February 2004
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Garden design using shade plants and shade trees   

Gardening Newsletter - February 2004, Issue 18

Intrigued by an article I recently read, I started to research hydrangeas. Of course I studied them at the New York Botanical Garden (NYBG) and sold them when I worked at a garden center. Nevertheless, I never heard anyone in the horticulture world discuss the differences in growing conditions the different types of hydrangeas prefer. Some need more shade than others and likewise some need more water. Yet, I am sure that many professionals aren’t aware of these differences. You will be though once you read this addition of the newsletter.

I realize that I am presenting a lot of detailed information in this newsletter. Many of you may not be interested in knowing the details of the different types of hydrangeas. You may just want to know which hydrangea will grow best in your garden. To help make it easy for you, I have included a table that briefly states the growing conditions and a few facts about each hydrangea. Once you find the hydrangea that is right for you on the table you can look up the specific details in the body of the newsletter.

If you aren’t very familiar with hydrangeas you may be wondering why I am devoting so much effort to them. Hydrangeas are absolutely beautiful plants. No shade garden should be without them. While many shade plants have tiny, pale flowers, hydrangeas have big, bold, and sometimes deeply colored flowers. Their foliage is attractive when the plant is not in bloom, making an excellent backdrop for other plants. In addition, the flowers can be dried and used to make flower arrangements. Unfortunately, deer do eat them though.

If you would like to learn more about hydrangeas join The American Hydrangea Society. Their web site is: http://www.americanhydrangeasociety.org

Rebecca Green
Horticulturist

Hydrangeas: The Untold Story

Many years ago my husband and I went to New Zealand. One of the things I remember best are the wonderful hydrangeas that were in a park in Auckland. As soon as we got home I ran to the local garden center and bought several hydrangeas. I didn’t know much about plants then and have no idea what kind of hydrangea I purchased. I planted the hydrangea along a fence in full sunlight. The poor things suffered terribly and looked miserable. My husband absolutely hated them. We moved about a year later, but I suspect they probably died shortly thereafter.

My early experience with hydrangea is probably similar to that of a lot of people. When I worked in a garden center a lot of customers purchased hydrangeas to plant in full sun. I tried explaining to them that hydrangeas are shade plants and need a lot of moisture. Of course the customers didn’t want to hear this and went ahead and purchased what they thought were the prettiest plants.

What I didn’t realize at that time, and neither did my colleagues, was that while hydrangeas are generally shade plants, there are a few that can tolerate full sun. In addition, once planted in your garden the flowers may look very different than they do in the container. This is because the plants adapt to the soil where they are planted.

The table below summarizes some of the key characteristics of the hydrangea you are most likely to find in commerce. Details about each of these hydrangea and how to propagate hydrangea and subsequently provided in this article.

Botanical
Name
Common
Name
Growing
Conditions

Hardiness
Zones

 Flowers  Pruning
Hydrangea macrophylla Bigleaf Shade, need moisture 5b-8 mopheads & lace caps, pink,    white & blue Immediately after flowering
Hydrangea arborescens Annabelle or smooth Morning sun, or all day partial shade, needs moisture 3-7 Like mopheads, white Fall, winter, or early spring
Hydrangea paniculata Paniculata or PeeGee Tolerates full sun, shade 3-8 Cone shaped, white Spring
Hydrangea quercifolia Oakleaf Tolerates full sun, shade, doesn't need much moisture 3-8 Cone shaped, start light green, turn white, then tan After flowering
Hydrangea anomala subsp. petiolaris Climbing Tolerates sun or shade 4-7 White Prune to keep small

Hydrangea macrophylla, commonly called bigleaf hydrangeas

Hydrangea macrophyllaThese are the hydrangea that most people are familiar with. They are popular for growing in pots. They are also the ones that are typically grown in southern gardens. Most hydrangea macrophylla are either blue or pink, some are white. Many are called lacecaps, which means they have both sterile and fertile flowers, which gives them a lacy appearance.

Hydrangea macrophylla is only hardy to zone 5b. If it becomes too cold the flower buds, which are formed on the tips of the branches, freeze and die. This is the most common reason why they sometimes don’t flower. They are also susceptible to late frosts that can kill the leaf and flower buds. They do best in zone 8. They are especially well suited to coastal conditions and the Pacific Northwest. However, they do need shade.

