Phyllis Heuerman
Frederick County Master Gardener Program

One of the questions most commonly asked of Master Gardeners is "Why won't my Hydrangeas bloom?" The hydrangeas they are most probably talking about are the Hydrangea macrophylla or the Big Leaf Hydrangea, or its cousin, the H. serrata. These are current the most popular hydrangeas. They are the only hydrangeas that bloom in colors other than white, with shades ranging from blue to pink, red and purple. They are also the only hydrangeas that change color depending upon the acidity of the soil.

To understand why these hydrangeas sometimes do not bloom it is important to know that all of the old varieties and many of the new ones bloom on flower buds formed the previous year. This coupled with the fact that many are not hardy or only marginally winter hardy in our area, may cause your shrub not to bloom. Sometimes the stems die to the ground over winter. This may be true even for cultivars that are labeled as hardy in Zone 6. Although new stems form in the spring, they have no flower buds on them. Sometimes the stems survive, but the flower buds die. This happens frequently if we have a frost after the buds have started to swell in March. Winter and early spring protection may save the flowers on these hydrangeas, but often they are just too tender, even with protection. The blooming hydrangea plants that are sold for gifts in the spring are often too tender to survive and bloom outdoors in our area. Look for your hardier shrubs in a nursery or the nursery department of a store.

There are a couple of other reasons why these hydrangeas may not bloom. Although hydrangeas like shade, particularly in the afternoon, they will not bloom if the shade is too deep. They must have some sun, in the morning, or filtered through the leaves of trees in order to bloom. The other factor that affects bloom is the timing of your pruning of the shrubs. If you want to prune a H. macrophylla for size or shape, do it right after the hydrangea blooms, just as the flowers begin to fade. If you wait too long or prune in the spring, you will be cutting off the new flowers buds for the next bloom period. They only pruning you should do in the spring is to remove dead wood.

For those of you who are about to purchase a new big-leaf hydrangea, there is good news. There are some old and new varieties whose flower buds survive the winter, or that form their flower buds in the spring. More improved cultivars are being developed every year. Some cultivars to try include Endless Summer, Blushing Bride, Nikko Blue, All Summer Beauty, Penny Mac, Early Sensation, Coreulea Lace and Dooley. You can find pictures of these varieties on the Internet, and also identify additional cultivars that will work in our area.

The second most frequently asked question about Hydrangeas is "How do I change the color of the flowers on my hydrangea?" The flowers of the H. macrophylla and serrata that we have been talking about tend towards the blue tones in acid soil and pink tones in alkaline soil. The degree to which they will change color varies with cultivar, but if you do not like the color you can adjust the soil pH. To make the flowers more blue, add aluminum sulfate, and to encourage pink add lime to the soil around the shrub. You may have to do this several times. Add the aluminum sulfate or lime in the spring when the leaves are starting to open, and again about one month later.

If you would like to try a more carefree hydrangea, I encourage you to look at the Hydrangea quercifolia, or Oakleaf Hydrangea. It is native to this region and is a beautiful shrub all seasons of the year. It is cold hardy to Zone 5. The species is an open deciduous shrub growing to 7 feet in height. There are good cultivars, including the compact 'Pee Wee" which grows to about 3 feet high. It has white flowers in the summer that fade to shades of pink and brown as they mature. The leaves turn red in the fall before they drop. The shrub has a beautiful open shape and brown, exfoliating bark that adds interest to your garden in the winter. It prefers light to medium shade, but tolerates sun or moderately dense shade. Another hydrangea that I am very fond of is H. paniculata. It is a large shrub that can be trained into a small single or multi stemmed tree. Flowers are generally white, but some tend towards pale pink or green, depending upon the cultivar. The flowers are great for drying. The shrub is very winter hardy.

No matter what hydrangea you choose to grow you will have a shrub with beautiful large flowers that are great as cut or dried flowers. If you select the right cultivar you will also have a relatively carefree shrub that only needs sufficient water, annual fertilization, and occasional pruning to shape and size.