General Care:

Hydrangeas are workhorses in the garden. They produce sumptuous petals of color to add beauty to your garden. But they need not be forgotten once their flowers have died. Hydrangeas can be captured easily in dried flower arrangements, providing the last gasp of the summer garden. Pick the blooms on a clear dry day in late summer and hang to dry. H. macrophylla and H. paniculata 'Grandiflora' are especially good for drying.

Most species of hydrangeas can be grown in either full sun or partial shade. They are resistant to most insects and diseases. They can be grown in a wide range of soil but prefer a rich, moist soil and should be planted where watering will not be a problem. Avoid dry windy sites, as their large, soft leaves lose water quickly, especially on hot, windy days, causing the foliage to wilt.

Flowers are produced on the new growth; therefore, it is important that hydrangeas be given some care each season to ensure that vigorous new growth occurs. Irrigate plants weekly to replace moisture loss. Prune flower stems after they have bloomed.

Fertilize hydrangeas in early spring with a complete granular fertilizer at the rate of 2 pounds per one hundred square feet. Apply fertilizer as broadcast top dressing. During long periods of drought, water thoroughly each week.

Pruning:

There are many different types of hydrangeas; many require different methods of pruning. First, one must know the identity of the hydrangea in the landscape, so the proper pruning procedure can be implemented.

Hydrangea arborescens 'Grandiflora' -- Hills-of-Snow Hydrangea
Prune hills-of-snow hydrangea to the ground line each winter or early spring because it flowers abundantly on new growth, and is frequently killed back during winter. If a larger shrub is desired (3+ feet) and/or it is not killed back over the winter, prune less severely. Remove certain branches to the ground; cut others back at varying heights from 1 to 3 feet.

Hydrangea paniculata 'Grandiflora' -- Pee Gee Hydrangea
This is the most commonly planted hydrangea because of its massive displays of large white flowers in mid-to-late summer. They gradually turn to pink and remain on the plant in a semi-dried condition long after the leaves have fallen. Pruning involves the removal of dead flowers, if unattractive, and an annual corrective pruning of vigorous shoots. Thin and/or cut back the previous season's growth in late winter or early spring, since flower clusters occur on newly developing branches. Without regular pruning, this hydrangea can rapidly become quite overgrown and out of scale in the landscape. It can, however, be developed into a single or multi-stemmed tree form.

Hydrangea macrophylla -- Hortensia or Florist Hydrangea
This is a commonly grown hydrangea with large globe-shaped flowers. It is frequently forced by florists and sold as an indoor pot plant during the spring season. Once moved outdoors, however, color is dependent upon the pH of the soil in which it is grown: blue if acid; pink if alkaline. There are also several white-flowered cultivars. Pruning can be accomplished at two different times. Late summer is more desirable, since most hortensia types flower only from the end buds of upright or lateral shoots produced during late summer and fall of the previous season. Prune as soon as the flowers have faded and strong shoots are developing from the lower parts of the stems and crown. Remove at the base some of the weaker shoots that are both old and new. Always try to keep several stems of old productive wood, with a sufficient number of stout new stems that will flower the following season. Early spring pruning (March), although acceptable, will result in the sacrificing of bloom for that growing season.

Pruning this species too late in the fall (after September) is harmful. New growth, both vegetative and reproductive, will not develop proper maturity. Hortensia is a good seashore shrub; flowering is more profuse in an open, sunny location. This, however, increases its vulnerability to bud killing. Winter protection of the plant should be initiated in December to preserve buds for next year's flowering. Tie the shoots together and wrap with burlap. If left unprotected, delay any spring pruning until the buds swell in order to determine which wood needs to be removed, and then cut back to below the point of injury.

Hydrangea quercifolia --Oakleaf Hydrangea This plant is grown primarily for its handsome oak leaf-shaped foliage, excellent fall color, attractive flowers and interesting winter bark. It is ideally suited to a lightly shaded or protected location, and if grown in an exposed site, it is subject to some winter dieback. Prune back in early spring to remove dead wood. Cut back to below the point of injury and remove old wood to the base.