If you live in one of the colder zones where Hydrangea macrophylla lives you will need to insulate your plant in the winter. Building a cage around the plant and gently packing the cage full of oak leaves does this. Oak leaves are considered the best since they don’t pack down as much as other types of leaves. However, you may find it necessary to add additional leaves to the cage area as the leaves settle. Especially make sure the tips of the branches where the flower buds are covered with leaves.

If you are growing your Hydrangea macrophylla in a container and live in the northern end of its hardiness you will need to bring it indoors for the winter. Unfortunately, hydrangeas have very aggressive roots and need to be planted in large containers that are heavy and difficult to move.

This shrub can get to be quite large. On average they are between 3 to 6 feet tall, but can grow to be 10 feet tall. They grow as wide or wider than they do grow tall.

Pruning should be done as soon as the flowering season is over. Once the current season’s flowers are spent the plant produces buds on the branch tips for the following season. Consequently, you want to prune before the new buds are formed to ensure you don’t cut off next season’s flowers. Never prune this plant in the fall.

Since these plants flower on new wood, when pruning remove all old flowering shoots down to a point on the stem where new shoots are sprouting. Do not cut off any new shoots.

Cultivars of Hydrangea macrophylla are divided into two groups: Lacecaps have flat flower heads with small, fertile flowers in the center and larger infertile flowers on the edges; Hortensias or mopheads have nearly spherical flower heads comprised of large sterile flowers.

Only hydrangea with naturally pink flowers can be made to bloom blue. If the soil is highly acidic (a pH of 5.5 or lower) where these plants are planted the flowers will turn blue. If the pH is greater than 5.5 these plants will bloom pink. White flowers are not affected by pH. If you live in the south you will not be able to grow a deep red flower.

There are several ways to make the soil more acidic to encourage blue flowers to grow. The most reliable way of changing a pink flower into a blue flower is to use one of the commercially prepared blueing agents. Adding organic matter, such as coffee grounds and citrus peel to the soil is a natural method of making your soil acidic. Alternatively you can add aluminum sulfate to the soil. However, care must be taken not to burn the roots of the plant when using aluminum sulfate. Add 1-tablespoon aluminum sulfate to a gallon of water. Before watering the plant with this solution thoroughly water it with normal water.

If your soil is naturally acidic the flowers will ten to be blue.

The fertilizer you use also influences the color of the flowers. Fertilizer low in phosphorus and high in potassium helps produce blue flowers. A fertilizer with the numbers 25-5-30 on it would help produce blue flowers. The last number, 30 in this example, is the amount of potassium. The middle number, 5 in this example, is the amount of potassium. You want the middle number to be low and the last number to be high. Although potassium encourages all types of plants to flower more, in Hydrangia macrophylla it will encourage the flowers to be pink.

Similarly the fertilizers you use may also help turn the flowers pink. If the middle number, which represents the amount of phosphorus, is high the flowers are more likely to be pink. A fertilizer with the numbers 25-10-10 would be good for pink flowers. The fertilizer called Superphosphate and bone meal would also help turn flowers pink.

If you want blue flowers don’t plant your hydrangea near the foundation of a building. The concrete can leach lime, making the soil more alkaline and the flowers pinker.

To make the soil less acidic, more alkaline, add lime. This will turn your flowers pink. You will need to add lime several times a year. You should try to keep you soil pH between 6.0 and 6.2.

Still it is difficult to control the shade of the color. Once a plant is transplanted the flower color may change as the plant adapts to the new soil around it. It is not uncommon to find a plant with multiple colors on it the year after it has been transplanted. If you plant your hydrangea in a container you will have more control over the soil chemistry and be better able to control the color of your flowers.

It is also difficult to change the intensity of the color. Color intensity is largely determined by the plant’s genetics, although weather and plant health may influence it somewhat. Consequently, fertilizing the plants may slightly intensify the color. The deeper the pink the deeper the blue color will be.

Hydrangea arborescens ‘Annabelle’, commonly called Annabelle hydrangea or smooth hydrangea

Hydrangea arborescens ‘Annabelle’Most Hydrangea arborescens that are sold are the cultivar ‘Annabelle’, as its flowers are considered the most attractive of all the Hydrangea arborescens. They are over ten inches in diameter and white. Changing the soil pH will not change the color of Annabelle’s flowers. The flowers start off pale green, then turn white, and finally turn tan. They are attractive for 6 to 8 weeks.

This hydrangea will grow to a height of about 3 to 5 feet with a similar width. It suckers freely and will cover large areas if it isn’t restrained.

Annabelle does best with morning sun and afternoon shade. It also does well with partial shade throughout the day. The blooms are short lived when planted in full sun.

