The hydrangea is a popular ornamental garden plant. They are typically grown for their large flowers. With more than 600 named cultivars, Hydrangea macrophylla (Mophead Hydrangea) is the most widely grown type. The flowers are primarily white. However, H. macrophylla can produce blooms in a variety of colors, including blue, red, pink or light/dark purple. These variations are influenced by the pH, or acidity, level of the soil. In fact, flowers from the same plant may appear as a gradient between two or more colors if the soil in which it is grown is inconsistent in pH.
Do All Hydrangeas Require Pruning?
Essentially, common varieties, such as the Mophead Hydrangea, do not require pruning until old age except to shape the plant or control its size. For these, removing dead stems is the only pruning needed to maintain the good health of the hydrangea. Dead branches can be removed any time of year. The same goes for dead or fading blossoms.
Pruning Mophead and Lacecap Hydrangeas
Those two varieties are most often seen in pink or blue and bloom on second-year growth, better known as old wood. These are the branches that have been on the plant since the previous season, not the newer growth from the current growing season. Without old wood to support bloom buds, these types will not flower during the summer. In warmer areas, early fall blooming would still be a possibility. However, in general, it is better not to over-prune the old wood.
When pruning or trimming, weak stems should be thinned out to manage both shape and size. Dead or crossing stems should be removed. As a rule, stems should be cut close to the ground. Removing dead canes and old growth can be done during dormancy. The more old wood that is removed, the fewer flowers the plant will produce in the following spring and summer. However, removing about one-third of old growth will revitalize the plant.
If needed for controlling the plant’s size, serious pruning should be done before late July. The plant can be expected to grow back quickly to its previous size. For that reason, hydrangeas typically need a lot of growing room.
Oakleaf and Endless Summer
Another old wood species is the oakleaf hydrangea (H. quercifolia), which differs considerably in appearance from the mophead variety and is native to the American Southeast. This plant should be pruned in the manner described above for optimal flowering.
A small group of hydrangea will bloom regardless of when they are pruned. These are called endless summer hydrangeas or ever-bloomers. Ideally, these should be pruned in early fall to control the shape and height of the hydrangea. They will continue to produce flowers in the following summer.
Pruning Other Varieties
Less common hydrangea varieties that bloom on new growth should be pruned in the spring and prior to the start of that season’s growth. Alternatively, they can be pruned after they go dormant in autumn. These varieties include the PeeGee (H. paniculata) and Annabelle (H. arborescence) types of hydrangea, both of which are starting to gain popularity. Be warned that over time, drastic pruning may cause these plants to weaken.
Removing Old Blooms or “Deadheading”
Removing dead flowers (deadheading) from the plant can be done at anytime without harming it or disturbing next year’s blooms. If blooms are removed to use in arrangements in June or July, they can have longer stems due to the bloom buds not yet setting for next year. After August 1, as a general rule, it is safer to cut very short stems to avoid harming any of the developing bloom buds. Late in the season, always cut above the first set of large leaves.