Hydrangea anomala petiolaris -- Climbing Hydrangea Climbing hydrangea is a desirable mid-summer flowering woody vine that attaches itself by aerial roots to brick, masonry or wood. It requires little or no pruning. If certain shoots have grown out of bounds, reduce their length in summer. Frequently, concern is expressed about climbing vines that may be inundating a tree and causing irreparable damage. There has never been a proven case of damage occurring from climbing hydrangea, however.

Color Confusions:

The color of the pink and blue varieties is determined by the degree of soil acidity. Pink varieties develop color best in the greenhouse at a soil pH of 6.5 to 7.0., while the best blue color occurs at pH 5.0 to 5.5.

The hydrangea flower is small and insignificant but is surrounded by showy bracts. The anthocyanin pigment will be pink or blue, depending upon the incorporation of either iron or aluminum in its molecules. Insufficient aluminum is absorbed by the plant at a pH of 6.5 to 7.0 so iron is used.

As the pH drops to 5.0 to 5.5, aluminum becomes more soluble, is absorbed by the plant and becomes part of the anthocyanin molecule resulting in blue coloration.

White hydrangeas contain no pigment in their sepals (although some may develop as the blossom ages). They may be grown as either pink or blue, the only indication being given by the small petals. Common white varieties are Sister Therese and Regula. Rose Supreme, Merritt's Supreme and Merveille are examples of varieties which can be blue or pink depending on soil conditions.

To maintain pink coloration, enough limestone should be incorporated in the soil at the time of planting to keep the pH at the above levels.

The aluminum necessary for blue color will become soluble in nearly any soil at a pH of 5.0 to 5.5. Depressing the pH with aluminum sulfate will also supply soluble aluminum. Since soluble phosphates precipitate aluminum, a fertilizer containing low phosphorous or none at all should be used (13-0-20 and 15-0-15 are examples).

While these methods of changing the flower color can be used, it's not a wise idea to do it indefinately. You risk severely altering the soil pH thereby making it difficult for the plant to utilize nutrients. Always get the soil tested before altering it. Refer to GreenShare Factsheet on soil testing for more information. If you really like different colors in your hydrangeas then get many different cultivars!

Holiday Plants:

If you obtain a potted hydrangea for Easter or Mother's Day in full bloom, keep the soil moist at all times as this plant has a high water requirement and tends to dry rapidly in the home. It should also receive direct light.

After the flowers fade, they may be removed and the plant treated as a house plant. When danger of frost is past, it can be planted in a sheltered location in the garden. Shelter is necessary because the hardiness of this plant is questionable in our northern gardens. This plant forms flower buds in the fall like forsythia and dogwood. It is these buds which freeze out or are destroyed in colder climates. Many times the plant will survive and produce green leaves, but no flowers. Planting in a sheltered location plus covering the plant with burlap will offer some protection. If you wish to improve the probability of flower bud survival, you may wish to try this.

When danger of frost is past, plant in a sheltered place in the garden. On the 4th of July, prune to 3 to 5 inches from the ground. The plant will produce short branches terminating in a dormant flower bud. Before the temperature falls to 25 degrees F, place a screen around the plant. Fill this with an insulating material such as coarse peat moss, vermiculite or bark. More material may be added if it settles during the winter. Remove this protection when the crocuses flower but protect on cold nights. If you are lucky, the flower buds will survive. In any event, you should have a pretty foliage plant.

If you are successful in over-wintering these varieties, fertilize with your usual garden fertilizer (such as 5-10-10). If you desire blue flowers, have your soil tested as a low pH is needed. 'Nikko Blue' is a common true blue variety. To lower the pH, apply aluminum sulfate or sulfur. Avoid over acidification and "playing" with the soil chemistry and plants. For pink flowers, limestone may be necessary to raise the pH since soils are naturally acidic in Southern New England. Always choose varieties for the colors you like.

If you are not successful in over wintering greenhouse varieties, hardy hydrangeas are available. Some are: Hills of Snow Hydrangea (H. arborescens Grandiflora, similar to the greenhouse varieties), Pee Gee Hydrangea (H. paniculata Grandiflora) or Climbing Hydrangea (H. anomala petiolaris).

Adapted by Rosanne Sherry, URI Master Gardener Coordinator, 7-98
Based on fact sheets from Cornell-Suffolk Co. Long Island New York Cooperative Extension, Connecticut Cooperative Extension and Virginia Cooperative Extension.