Annabelle grows in a variety of climates. It is hardy to zone 3. Although it is often difficult to grow hydrangea in frost-free areas, Annabelle does quite well in the San Francisco. This may be because San Francisco’s climate is cool compared to that of many frost-free areas.

Annabelle looks a lot like a mophead hydrangea, Hydrangea macrophylla. However, it is much more cold hardy than the mopheads. If you live in a cold location you will probably have better success with Annabelle than the mopheads.

The biggest challenge when growing Annabelle is to keep the stems upright. Typically when it rains the flowers become so heavy with water the stems bend down to the ground. Three ways to possibly keep this from happening are:

  1. Plant three or more plants close together. As the plants become larger they will grow together and help support each other. This hydrangea is sometimes planted as a hedge.
  2. Don’t heavily prune the plant. As the stems age they will grow stouter and will be able to better support the heavy flowers. Also, the flowers will be slightly smaller and not as heavy. However, in northern climates the stems may die back completely to the ground, necessitating drastic pruning.
  3. Put a tomato cage around the plant in early spring. As the shrub grows it will hide the tomato cage.

Fortunately, this hydrangea blooms on new wood. I say fortunately, because the branches can look terrible in the winter. For a tidy appearance, cut this plant to the ground in the winter. In the spring it will send up new shoots that will form flower buds.

In theory Hydrangea arborescens is susceptible to many insects and diseases. In reality most gardeners find them to be very trouble free plants.

Hydrangea paniculata, commonly called paniculata hydrangeas

Hydrangea paniculataThe word “paniculata” refers to the fact that the flowers of this plant are usually panicle-shaped. That is, they are shaped like a cone. While most Hydrangea flowers are shaped like a ball, oakleaf Hydrangea (botanically Hydrangea quercifolia) also have cone shaped flowers. The way to differentiate between paniculata and oak leaf is the leaves of oak leaf Hydrangea strongly resemble the leaves of oak trees, which look like they have protruding fingers.

This hydrangea’s flowers are always white. Unlike some other varieties of hydrangea, altering the soil pH will not make them turn pink or blue. Their flowers are in their glory in August and September.

Hydrangea paniculata are often called PeeGees. They get this nickname from the botanical name of the most popular cultivar of Hydrangea paniculata, Hydrangea paniculata ‘Grandiflora’ (P.G.). There are other cultivars of Hydrangea paniculata that are not PeeGees, but are frequently call that, even by people in the profession. This is a case where using a plant’s nickname or common name, such as PeeGee, may not get you the exact plant you are looking for. It is always best to use the complete botanical name when referring to a plant.

Frequently Hydrangea paniculata are grown on a standard, such that they look like tiny trees with a thin trunk. As most small trees grow to be much bigger over time, this form of Hydrangea is excellent when a tiny tree is needed in a small space.

This hydrangea should be pruned in the spring. The shoots of the past year’s growth should be pruned to an inch or two of their bases. When new shoots develop in the spring they should be reduced in number. This will ensure the remaining new shoots develop fine flowers.

Hydrangea paniculata are not very particular about their growing conditions. They grow as far north as Zone 3, which is much farther north than most. In fact, it does best in the cooler climates. They also tolerate full sun, while most hydrangeas like shade. They are excellent for low water gardening. This is perhaps one of the easiest hydrangeas to grow.

Hydrangea quercifolia, commonly called oakleaf hydrangea

Hydrangea quercifoliaHydrangea quercifolia is native to the United States. It is abundant in Georgia. The only other hydrangea to be native to the United States is Hydrangea arborescens.

This is my favorite hydrangea! The flower heads are shaped like large clusters of grapes, which are made up of both small fertile and large infertile flowers. They always start out white, but fade to a light pink color. The leaves are quite large and shaped like oak leaves. In the fall they turn a wonderful deep burgundy color.

The word ‘quercifolia’ is the botanical Latin for oak, referring to the shape of the leaves.

The typical oakleaf will grow to be between 4 to 6 feet tall and spread as wide or wider. However, in the south, where these shrubs flourish, they can grow to be 10 to 12 feet tall. Like many hydrangea, oakleaf suckers and can grow extremely wide.

This is the ideal plant for the garden that has dry, dark shade. Although many plants become lanky in dark shade, Hydrangea quercifolia grows quite dense. However, the fall color of the leaves will not be as striking in deep shade as if you grew it in partial shade. It also does very well in sandy soil, which is the driest type of soil. Most hydrangeas need a lot of water. As such, this is a good plant to use in a remote location where you cannot easily water. This hydrangea is susceptible to rot root if it gets too much water.

