Hydrangeas are beautiful old-fashioned shrubs that flower in summer and fall. There are many different species and hundreds of varieties available to choose from, but the one that most people envision when they think of hydrangeas is the bigleaf hydrangea (Hydrangea macrophylla), also called florist, hortensia, mophead or lacecap, depending on variety.
Unfortunately, the vision of large mounded blooms of pink or blue covering a floriferous shrub all summer long is not always the reality. Two of the most frequent questions I get from gardeners are, “Why doesn’t my hydrangea bloom?” and “How and when do I prune my hydrangeas?” In this column, I will address the first question; in my next column, I will focus on proper pruning.
In figuring out why it doesn’t bloom and how to prune, it is important to understand that bigleaf hydrangea blooms on “old” wood, which means that the flower buds are formed during late summer and fall on currently growing stems but will not bloom until the following season, generally in June and July.
So those flower buds have to survive both Old Man winter and the well-meaning pruner in order to bloom. And although improper pruning can be one of the main reasons a bigleaf hydrangea does not flower, there are several other possible causes of failure.
A primary cause is winter hardiness of the flower buds. Many varieties, grown in greenhouses for Easter, what we call florists hydrangeas, are not fully hardy in our Zone 6 gardens. The stems are killed back to the ground during winter, but the roots survive and new stems emerge in spring. Flower buds may form on those stems, but they will die back the following winter and never bloom.
Other varieties are fully bud-hardy in our Zone 6 winters, as long as the plant is fully dormant. But with the arrival of warmer temperatures in early spring, dormancy is broken, the plant resumes growing, and hydrangea flower buds begin swelling. Those flower buds are damaged at 27 degrees F and killed outright at 20 degrees F; so if a late spring freeze hits, there may be few if any flowers that summer.
As nice as the weather was last week, its mild warmth pushed a lot of plants out of dormancy and into early growth, which may lead to bud damage later on if temperatures drop below freezing again, as they usually do in our springs.
Cultural factors may also affect flowering performance of bigleaf hydrangeas — too much or too little of something, whether it is light, moisture or nutrients. Hydrangeas won’t flower well in deep shade; they prefer dappled sunlight, such as at a woodland’s edge; or morning sun and afternoon shade, such as on the east side of a building; or even full sun, as long as soil and moisture are good. They will not grow or flower well in competition for moisture and nutrients with many broadleaf trees such as oaks, maples and beeches.
They need well-drained but moist soil, preferably acidic, enriched with abundant organic matter (leaf mold, compost or well-aged manure) and layered with organic mulch to a depth of 2 inches. They demand consistent moisture throughout the summer, at least 1 inch per week, and will quickly let you know if moisture is lacking by wilting. Droughty summer conditions may lead to poor bud set and few if any flowers the following summer.
They generally require higher levels of nutrients than many other shrubs, but excessive levels of a high nitrogen fertilizer may lead to long weak shoots that don’t set flower buds. It’s best to broadcast a general purpose (10-10-10) or slow-release organic fertilizer in early spring before the plant leafs out. Never fertilize bigleaf hydrangeas in late summer or fall; this may encourage weak growth that is then winter-killed, which means no flowers next summer.
The flower color, pink or blue, is determined by the availability of aluminum in the soil solution to the roots; the availability of aluminum is, in turn, determined by the pH of the soil. In highly acidic soils, with a pH of 5.5 or below, aluminum is available and the flowers will be blue. In weakly acidic or more alkaline soils, with a pH of 6.0 to 7.5, aluminum is not soluble and flowers will be pink, rose or red.
You can apply aluminum sulfate and water it in to provide the aluminum needed, and although hydrangeas are tolerant of high levels of aluminum, many other plants are not. Too much aluminum in the soil can dwarf or kill plants, even hydrangeas.
Trying to bring the pH down with garden sulfur, in order to make aluminum more available, is a slow process, and one that expert Michael Dirr characterizes as “borderline crazy.” His advice: In higher pH soils, learn to live with and enjoy the beautiful pink, rose and red colors.
In my next column, I’ll tackle how to prune hydrangeas for best flowering and hydrangea alternatives for the hydrangea-challenged gardener.
Annette MaCoy is the consumer horticulture extension educator and master gardener coordinator for Penn State Cooperative Extension in Cumberland County. If you have garden questions, contact Annette at 240-6500 or by e-mail at firstname.lastname@example.org.