Hortensias, commonly known as hydrangeas, are flowering shrubs that enhance garden spaces with their abundant and diverse blooms. These plants offer a wide array of forms, from compact varieties ideal for containers to large shrubs that create stunning landscape features. Their ability to produce a profusion of flowers throughout the growing season makes them a favored choice for gardeners seeking continuous color. Understanding their specific needs helps cultivate healthy, beautiful plants.
Understanding Different Hydrangea Types
Different hydrangea types have varying care requirements. Bigleaf hydrangeas ( _Hydrangea macrophylla_ ), including Mophead and Lacecap varieties, are known for their large, showy flowers. These types bloom on old wood (previous year’s growth). Panicle hydrangeas ( _Hydrangea paniculata_ ) have cone-shaped flower clusters and tolerate more sun than other types. These varieties bloom on new wood (current season’s growth).
Smooth hydrangeas ( _Hydrangea arborescens_ ) bloom on new wood, creating large, rounded flower heads. Oakleaf hydrangeas ( _Hydrangea quercifolia_ ) are distinct for their oak-like lobed leaves and elongated, conical flower clusters on old wood. Their foliage turns red and purple in autumn. Mountain hydrangeas ( _Hydrangea serrata_ ) are related to Bigleaf hydrangeas but are more compact and cold-hardy, blooming on old wood. Climbing hydrangeas ( _Hydrangea anomala petiolaris_ ) are vines that scale walls and trees, producing white, lacecap-like flowers on old wood.
Essential Care for Thriving Hydrangeas
Most hydrangeas prefer morning sun and afternoon shade, especially in warmer climates. Intense afternoon sun can scorch leaves and stress plants, particularly Bigleaf and Smooth varieties. Panicle hydrangeas, however, tolerate more direct sun exposure.
Hydrangeas thrive in well-draining soil rich in organic matter. A slightly acidic to neutral soil pH is preferred, though pH significantly influences Bigleaf hydrangea bloom color. Consistent moisture is important, especially during establishment and dry spells. Watering deeply two to three times weekly, rather than shallowly, encourages a strong root system.
Applying a balanced, slow-release granular fertilizer in early spring supports healthy growth and flowering. Fertilizers for flowering shrubs, with an N-P-K ratio like 10-10-10 or higher phosphorus, are suitable. Avoid over-fertilizing, as too much nitrogen can promote leafy growth at the expense of flower production.
Mastering Hydrangea Pruning
Correct pruning maintains hydrangea health and encourages blooming, with timing dependent on the type. Hydrangeas that bloom on old wood, such as Bigleaf ( _Hydrangea macrophylla_ ), Oakleaf ( _Hydrangea quercifolia_ ), Mountain ( _Hydrangea serrata_ ), and Climbing ( _Hydrangea anomala petiolaris_ ), should be pruned immediately after flowering in late summer. This allows time for new growth and bud set for the following year. When pruning these types, focus on removing dead, damaged, or weak stems, spent flowers, and crossing branches to improve air circulation.
Conversely, hydrangeas that bloom on new wood, including Panicle ( _Hydrangea paniculata_ ) and Smooth ( _Hydrangea arborescens_ ), are best pruned in late winter or early spring before new growth begins. As flowers develop on current season’s growth, pruning at this time will not remove potential blooms. These types can be pruned aggressively to control size, shape, and promote larger blooms. Removing up to one-third of the plant’s height can rejuvenate older plants and encourage robust new stems.
Some Bigleaf hydrangeas are reblooming varieties, producing flowers on both old and new wood. These offer pruning flexibility; light deadheading of spent blooms throughout the season encourages additional flowering. If significant shaping or size reduction is needed for reblooming types, it is best to prune them after their first flush of blooms in early summer.
Adjusting Hydrangea Bloom Color
The ability to change Bigleaf hydrangea ( _Hydrangea macrophylla_ ) bloom color is tied to soil pH. To achieve blue blooms, the soil needs to be acidic, with a pH between 5.0 and 5.5. This acidity makes aluminum available to the plant, responsible for blue coloration. Gardeners can lower soil pH by applying aluminum sulfate or elemental sulfur.
For pink blooms, the soil needs to be more alkaline, with a pH ranging from 6.0 to 7.0. In this pH range, aluminum becomes less available, allowing pink pigments to dominate. To raise soil pH, garden lime can be incorporated. Changes in soil pH may take months to show effects on bloom color, requiring consistent application.
Not all hydrangeas can change bloom color based on soil pH. White-flowered varieties, like Panicle ( _Hydrangea paniculata_ ) and Smooth ( _Hydrangea arborescens_ ), do not contain pigments that react to aluminum. Therefore, their blooms remain white regardless of soil pH adjustments.
Troubleshooting Common Hydrangea Problems
Hydrangeas are resilient, but can encounter issues affecting growth and flowering. Lack of blooms is a common issue, stemming from several factors. Improper pruning is a frequent cause, especially for old wood blooming varieties where flower buds may have been removed. Late spring frosts can also damage tender new growth and developing flower buds, leading to reduced or absent blooms. Insufficient light, less than four hours of direct sun daily, can also hinder flower production.
Leaf issues signal environmental stress or nutrient deficiencies. Yellowing leaves, or chlorosis, can indicate an iron deficiency, caused by overly alkaline soil making nutrients unavailable. Overwatering or poor drainage can also lead to yellowing leaves and root rot. Browning leaves, around the edges, may suggest underwatering or excessive sun exposure, causing leaf scorch.
Hydrangeas can also be susceptible to certain pests and diseases, though severe infestations are uncommon. Common pests include aphids, small insects that cluster on new growth, dislodged with a strong water spray or treated with insecticidal soap. Powdery mildew, a fungal disease appearing as a white, powdery coating on leaves, often occurs in humid conditions with poor air circulation. Adequate spacing and avoiding overhead watering can help prevent its spread.