Hydrangea quercifolia blooms on old wood, wood that has been on the plant for at least nine months. As such, you should prune them immediately after flowering. If you prune them in late fall or early spring you will cut off the flower buds.

There are numerous cultivars of Hydrangea quercifolia. The growers have bread these plants to be supposedly superior to the native plant. The native Hydrangea quercifolia has single flowers, while many of the cultivars have doubles. Doubles have more flower petals, making the flowers look more voluptuous. ‘Snowflake’ is the most common double and has a very long blooming time. ‘Harmony’ is another double, but may be a little difficult to find. ‘Sikes Dwarf’ and ‘Pee Wee’ are singles that only grow to be about four feet high and wide.

Perhaps as important or maybe more so than the flowers is the fall color these plants provide. They can turn wonderful shades of red, orangish brown and purple. In the south the leaves typically stay on the plants until late November or December, and even until February in mild years.

Hydrangea quercifolia is hardy to Zone 4b/5a. However, it needs sunny hot summers to bloom well. That is why it is so prevalent in Georgia.

Hydrangea quercifolia is difficult to grow in containers. It is susceptible to leaf blight while in containers. However, when planted in the ground it is quite trouble-free.

Hydrangea anomala subsp. petiolaris, commonly called climbing hydrangea

Hydrangea anomala subsp. petiolarisThis hydrangea is a vine. It is a deciduous, woody climber, clinging by aerial roots. It bares white flower heads comprised of fertile and infertile flowers. The sterile flowers are larger and shower and encompass the small fertile flowers. It can grow to be between 50 to 80 feet long, but can be kept smaller by pruning. It is native to Russia, Korea, Taiwan, and Japan.

Climbing hydrangea will grow in either full sun or partial shade. It is adaptable to many kinds of soil, but prefers well-drained, moist soil. It grows between zones 4 and 7, not caring for the south.

Climbing hydrangea puts on a fabulous flower show for approximately two weeks in late June or early July. The flowers are between 6 and 10 inches in diameter and very sweet smelling.

The leaves are a glossy dark green and very lovely. Sometimes they do not change color in the fall, falling off while they are still green. However, occasionally they will turn a stunning yellow color in the fall.

The bark of this vine looks like peeling cinnamon sticks. It is very attractive in the winter when it looses its leaves.

This non-aggressive vine adheres by root-like tendrils. This makes it an excellent vine to grow on the side of buildings or up trees. However, it gets very woody and heavy. Consequently, it needs to be well supported. It is considered by many in the horticulture profession to be the best climbing vine.

Unlike some other vines, such as ivy (botanically called Hedera helix), climbing hydrangea is three-dimensional. While ivy grows flat against the structure it is climbing, the branches of climbing hydrangea protrude, creating a three-dimensional effect and interesting shadows.

When it is first planted, climbing hydrangea is a slow grower. However, as the plant settles in to its new location it will start to grow quicker (approximately 2 feet per year) in subsequent years.

Being native to the orient, climbing hydrangea is very effective in the oriental garden. However, it looks wonderful in virtually any style garden.

Climbing hydrangea has no serious pests.

Propagating Hydrangeas

It is easy to start new Hydrangea plants from the ones you currently have. There are several methods of doing it.

Rooting in Water

Place a cutting in water and leave it there for a long time. Occasionally the cutting will start to grow roots.

Rooting Cuttings

Take a cutting from a branch that flowered this year. The cutting should be between 5 and 6 inches long.

Remove the lower leaves off of the cutting so there are only leaves at the very top.

Of the remaining leaves, cut the largest leaves in half. 4. Dip the cuttings in rooting hormone and insert them in a sterile growing medium, such as damp vermiculite. Keep the medium moist, but not soggy. Water only when the top of the soil feels dry. 5. Cover the container with plastic. Do not let the plastic touch the leaves.

The cuttings should be put in a bright location, but never in direct sun-light.

Ground Layering

Select a branch near the ground. Do not cut the branch off of the plant.

Remove the leaves where the branch touches the ground and scrape a small amount of the bark off the bottom of the branch.

Dig a trench and lower the branch into it. Cover the branch with soil.

Put a large stone or brick on top of where the branch is buried.

Water occasionally.

Once the branch has grown roots it may be cut from the parent plant and relocated.

That's all for now. If you have any comments or questions on the newsletter, please email us at

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