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How-to's for Hydrangeas

Hydrangeas are a popular shrub for the homeowner. From mid-summer until fall, they produce a glorious show in gardens. Versatile shrubs, they thrive in both sandy coastal soils and in shady woodland sites.

When we think of hydrangeas we usually think of bigleaf hydrangeas (Hydrangea macrophylla) and mountain hydrangeas (Hydrangea serrata). These are deciduous shrubs that are native to seacoasts and mountain valleys in Japan and divided into two types by the shape of their flowers. Mophead types (sometimes called hortensias) have large, ball-shaped, flower clusters. Lacecap types have flat, delicate clusters.

These shrubs are hardy from zones 5 to 9 and range from 3 to 6 feet tall. They present few challenges in well-drained soil that is amended with organic matter. The biggest problem that gardeners face is lack of flowers.

Here are some simple steps for troubleshooting:

  1. One common cause for lack of flowers is too much shade. Hydrangeas do well in partial shade provided by tall deciduous trees. Plant your hydrangeas in an area where they receive morning sun. They also thrive in full sun but may need extra water on hot summer days. (Bigleaf hydrangeas survive full sun in coastal areas once they are established.)
  2. Cultural practices are sometimes the cause of the lack of flowers. If hydrangeas are given too much, high-nitrogen fertilizer, they may produce lush foliage with few flowers. Good watering practices are also important for healthy plants. Hydrangeas benefit from 1 inch of water a week during summer. Bigleaf hydrangeas in full sun may need up to 2 inches a week during the hottest summer days.
  3. Winter dieback can be a problem in colder regions. Plant your hydrangeas in a sheltered spot or try creating a burlap windscreen or a burlap frame filled with dry leaves. Winter temperatures tend to be more constant on a north- or east-facing site. The south and west side of your property will heat up in the winter sun and may cause hydrangea buds to open prematurely.
  4. The biggest culprit for a lack of flowers is incorrect pruning. Bigleaf hydrangeas flower on old wood. They need minimal pruning, but if you would like to reduce the size of the plant, technically the best time to prune these shrubs is immediately after flowering. However, if cold winters kill the tips of the branches, prune in spring instead. Before the hydrangea starts to leaf out, remove deadwood and cut the stems back to a healthy set of buds. If you have an established shrub, you can also take out several of the older stems at this time. This will alleviate congestion and encourage new growth. Make sure that you do not remove too many buds, otherwise you will loose your blooms. 

New Ever-blooming Cultivars:

If all of this sounds too complicated there is hope for the hydrangea enthusiast. New cultivars have been introduced in recent years that flower on both old (last season's) and new (this season's) stems. These are sometimes referred to as ever-blooming hydrangeas. They flower almost continuously throughout the season.

These ever-blooming cultivars include: Hydrangea macrophylla 'All Summer Beauty', 'David Ramsey', 'Decatur Blue', 'Endless Summer', 'Mini Penny', 'Oak Hill', and 'PennyMac', Hydrangea serrata 'Blue Deckle', and 'Coerulea Lace'.

There are other vigorous old and new cultivars that flower over a very long season. These are sometimes called free-flowering hydrangeas. They include Hydrangea macrophylla 'Altona', 'Ami Pasquier', 'Europa', 'Forever Pink', 'Frillibet', 'Générale Vicomtesse de Vibraye', Lilacina', 'Lanarth White', 'Madame Emile Mouillere', 'Mousseline', 'Nikko Blue', and Hydrangea serrata 'Fuji Waterfall'.

Other good mountain hydrangea (Hydrangea serrata) cultivars include: 'Beni-gaku', 'Geisha Girl', 'Kiyosumisawa', 'Miranda', 'Miyama-yae-murasaki', 'Tiara', 'Woodlander' and 'Yae-no-amacha'.

The Relationship of Soil pH to Hydrangea Flower Color:

The color of bigleaf hydrangea blooms is not only determined by the particular cultivar, but also by the amount of aluminium in the soil and the soil pH. The soil pH will determine how available aluminium is to the plant. Acidic soil will  give you blue flowers (aluminium available to the plants) and alkaline soil will give you pink flowers (aluminium unavailable to the plants).

Warning: It is not always easy to manipulate the color of your hydrangea and some of them will not change their color regardless of what you do. On the other hand some of them will change their color if you adjust the soil pH and even provide you with both pink and blue flowers during the same season.

To decrease the acidity of your soil (to change flowers from blue to pink), add hydrated lime to your soil in the spring. To increase the acidity of the soil (to change flowers from pink to blue), add aluminum sulfate to your soil in the spring (follow directions carefully on label and don't over-do it.) Otherwise, pay attention to the three numbers on your fertilizer. The middle number is phosphorous. High phosphorous will help lock the aluminum in the soil providing your with a pink hydrangea, while fertilizers that are low in phosphorus and high in potassium will help turn them blue.

Other Hydrangeas to Consider:

Aside from the ever-popular, bigleaf and mountain hydrangeas, there is a wonderful selection of other species for the home gardener to choose from.

The oak-leaf hydrangea (Hydrangea quercifolia) is slowly becoming a staple of the East coast garden and it is hardy to zone 4. This is beautiful, southeast, North American native has ornate, peeling bark, large-flowering panicles that age well, and wonderful red fall color. It flowers on old wood and should be pruned immediately after flowering. It is a low-maintenance shrub that needs only a little pruning; simply remove dead wood and cut back a few stems when necessary to
maintain a full, healthy plant. It thrives in partial shade, but can tolerate quite a bit of sun. Some good cultivars are 'Alice', 'Amethyst', 'Pee Wee', 'Snow Flake', and 'Snow Queen'.
Another hardy hydrangea is the smooth hydrangea (Hydrangea arborescens). An East coast native, this hydrangea does best in partial shade to full sun. it grows from 4 to 6 feet tall and up to 6 feet wide. Smooth hydrangeas flower on new wood. You can treat them just like a perennial and cut them back in the spring. Some popular varieties are 'Annabelle', 'Grandiflora', and White Dome™.  
Panicle hydrangeas (Hydrangea paniculata) have long been a staple of the home garden. They can be grown as a shrub or a small tree. They flower on new wood and can be pruned in the spring according to the desired form. They can be kept small by cutting back to 1 to 3 feet or allowed to grow much larger. For a small tree, prune the crown of the tree to 3 to 5 main branches. As with all hydrangeas, remove any weak or dead branches. Panicle hydrangeas tend to flower later in 
the season than other species. The large flower panicles can range from white to pale pink to lime green. Cultivars include: 'Burgundy Lace', 'Chantilly Lace', 'Grandiflora', 'Limelight', 'Little Lamb', 'Pee Wee', and the late-flowering 'Tardiva'.

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