Date: January 11, 2001

Growing Hydrangeas in Containers

Dick Bir
NC Cooperative Extension

Introduction: In North Carolina, two native species, Hydrangea arborescens and quercifolia, are commonly container grown. In addition, at least two non-native species, Hydrangea macropylla (including varieties macrophylla, normalis and serrata) and Hydrangea paniculata are commonly grown in North Carolina. Excellent quality plants of the species and most cultivars of these four species can be grown within North Carolina if attention is paid to basic cultural practices. However, not all of these species grow well under the same conditions.

All cultural practices must be met, not just some, to grow the best quality plants. A day or two of carelessness may not kill the crop but it may reduce quality to the point where plants are unsalable.

Common names and a popular cultivar for these species are as follows: Hydrangea arborescens - Smooth or Wild Hydrangea, 'Annabelle'; Hydrangea macrophylla var macrophylla - Bigleaf, florist or French hydrangea (mopheads or hortensias), 'Nikko Blue'; H. macrophylla var normalis (lacecaps). 'Veitchii'; H.macrophylla var serrata - Mountain hydrangea. 'Preziosa'(var serrata has both mophead and lacecap flowers); Hydrangea paniculata - most often called "PeeGee's" but 'PG' is just one cultivar . . . var. grandiflora is 'PG'. 'Tardiva'; Hydrangea quercifolia - Oak leaf hydrangea. 'Alice' or 'Snow Queen.'

Light: Hydrangea arborescens and paniculata can grow well in full sun and may become leggy if grown in dense shade. However, both use large quantities of water when grown in full sun and should not be allowed to wilt. As a result, some growers produce these plants in light shade or in areas where plants will receive natural afternoon shade.

Hydrangea macrophylla and quercifolia should be grown under at least 30 % shade except in the coolest areas of NC. Most are grown under 50% or greater shade in the coastal plain and piedmont.

Water: Hydra, Greek for water (as well as a many headed monster slain by Hercules), are the first five letters of the species name. Hydrangeas need abundant supplies of available water in order to grow and develop properly. Hydrangea arborescens and paniculata need less water than macrophylla or quercifolia when grown under similar conditions. Although tolerant of moisture stress, both drought tolerant species will drop older leaves after bouts of moisture stress. Many cultivars of Hydrangea macrophylla and quercifolia will produce large, lush leaves in deep shade with abundant moisture and fertilizer. However, unless there is adequate air drainage problems with powdery mildew, slugs and even grey mold may develop. If the potting medium is kept constantly saturated, root rot diseases are likely to develop. A key to growing good quality hydrangeas in containers is to keep the roots wet enough but not too wet.

Potting Mixes: Standard pine bark based potting mixes have performed well for hydrangeas across North Carolina. Adding sand and/or sphagnum peat and other amendments is done by individual growers in order to manage irrigation properly. Except where all pH management, calcium and magnesium needs are met through the irrigation water, dolomitic limestone is generally added to the potting mix. Liming rates suitable for azaleas, camellias or rhododendrons are generally used for blue flowered Hydrangea macrophylla and all quercifolia. Hydrangea arborescens, paniculata and the pink flowered macrophylla are usually limed to a pH more suitable for other deciduous shrubs, i.e., around pH 6.5. However, both arborescens and paniculata are tolerant of acidic potting mixes. Chlorotic foliage can be expected in all of these hydrangea species if pH gets too high. Avoid excess liming of potting mixes and check irrigation water for pH, lime and salinity.

Fertilizers: Greenhouse and florist hydrangeas have been grown using completely soluble liquid fertilizers for years. However, the most commonly used hydrangea cultural practices used in NC are overhead irrigation together with controlled release fertilizers (CRF) applied at or following potting or in the spring. The CRF should last throughout the growing season so choose a release pattern to fit the crop and the climate, i.e., 180 days in the mountains and 270 days or longer in the coastal plain and piedmont for early spring potting.

Rates of CRF application depend upon the size of the plant, irrigation techniques and the plant species/cultivar. Rarely are high rates of fertilization needed when growing any but the macrophyllas. H. macrophyllas will respond to the label high rate and tolerate even higher rates of fertilizer application but rates higher than those listed on the label are usually not economically beneficial.

If chlorosis becomes a problem with any species, check for causes. It is not uncommon to spray chelated iron solutions or use a fertilizer containing extra iron when growing hydrangeas in containers but lack of iron may not be the reason for the chlorosis. Treat symptoms but find causes.

Blue flowered macrophylla will not flower blue without available aluminum, iron and acidic potting mixes (pH around 5.5 or lower). Most often, spraying or irrigating with an aluminum sulfate solution or top dressed when flower buds become visible can enhance the blue color of normally blue flowering cultivars that appear lavender or pink when grown in containers. Once planted into most NC soils, as soon as roots extend into the native soils the flower color will become the appropriate blue. Pink flowered macrophylla should have no aluminum or native soil available to the roots. In addition, potting mix pH should be maintained at pH 6.5-6.8.

Pests: The biggest problems with container grown hydrangeas involve too much or too little light and water. Next is over fertilization and growing plants so close together that inadequate air flow occurs.

Occasionally mites, caterpillars, grasshoppers, beetles and slugs are a problems but rarely do they occur on container grown plants that are not too crowded, potbound, moisture stressed and/or sunscalded. Pest problems are usually easily managed by consulting the Ag Chemicals Manual if other cultural problems are kept under control.

Pruning: All of these shrubs grow rapidly in containers and may need to be encouraged to branch to fill the pot. Generally hedge clippers are the pruning tool used in commercial nurseries, i.e., liners are hedged. Large, full liners should be put in containers. Even then, a mid season pruning is often necessary to increase plant density. This is most frequently done right after the propagation crew has taken all the cuttings they need for next years crop. Please check with the propagator before pruning.

Flowering will occur on most of these plants when they get large enough. Hydrangea arborescens and paniculata will usually flower the season they are planted into 3 gallon pots and allowed a full season's growth. Despite the alleged fact that Hydrangea macrophylla and quercifolia form flower buds one year but do not flower until the next year, it is not uncommon to see flowers on plants of these species undergoing rapid growth in a container nursery under shade. These plants will revert to "normal" flowering patterns by the second year they are in the landscape.

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