Garden Myths - Learn the truth about gardening

Hydrangea Myths

Hydrangeas are very popular shrubs for the garden, but they can be a bit tricky to grow if you listen to all of the hydrangea myths on the internet and in books. In this blog I will look at the truth behind some of the more common hydrangea myths. Once you have the facts, you will find that hydrangeas are easy to grow.

Hydrangea myths, is it blue or pink?

Hydrangea myths, is it blue or pink?

Hydrangea Myths

Some of the myths presented here apply to all hydrangeas. Others deal with only one or two hydrangea types. I have discussed the hydrangea types in Hydrangea Identification which will help you determine which type you are growing. Armed with that knowledge, the following myths will make more sense.

Hydrangea Need Lots of Water

This is probably the most common hydrangea myth and stems from the name, hydrangea. You see comments all the time such as, “you know, it needs lots of water – that is why they are called hydra”.

The name hydrangea is made up of two Greek words “hudro”, meaning water, and “angeion”, meaning vessel.

This reference to the ‘water vessel’ probably refers to the cup shape of its seed capsule and not to a requirement for water. In fact hydrangea do not need more water than other shrubs. H. macrphylla and H. quercifolia do enjoy more moisture but they don’t need to be grown wet.

Fertilizer will Make Hydrangea Bloom

The idea that extra fertilizer will make plants bloom is a myth. If the soil is not deficient of a nutrient, adding fertilizer will not make a plant bloom. Neither will bloom boosters and bone meal.

Too much fertilizer, particularly high nitrogen fertilizer, can result in beautiful leaves but few, if any, flowers.

To better understand why your hydrangea does not flower, have a look at this post; Why Hydrangea Don’t Flower?

Fertilizer Will Change the Color of Blooms

Not true. See the next section on pH.

pH Changes Bloom Color

This statement is definitely not true for most hydrangea. Most of them are white, changing to greenish or pinkish colors when they age. It is only the H. macrophylla and H. serrata that show blue or pink depending on soil conditions. H. paniculata and H. arboescens will not change color with a change in pH.

The change of color between blue and pink does not happen because of a change in pH. If you have soil that contains no aluminum, you will never have blue flowers, no matter what the pH is. It is the aluminum in the soil that makes the flower blue.

So why do so many people recommend changing the pH of soil to cause a color change? To understand this better we need to look at what happens in soil when pH changes. In alkaline conditions, the aluminum molecules are tightly held by the soil so that they are not available to the plant roots. It is as if there is no aluminum in the soil, and flowers are pink (ref 3).

As the pH in soil becomes more acidic (below 5.5), aluminum is released from the soil and made available to the plant. The plant absorbs the aluminum, transports it to the flowers, turning them blue. If your flowers look purple, they are getting some aluminum but not enough to create the real blue color so many people want.

Excess phosphorus will also tie up aluminum making it unavailable to plants (ref 2).

Adding Aluminum Creates Blue Blooms

This is partially true and partially false, and of course only applies to the macrophylla types.

Adding aluminum to soils that are alkaline will not make blue flowers. As discussed in the previous section the aluminum will just be absorbed by the soil and plants can’t use it.

Some soil, such as potting mix, can be deficient of aluminum. Even when this soil becomes acidic, you won’t get blue flowers unless aluminum is added to the soil. In this case the statement is true.

Adding Iron Prevents Chlorosis

Chlorosis shows up as a yellowing of the leaves and veins. This is not due to an iron deficiency. What people are referring to when they talk about an iron deficiency is ‘interveinal chlorosis’, where there is a yellowing of the leaf spaces between the veins. This is clearly seen on the lower leaves in this photo.

Oakleaf hydrangea showing interveinal chlorosis on the lower leaves, by Robert Pavlis

Oakleaf hydrangea showing interveinal chlorosis on the lower leaves, by Robert Pavlis

This symptom can be caused by an iron deficiency, but it can also be caused by other nutrient deficiencies. It can even be caused by high levels of nutrients such as phosphate, manganese, copper or zinc. In alkaline conditions iron is converted to an insoluble form which is not available to plants. Hydrangea prefer a slightly acidic conditions where they have easier access to iron.

