Changing Hydrangea Colors

Color Depends on Soil pH, Aluminum

Let's Dance Rhapsody Blue hydrangea (image) can be pink, purple or blue. It changes with conditions.
My Let's Dance Rhapsody Blue hydrangea was purple in its second year. David Beaulieu

Are the flowers on this bush really blue, as the name suggests? Or are they pink? Or mightn't we term the plant a purple hydrangea? Well, that depends, as I explain in this article. Let's begin by breaking down its full name, which is Hydrangea macrophylla Let's Dance® Rhapsody Blue.

That first part (in italics) is the genus and specific epithet, and it tells us that this plant is one of the so-called "Bigleaf hydrangeas." The second part (in bold) is the series it belongs to, as dubbed by the plant developers who brought us this and closely-related new bushes. Finally, "Rhapsody Blue" identifies this particular plant in that series.

Plant Description

Rhapsody Blue hydrangea is a deciduous shrub  that matures to a height of about 2-3 feet, with a similar spread (a size that qualifies it as compact). It belongs to the group of reblooming hydrangeas (see below). The H. macrophylla species, which is native to the Far East, comprises two groups, the mopheads (sometimes called "hortensias") and the lacecaps, terms that refer to the appearance of the flower heads; Rhapsody Blue is a mophead. They can be used in landscaping as specimens, as edging plants, and in mixed shrub borders.

Note: As for all hydrangeas, what's showy on the flower heads (corymbs) of these shrubs are called, technically, "sepals" (i.e., not petals).

How to Change This Hydrangea's Color to Purple (or Pink or Blue)

The picture of Rhapsody Blue that you see above shows a purple hydrangea. "How can that be?" perhaps you ask. "Why isn't it blue?" The answer is that, like other members of the H. macrophylla species, the flower color is variable. It varies from pink to blue (with purple standing in between the two extremes). Coloration will depend on the presence (and quantity) or absence of aluminum in the soil, in conjunction with soil pH:

  • More aluminum turns the sepals blue
  • A little less changes them purple
  • An absence of aluminum makes them pink

Why did I say above "in conjunction with" soil pH? Well, as horticulturist, Tim Wood points out, aluminum is made more accessible to plants in acidic soils. So aluminum may be the doorway to changing hydrangea color to purple or blue, but it is soil pH that holds the key to the door.

What does all of this mean to you, the grower? First of all, it means that, if you'll be striving for a particular color, you'll need to ascertain the pH of your soil. You can perform a soil test using a kit (easily obtained from a good garden center) or send in a sample of your soil to your local cooperative extension and have them do the test for you.

Secondly, if you desire your H. macrophylla to be pink, your soil will have to be alkaline. If you need to make an acidic soil alkaline, as one Master Gardener suggests, "Add dolomitic lime, to raise the soil pH to about 6.0 to 6.5." She warns, however, that raising alkalinity to higher levels than that could have negative consequences for your garden.

Thirdly, those who wish to change hydrangeas to purple or blue will need to make their soil more acidic (if it isn't suitably so already) and must ensure that it contains a sufficient amount of aluminum. You can accomplish both goals by applying aluminum sulfate to the soil in question in late fall or in early spring. You may not achieve these results immediately, a fact to which my own experience (as follows) testifies:

The first year I grew Rhapsody Blue hydrangeas, the flower color -- belying the name – was pink. In November of that year, I applied aluminum sulfate to the soil. In the following year, the result (shown in the picture above) was purple flowers. Future applications will eventually result in blue flowers, all else being equal.

Note: There are many groups of hydrangeas, and it's generally only the H. macrophylla group that allows you to experiment with color in this way. Examples of hydrangeas with fixed coloration (that is, their color is what it is, and no amount of monkeying around with the soil will change that) are:

By the way, have you ever heard that, if you place pennies around one of the H. macrophylla bushes, it will turn the color of the flowers to blue? C.L. Fornari debunks this as a myth in her book, Coffee for Roses, a work that exposes seventy-one gardening myths.

Let's Dance® Rhapsody Blue One of the Reblooming Hydrangeas

You may be familiar with the idea that some bushes flower on old wood (i.e., the prior year's growth), while other shrubs bloom on new wood (i.e., the present year's growth). But did you know that some kinds of hydrangeas fall into both of these categories? Well, it's true, and there's even a fancy name for this phenomenon: "remontant." These kinds of hydrangeas flower the first time around on the prior year's growth, then rebloom later on the present year's growth.

It's a nice bonus, obviously, to have a shrub rebloom for you. The more color you can achieve in your landscaping, the better, right? Those of us who bother with planning sequence of bloom will certainly value the added color. Along with growing long-blooming perennials, selecting shrubs that rebloom aids us greatly in realizing our goal of continuous color in the yard.

Let's Dance® Rhapsody Blue is one type of reblooming hydrangea. Wood mentions the "cultivars 'Endless Summer', Let's Dance Moonlight, Let's Dance Starlight, and 'Forever and Ever'" as other types. Horticulturist, Michael Dirr notes another member of the Let's Dance series -- 'Big Easy' -- while also listing the following as members of the Forever and Ever series:

  • Double Pink           
  • Original
  • Peppermint 

Care for Rhapsody Blue Hydrangeas: Location, Fertilizing, Pruning

The shrubs can be grown most easily in growing zones 5-9. Grow them in a moist but well-drained soil. The usual recommended location for them is in full to partial sun (qualified by saying that locations with partial sunshine will be best for Southern gardeners). I have found, however, that mine tend to wilt easily during hot summers in their present location, which receives partial sun (including sunshine during mid-day, which is the hottest part of the day). So I am considering moving mine to a partially shaded location.

Wood warns against fertilizing with phosphorus if you seek to change color to purple or blue on these hydrangeas. He explains that if the soil is high in phosphorous, the aluminum that you need for the purple or blue color won't be available to your bushes, so they will most likely be pink. But do continue fertilizing with aluminum sulfate every year either in late fall or early spring (if it's the blue or purple color you desire).

Because Rhapsody Blue blooms on both old and new wood, it's fussy about pruning time, granting you just a narrow window of opportunity. On the one hand, you won't want to prune too early, otherwise you'll lose the first flush of flowers (those that bloomed on the old wood). On the other hand, if you wait too long, it may be too late for the buds to set on the new wood (the buds that you'll be counting on for next summer's first flush of flowers). In this sense, it poses a pruning challenge similar to that of a clematis vine such as Dr. Ruppel clematis. Mine (zone 5) first blooms in late June. The sepal color fades somewhat by early August, so that's the best time for me to prune.