For Release On Or After 05/14/10

By Dan Gill
LSU AgCenter Horticulturist

With flower clusters like fluffy balls of cotton candy, hydrangeas are among the most popular summer-flowering shrubs for shady gardens. From now through July, huge flower heads of pink, blue and blends of those two colors appear above the rich green leaves.

Potted hydrangeas are popular gifts for Mother’s Day. When their flowers fade, these gift plants can be planted outside where they will provide beautiful flowers for years to come.

Hydrangeas are deciduous shrubs – they drop their leaves in winter – originally native to Japan. As their name implies, hydrangeas need an evenly moist soil and are intolerant of dry conditions. Allowing hydrangeas to wilt excessively, either in pots or in the ground, will shorten the life of the flowers and may cause the foliage to develop scorched edges.

You can display your potted gift hydrangeas in a bright, sunny window – and make sure the soil stays moist so the plant doesn’t wilt. When the flowers begin to look unattractive, cut off the flower heads and plant the shrub in an appropriate location in your landscape. Blooming hydrangeas may be available at local nurseries and could be purchased and planted into garden beds now.

Locate hydrangeas in a spot where they receive some shade during the day. A shady (two hours of direct sun) or partly shaded (about four hours of direct sun) bed is ideal. Avoid hot, sunny, dry areas or beds that are baked by the afternoon sun. Morning sun is much preferred by these plants, so an eastern exposure is excellent.

Hydrangeas do best in beds that have been amended with generous amounts of organic matter, such as compost, rotted manure or peat moss. Dig in a 2- to 4-inch layer of organic matter during bed preparation. The organic matter helps retain moisture in the soil.

Once planted, hydrangeas must be kept well watered. Water them deeply and thoroughly two or three times a week while they get established and once or twice a week later on if the weather is dry. Hydrangeas also benefit from a 2- to 3-inch layer of mulch, such as leaves, pine straw, cypress mulch or pine bark, over their root system to help maintain moist soil.

Interestingly, the colorful parts of the flower head are not part of the flowers at all. What you might think of as petals are actually modified sepals – or modified leaves. The actual flowers are tiny, inconspicuous and located in the middle of the four showy sepals. The flowers that have showy sepals are called sterile flowers. The mophead or hortensia hydrangeas have huge, round heads of sterile flowers. Another group, the lacecap hydrangeas, has a center cluster of small, fertile flowers, which are not showy, surrounded by a ring of attractive, sterile flowers. The popular variegated hydrangea is a lacecap type.

When the colorful flower heads turn green, it signals the ideal time to prune your plants. First, cut off the faded flower heads along with about 4 to 6 inches of stem. If you’re trying to reduce the size of your bush, cut the heads with longer stems.

If there are any especially tall shoots, or if you need to cut the whole bush back to reduce its size, you may continue to cut back shoots to achieve the desired results. Pruning stimulates new growth, creating a fuller, shapelier bush. This type of pruning must be done by the end of July. Hydrangeas set their flower buds for the next year in late summer, so any pruning after that removes the flower buds and reduces or eliminates flowers for the next year.

The flower heads of hydrangeas are rather unique. Depending on how acid or alkaline the soil the plant is growing in, the sepals can be different colors. If you have pink hydrangeas and you want them to be blue, treat the soil around the bushes with aluminum sulfate in March and again in October each year. Gradually, over time, the flower heads will turn blue. If your hydrangeas are blue and you want them to be pink, treat the soil around the bushes with lime following the same schedule. It may take a couple of years to be fully effective. The intensity of the color is controlled by genetics and depends on the variety you are growing. White hydrangeas do not have pigment in their sepals and are white regardless of the soil pH.

Don’t forget our native oak leaf hydrangea. With its large, pointed cones of white flowers that age to dusty rose and its dark green, attractive leaves, it also deserves consideration for shady areas of your landscape. Larger-growing than standard hydrangeas (it will grow up to 6 feet), oak leaf hydrangeas are also deciduous and have the added attraction of orange-to-burgundy fall color.

Also worth growing are Grandiflora (or Peegee) hydrangea with pointed clusters of white flowers on a larger, more tree-like shrub (up to 15 feet) and Annabelle hydrangea, which produces enormous white flower heads on bushes about 4 feet tall. Both of these hydrangeas differ from garden hydrangeas – they bloom on new growth and are pruned in late winter or early spring.

Rick Bogren