep the flower buds alive.

My hydrangea is a Bigleaf Hydrangea called 'Endless Summer'. It will bloom on both new and old wood. It is also planted in a protected area near my house. So too much cold shouldn't affect its ability to bloom.

Hydrangeas are generally considered to be an understory plant, thriving in partial shade. They do need some sun in order to flower well. Morning sun and afternoon shade is ideal for many hydrangeas. Too much shade can decrease flowering or inhibit it altogether. My hydrangea is on the north side of the house, and it gets morning sun and is shaded most of the afternoon. Too much shade doesn't seem like the likely cause of its refusal to flower.

Excessive nitrogen fertilizer application can be a problem for any flowering plant, not just hydrangeas. Nitrogen promotes growth of leaves rather than flowers. I hadn't applied any fertilizers while my hydrangea was refusing to flower.

This left me puzzled. According to the sources I had consulted, there was no clear reason my hydrangea wouldn't flower. So like a lot of gardeners and Cubs fans, I decided to "wait 'till next year".

The year after I planted my hydrangea, it had absolutely no flowers. Not even one. I waited again. The next year I had one small cluster of flowers. A small victory. Every year since, my hydrangea has been loaded with flowers-- some of the branches often look like they will break.

My opinion is that given proper temperatures, sun exposure, and nutrients, a hydrangea that refuses to flower just needs time. That's a hard point to swallow in our hectic society, but Mother Nature just may have some lessons to teach us if we take time to listen.

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