For the Love of Hydrangeas

I would have to say that 9 out of 10 clients tell me they want hydrangeas in their garden.

Everyone LOVES hydrangeas. Some who have done their research may even go as far as telling what variety of Hydrangea they want. And then there are the people who just say, “I want the blue ones”. Nowadays, the possibilities are endless. There is even a variety called “Endless Summer”, but more on that later.

Since I mentioned Blue Hydrangeas, let’s start there.

Hydrangeas are fascinating in that, unlike most other plants, the color of their flowers can change dramatically based on the pH of the soil. Of course, I should add that not all hydrangeas produce blue flowers. Hydrangeas with white or cream flowers, such as Annabelle hydrangeas, Oakleaf hydrangeas and members of the PeeGee family, can only produce white or cream flowers. Often their blooms take on a pink tinge at the end of the season, but that’s about as colorful as they get.

Because the soil pH determines the bloom color, the various names given to some types of hydrangeas means very little when it comes to bloom color. For instance, Nikko Blue, Pretty in Pink, Forever Pink and Blue Deckle, all have an almost equal chance of blooming pink or blue, depending on the soil they are planted in.

Hydrangeas with bloom colors that range from pink through blue and purple usually belong to the Hydrangea cultivars known as mopheads and lacecaps.

As I mentioned above, these types of hydrangeas have the interesting ability to change the color of their blooms based on the chemistry of the soil. When grown in alkaline soil, the bloom colors are pinker. When grown in acidic soil, the bloom colors are bluer.

So remember that even if you purchase a hydrangea in bloom, you cannot be sure the plant will produce the same color flowers once it’s growing in your garden.

Can you change the color of your hydrangea?

Yes, you can. Though having said that, it is much easier to change a hydrangea from pink to blue than it is from blue to pink. Changing a hydrangea from pink to blue entails adding aluminum to the soil. Changing from blue to pink means subtracting aluminum from the soil.

Changing from blue to pink:

For hydrangea blooms to be pink, the plants must not take up aluminum from the soil. To achieve this result, you can add lime to the soil. This will help to raise the pH. Since hydrangeas take up aluminum best at lower pH levels, raising the pH will help to keep the bluing effect of aluminum out of the hydrangea’s system.

Use a fertilizer with high levels of phosphorus.

Phosphorus helps to prevent aluminum from creeping into the system of the hydrangea. Choose a fertilizer close to the ratio of 25/10/10 (Phosphorus is the middle number).

Blue Hydrangea

To obtain a blue hydrangea, aluminum must be present in the soil. To ensure that aluminum is present, aluminum sulfate may be added to the soil around the hydrangeas.

Authorities recommend that a solution of 1/2 oz (1 Tbsp) aluminum sulfate per gallon of water be applied to plants (which are at least 2-3 years old) throughout the growing season. Water plants well in advance of application and put the solution on cautiously, as too much can burn the roots.

Adding aluminum sulfate will tend to lower the pH of the soil. Another method for lowering the pH is to add organic matter to the soil such as coffee grounds, fruit and vegetable peels, grass clippings etc.

If the soil naturally contains aluminum and is acid (low pH) the color of the hydrangea will automatically tend toward shades of blue and/or purple. As above, the choice of fertilizer will also affect the color change. A fertilizer low in phosphorus and high in potassium is helpful in producing a good blue color (25/5/30 is good. Potassium is the last number). Superphosphates and bone meal should be avoided when trying to produce blue.

Pruning Hydrangeas

I have often had clients say to me, “My hydrangeas did not bloom at all this year.” As mentioned above, if they are referring to the old-fashioned mopheads and lace caps, the issue often has to do with pruning and when the pruning was done.

You see, mopheads and lace caps bloom on old wood. Which means the buds are set at the end of the season and assuming they make it through the winter they will bloom in spring on last years’ stems, hence old wood. If you prune the hydrangea in spring before the flowers, there go all those set buds and there go all your flowers for the year. So if you really need to prune your hydrangea for shape or size, than be sure to do so immediately after the flowers fade and then not again until they flower the following year.

The inverse is true of many of the newer forms of hydrangeas that bloom on both old and new wood and therefore can be pruned at any time of year.

Following are some common types of hydrangeas and when they should be pruned:

Big-leaf Hydrangeas, Hydrangea Macrophyla: These are the most common species and include the popular mopheads and lacecaps in wide-ranging colors of blue, violet, pink, purple, red and white. They bloom in early summer on old wood, so prune them after flowering.

Oakleaf Hydrangeas, Hydrangea Quercifolia: This native hydrangea features cone-shaped white blooms that turn a beautiful shade of russet in late summer. It, too, blooms on old wood so should not be pruned until after flowering.

Panicle Hydrangeas, Hydrangea Paniculata: Often pruned as tree forms, these shrubs (Tardivas, PeeGees) explode with panicle-shaped white flowers in mid- to late summer. Because they bloom on new wood, prune these in early spring before they sprout new foliage.

Hydrangea Arborescens, ‘Annabelle’: Featuring globe-shaped blooms in spring that start out chartreuse and later turn white, this variety blooms on new wood as well so prune in early spring.

“Reblooming” Hydrangeas: These are a new generation of hydrangeas bred to flower more than once throughout the growing season. They offer wide-ranging colors, and one of the most popular cultivars is “Endless Summer.” So because they bloom both on old and new wood, these hydrangeas can be cut back at any time.

There is so much more to say on hydrangeas, stay tuned…