If the problem is high levels of other nutrients, or a high pH, adding iron to the soil will not prevent interveinal chlorosis in hydrangea. To learn more about this have a look at Chlorosis in Plants – Is It Iron Deficiency?

My soil is slightly alkaline at pH 7.4. Hydrangea tend to have green leaves in spring and early summer but as the summer progresses, the lower leaves start to show interveinal chlorosis. I suspect the plant is short of iron and is moving it from older leaves to the newer top leaves – I don’t know this for a fact, but plans do move some of the nutrients in this way. How do I deal with this problem? I do nothing. The plants grow just fine and bloom well. Why solve a problem that does not exist.

Hydrangea Hardiness is Misleading

Hardiness zones for hydrangea can be very misleading. They almost always indicate the hardiness of the plant. Many H. macrophylla are hardy in zone 5 and are sold here all of the time. The problem is that the roots are hardy, but the stems and buds are not. Since this plant flowers on last years growth and that is killed in winter, the plants hardly ever flower in zone 5.

The same holds true for the oakleaf hydrangea. It is quite hardy in zone 5 but rarely flowers in zone 5.

This really is not a problem with plants that flower on new growth, like paniculata and arborescens types, but if you are buying one that flowers on old wood, check the bud hardiness – or you might never see flowers on the shrub.

Hydrangea Macrophylla are Blue or Pink

This species also has purple and white cultivars. Purple is not surprising but white ones are less uncommon.

When people ask for help, on social media, to get their hydrangea to flower it is common to ask them if it is a blue or pink one. If they answer, no, it is assumed it is not a macrophylla and incorrect cultural information is provided.

White flowers do not help in identifying the type of hydrangea.

If you are trying to identify the type of an hydrangea, have a look at Hydrangea Identification which will help ID a plant without flower color.

Hydrangeas Turn Blue if You Bury a Nail Under Them

People claim that a lot of things can turn an hydrangea blue, including razor blades, hairpins and copper pennies. To have any chance of working, these items need to be made of aluminum or at least have a significant amount of aluminum in them. Most nails are made of steel, not aluminum.

Secondly, the item would need to corrode quickly enough so that the aluminum gets into the soil. Aluminum is very stable and corrodes slowly. Aluminum nails exist mostly because they don’t corrode.

Copper seems like an odd choice, but there is some sound chemistry here. Copper would link up with hydroxyl ions which in turn lowers the soil pH, thereby freeing up aluminum. The problem is that copper is very stable and corrodes very slowly. Copper pennies does not change soil pH.

It is better to use aluminum sulfate. The sulfate will acidify the soil, and the added aluminum makes sure that there is enough aluminum in the soil. Personally, I would not use aluminum sulfate because aluminum is toxic to many plants.

Hydrangeas Need Hard Pruning Every Year

Macrophyllas will bloom well with light pruning of the current years wood – hard pruning is not required.

According to Michael A. Dirr (ref 2), an expert on hydrangea, macrophylla produce flower buds all along the stem. Normally only the apical buds, the ones at the tip of the stem develop into flowers. But if you cut the stems back, the remaining buds near the cut will flower. In fact, even if you cut them to the ground in spring, they will still flower, but very low to the ground.

The oakleaf hydrangea may not flower if pruned to the ground.

Other types of hydrangea do not need to be pruned at all. With no pruning they will be taller each year. Alternatively, you can prune to any height that suits you, even right to the ground.

No hydrangea needs hard pruning – that is an hydrangea myth.

Hydrangeas Won’t Grow in Pots

This is not true. They actually grow quite well in pots, but this type of culture is better suited for the smaller cultivars.

Sun or Shade – Which is best?

There seems to be lots of advice about how much sun or shade hydrangeas need. The problem with this advice is that it rarely includes the type of hydrangea and the climate. Unless both of these are stated, the advice is suspect. The amount of moisture in the soil is also important.

The following is a general guide.

