Neil Sperry: The challenges of growing hydrangeas

It’s pretty amazing what a difference 25 or 50 miles can make in the plants you can grow. Hydrangeas (Hydrangea macrophylla) are staples of East Texas landscaping, yet they play out as you drive west onto the Blackland Prairie soils here in North Central Texas.

And, it can get even more precise than that. There are spots in the Mid-Cities in DFW where soils are sandy and slightly acidic, and hydrangeas will grow fairly well, but just a mile or two down the road, the soil shifts back to the Blackland side and the opposite is true.

What you need to know

These plants like sandy, acidic soils and they don’t do well with black, alkaline clay. Most of us locally were what my Ohio-bred wife calls “behind the door” when the good soil was passed out. Which means if we want to grow the colorful types of hydrangeas, we’d better be ready to commit to some major soil amendment efforts.

There’s also a strange fact you need to know about hydrangeas and their “flowers.” What you’re seeing as pink or blue blossoms are actually floral bracts (modified leaves). The tiny flowers are concealed down inside them. Poinsettias, bougainvilleas and dogwoods are other common plants that produce showy bracts that many mistake for flowers.

And there is another, even stranger fact that you might enjoy learning about hydrangeas and their floral bracts. These plants are like litmus paper. Their bracts will be blue in acidic conditions (such as East Texas soils), and they will be pink in alkaline soils — and with alkaline irrigation water (what we have locally).

Growers can vary the colors of their hydrangeas’ bracts by adding aluminum sulfate to lower the pH (make the soil more acidic), or they can add lime to raise the pH (push the soil toward the alkaline side) to send flowers to the pink side. However, that comes with a risk, because if you don’t do quite enough, the plant’s bracts may end up an unattractive shade between blue and pink and leaning toward brown.

Checklist for success

But all that’s not why I called this hydrangea meeting. The goal is to hone in on how local gardeners might improve their chances of succeeding with this temperamental plant.

In my own experiences, hydrangeas grow best where they receive morning sun (until 10 or so) and shade the balance of the day. In areas with black, alkaline soils, they should be planted much like azaleas. Dig a hole 15 to 18 inches deep and 3 feet wide for each plant.

Remove all of the native soil. Fill the hole with a mix of half sphagnum peat moss and half finely ground pine bark mulch, and mound more of that mix to 12 or 15 inches above the surrounding grade. That will give your plant good drainage and an acidic growing medium. You will need to water it slowly and carefully, however, because the mound will tend to shed irrigation and rainfall.

Hydrangeas dry out more quickly than almost any other shrub that you’ll grow. That’s partly because they prefer moist soils and cooler summer weather, but it’s also because of their very large leaf surfaces. It simply takes a lot of water to keep them plumped up and happy.

And since local irrigation water tends to be highly alkaline — not the friend of hydrangeas — you may see signs of iron deficiencies showing up. Iron is insoluble in alkaline conditions, and it’s a critical part of the chlorophyll molecule, so when alkaline irrigation water combats the original acidity of that planting mix you prepared, you can expect iron chlorosis to appear after a few years.

On the plus side, very few insects or diseases bother hydrangeas.

You’ll want to trim and reshape the shrubs only after they finish blooming. Try to do as little pruning as possible, though. Strong vegetative growth made late in the growing season can come at the cost of having no flowers the following spring. Florists refer to those vegetative stems as “blind shoots,” and they can defeat the whole purpose of raising the plant for its floral bracts.

A hydrangea with a new hue

As frustrated as I’ve been sometimes with florist-style hydrangeas in my own landscape, I’m a huge fan of a sister plant, oakleaf hydrangea (Hydrangea quercifolia). It’s a real shrub, with woody branches and a long life expectancy.

It grows in shade, preferring not to be exposed to sun after mid-morning, if possible. Its leaves are dinner plate-sized, and the long flowering panicles are creamy white. It blooms during April, and it’s a mainstay among North Texas’ shade flowering shrubs. As with its sister, however, it does consume a lot of water, so plan and plant accordingly.

This story was originally published April 9, 2015 at 1:18 PM with the headline "Neil Sperry: The challenges of growing hydrangeas."