When should I Prune my Hydrangea?……….It Depends.

by Martha Engelke, EMGV
Previously published in
The Daily Reflector, Greenville, NC

Several years ago, I waited for my large blue hydrangea to bloom like it did every year. But, not a bloom emerged even though the plant looked healthy. After a bit of searching, I realized that the pruning I did in early March, prevented the blooms from forming.

Hydrangea macrophylla, the name of the blue and pink hydrangeas often seen in North Carolina, bloom on old wood which means that they need to be pruned right after they finish flowering, usually in August. Waiting until March, as I did, meant that I cut off the potential blooms for the upcoming year. To prune H. macrophylla, remove dead, diseased, or broken branches and trim the bush to maintain its shape. Harsh pruning is not recommended. H. macrophylla come in a variety of colors, usually pink, purple, or blue. The color of the flower is dependent on the soil Ph. Acidic soil (pH.=5.0-5.5), results in blue flowers and a higher pH. (6.0-6.5) results in pink flowers. If you want to amend the soil to achieve a particular color, it’s best to first get a soil test. Aluminum sulfate can be added to acidify the soil, and lime will produce a more alkaline soil.

Most of us have hydrangeas that are “Mopheads” which are large ball shaped flower clusters. More recently, the “Lacecap” version has become popular. Lacecap hydrangeas have flat clusters of small flowers surrounded by a ring of more prominent flowers. It’s important to remember that even if you bought your H. macrophylla at a nursery where it was in full sun, it likely with do better in a location with afternoon shade.

Another common hydrangea that blooms on old wood is the Oak Leaf Hydrangea (H. quercifolia). They are considered native to the Southeast United States. Oak Leaf Hydrangeas can be 8-10 feet tall and just as wide, depending on the cultivar. Be sure to check the mature size before you plant. Oak leaf hydrangeas have leaves that look like the oak tree and the flowers are usually large white panicles. Pruning should be done when they finish blooming and severe pruning is not recommended.

In contrast, there are other hydrangeas that bloom on new wood. The Smooth leaf or Wild Hydrangea (H. arborescens) is native and often seen growing on moist or rocky wooded slopes, ravines, streambanks, and bluff bases. The blooms are white and not affected by the pH of the soil. H. arborescens should be pruned back close to the ground in late winter to encourage vigorous stem growth. If not pruned back, any weakened and/or damaged stems should be removed in early spring.

Another popular hydrangea that blooms on new wood is H. paniculata. Popular cultivars are “Limelight” and it’s smaller sibling “Little Lime”. This type of hydrangea blooms later than other hydrangeas and can tolerate more sun. The flowers can be 6-12 inches long. They begin as a light green, turn to white, and then fade to green or pastel pink. The color of the flowers is not affected by pH. Pruning of H. paniculata should occur in early spring just as the leaves are beginning to show. Flower head size can be related to pruning. With more aggressive pruning, flower heads will be larger. Less aggressive or tip pruning can result in smaller but more numerous flower heads. The only problem is that I have found that deer absolutely love Limelight hydrangeas. So, think about where to plant them and how you might discourage deer.

One of the exciting things about hydrangeas is that with careful selection of different cultivars there can be blooms through late spring to late fall. They also make great dried flower arrangements. In late summer when they begin to turn color, pick them, put them in a vase with about 1” of water and ignore them. They will dry to a color that is similar to when they were picked and they will last for months or years.

Additional References:
Pruning woody ornamentals

Soil Testing and pH

Discouraging deer


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