In US climatic zones of 6 or colder:

  • All hydrangeas can take full sun provided they have enough moisture.

 

In US climatic zones 7 and warmer:

  • Some shade should be provided for H. macrophylla, H. serrata and H. quercifolia. H. quercifolia would do best in full shade. H. arborescens can take full sun in warm climates but probably does best with some shade. H. paniculata can take full sun even if dry.

Flowering Times Not Correct

Lots of references will provide the flowering time for a particular cultivar. The problem with this information is that flowering time varies depending on climatic zones.

For example, Michael Dirr (ref 2) provides the following flowering times for H. quercifolia.

  • Thomasville, Georgia – late April
  • Athens, Georgia (200 miles north of Thomasville) – mid May
  • Chicago, Illinois – late June

You also can’t count on the flowering times of plants in a nursery since they flower based on the climate where they were grown. Particularly in the north, many plants are grown farther south.

Single-node Cuttings Work

The standard recommendation for propagating hydrangeas, especially macrophylla, is that double-node cuttings should be used. Although this works, it is not necessary.

Hydrangea cuttings root both at nodes and internodally (between nodes). A single node works well. Hydrangea can even be propagated by split-node cuttings. These are single node cuttings, which are split vertically along the stem so that each half contains one dormant bud. This method produces twice as many cuttings from the same material.

Annabelle and PG Confusion

Annabelle is the cultivar name for one of the most popular arborescens type. It has been around for a long time and is found in many gardens.

PG is the common name for Hydrangea paniculata ‘grandiflora’. PG are the initials of paniculata and grandiflora. It too has been around for a long time and is very popular.

Over the years these plants have become so popular that their name is used to identify the type of hydrangea. So Annabelle is used for any arborescens hydrangea, and a PG for any paniculata type. This does not sound like a big deal, but it leads to all kinds of incorrectly labeled plants. Anyone with a white paniculata sticks the name PG on it. When you buy a PG, you have no idea what you are buying.

The names Annabelle and PG should only be used for the correct cultivar.

More Hydrangea Myths

If you know of any other hydrangea myths please add them to the comments below.  If I can validate them I will add them to this post.

Learn More About Hydrangeas

To understand hydrangeas better have a look at these other posts.

Hydrangea Identification – learn how to identify the type of hydrangea without seeing the flowers.

Why Hydrangea Don’t Flower? – understand why your hydrangea is not flowering.

References:

  1. Why Don’t My Hydrangeas Flower?; https://www.ces.ncsu.edu/fletcher/staff/rbir/hynonflower.html
  2. ‘Hydrangeas for American Gardens’, by Michael A. Dirr
  3. Chemistry of Hydrangea Colors; http://www.americanscientist.org/issues/pub/2014/6/curious-chemistry-guides-hydrangea-colors/1

 

Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

7 Responses to 'Hydrangea Myths'

  1. Kat says:

    Thank you for the great info and pics. For the chlorosis – I have very sandy soil, zone 5, guessing the same pH approx as you, in the past I have tried adding a cupful of epsom salts, magnesium sulfate which has seemed to help quite a bit? Look forward to your comment, with thanks

    • Sandy soil can have a very different pH than clay soil. The sulfur in Epsom salts may acidify soil a bit, but the magnesium is probably not doing anything to resolve a chlorosis problem.

  2. sandra shean says:

    Thanks for the great info in hydrangea’s !!! I planted a blue one I got from my girls a few years ago, and it never bloomed again except for this year, I got one beautiful blue flower… Why after so many years ??

  3. Karen P says:

    Thanks for all the good info on hydrangea’s. I have had luck with
    them but always thought they were very acidic plants and so I put my leftover coffee on them trying to turn them blue. I now know better.

  4. Mia says:

    I appreciate the info about pruning in particular. It’s a pervasive myth that some hydrangeas “have to be” pruned in fall in order to bloom and others “have to be” pruned in spring in order to bloom. It wasn’t until I read an interview with a hydrangea grower who said that they would bloom just fine without any pruning. My life was made easier!

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