Feeling Blue About Knowing When to Prune Your Pink Hydrangea?
You’re not alone. Most homeowners become lost within the myriad of hydrangea varieties and the science behind changing hydrangea flower color. But don’t despair, with a few timely pruning tips and a little chemistry you can have beautiful hydrangea flowers with your choice of colors.
Knowing when to prune hydrangea requires knowing the basic group that your hydrangea fall into. This is important because some hydrangea produce flower buds in late summer for the following spring (old growth), while others produce flower buds in the spring (new growth). Therefore, if you prune a hydrangea in late September it will produce flower buds in late summer for the following spring. In addition, you will have pruned off many if not all of the flower buds for next year. As an aside, hydrangeas do not have to be pruned unless they are too large for their location.
The basic hydrangea species are, Macrophylla, Arborescens, Paniculata, Serrata, Aspera and Quercifolia. Of these, macrophylla are the most common and includes varieties like Nikko Blue, Endless Summer, Blue Wave, Pia and Glam Rocket and many Lace Cap types. Arborescens are popularized by the variety Annabelle, Invincible Spirit and Hayes Starburst. Paniculata include the old Pee Gee varieties as well as Tardiva, Lime Lite, and Little Lite. Serrata, a group of almost all lace caps, have serrated leaves. Aspera include a small group of lace cap types with very fuzzy leaves. Quercifolia include all Oak Leaf Hydrangea.
So when is the right time to prune each group? A simple rule of thumb is that any plant can be pruned right after flowering. This excludes the special variety, Endless Summer. It is the only Macrophylla that produces flower buds in both late summer for the following summer (old growth) and spring (new growth). Therefore, this hydrangea can bloom well into November as long as the temperatures remain above freezing at night. On the other hand, most hydrangea experts suggest pruning the Macrophylla group no later than August since the buds for this group begin set in mid to late summer for the following spring.
The Arborescens species can be pruned any time between the fall and very early spring. It is risky to prune an Arborescens after the leaves begin to emerge in spring. To be on the safe side, I prune my Arborescens in either late October or late March. The Panicualta species can be pruned exactly like Arborecens, between fall and very early spring. Like the Macrophylla species, Oak Leaf Hydrangea, the Quercifolia species and the Serrata species must be pruned no later than August. The Aspera species should be pruned back lightly every spring before the leaves emerge.
Essentially, the perfect time to prune your hydrangea falls into two time periods, no later than August and between fall and very early spring. Your job as a consumer is to be aware of the specie that you purchase so you know when it is the right time to prune it.
The following chart includes the four specie groups, popular hydrangea varieties, and the pruning group that each fall into. Keep in mind that all plants are classified by genus, specie and variety. For example, Hydrangea is the genus name, macrophylla, arborescens, paniculata and quercifolia are the specie names and Annabelle, Nikko Blue and Endless summer are examples of varieties. Hydrangea macrophylla ‘Endless Summer’ is an example of one complete name.
GROUP 1 – PRUNE NO LATER THAN AUGUST
Hydrangea (genus) Macrophylla (specie) (varieties) – Endless Summer, Nikko Blue, Alpengluhen Blue Wave Lace Cap, Altona, Amethyst, Blue Danube, Brestenburg, Gentian Dome, Europa, Gerda Stieniger, Harlequin, Coerulea Lace Cap, Fasan Lace Cap and many more.
Hydrangea (genus) Quercifolia (specie) (varieties) – Alice, Snow queen, Snow Flake
Hydrangea (genus) Serrata (specie) (varieties) – Preziosa, Miranda, Midoriboshi Temari, Kiyosumi, Hokaido
GROUP 2 – PRUNE IN FALL OR VERY EARLY SPRING
Hydrangea (genus) Arborescens (specie) (varieties) – Annabelle, Invincible Spirit, Radiata, Hayes Starburst
Hydrangea (genus) Paniculata (specie) (varieties) – Tardiva, Lime Light, Little Lime, Pee Gee, Chantilly Lace, Grandiflora, Pee Wee, Pink diamond
Hydrangea (genus) Aspera (specie) (varieties) – Kawakami, Villosa, Rocklin, Robusta
Soil pH and Flower Color
Knowing when to prune your hydrangea is important, but it’s just as critical to know how to, or at least attempt to, control flower color. Soil pH determines the color of most Macrophylla and to a lesser degree, the color of oak leaf hydrangea. Arborescens and Paniculata flower colors are generally not affected by soil pH. Invincible Spirit, and Arborescens, may be slightly affected by soil pH, but only through the intensity of pinkness. Therefore, it will not turn blue if the soil pH is lowered.
So what is pH? pH literally stands for the Power of Hydrogen – its logarithmic power since the pH scale is logarithmic. For example, a pH of 2 is ten times more acidic than a pH of 3 and 100 times more acidic than a pH of 4. Why hydrogen? The ratio of hydrogen (acidic) to hydroxide (Alkaline) ions in the soil determines soil pH and establishes which elements are free for plant absorption. If the soil pH is alkaline, above a pH of 7, hydrangea flower color will begin to turn pink, depending on the actual variety of hydrangea. If the soil pH is acidic, below a pH of 7, hydrangea flower color will begin to turn blue, depending on the actual variety. Some varieties of Macrophylla will never turn a deep blue or a wine color, like Endless Summer, which contains a number of sub-varieties that are extremely pale in color. The difference between pale blue and pale pink in full sun can be perceived as white to some degree. Nikko Blue that is grown in acidic soil conditions will turn extremely blue. My favorite hydrangea variety, Alpengluen, will contain a range of lime green, wine and blue-bluish purple on every flower in acidic soil. From a design perspective, it is important to know which color scheme you are looking for and the relative range of colors that each variety of hydrangeas are capable of producing.
How do we alter soil pH? We always begin with a thorough soil test prepared by an extension agency or a lab that is experienced with soil testing. The lab must not only test current pH, but also test the soil’s buffering capacity. Buffering capacity defines a soil’s resistance to pH change; for example, there may be a large well of hydroxyl ions (alkaline) in your soil. The sheer quantity of hydroxyl ions will out balance the relatively small amount of hydrogen ions (acidic). This is vital because your efforts to change pink hydrangea to blue may be ineffective. What can you do in this instance? You either live with the color that you have or you can invest a fair amount of time and money each season to attempt to temporarily offset the soil’s buffering capacity.
To raise the pH of acidic soil, we use lime. Dolomitic lime is best applied after a soil test in the fall, which allows it to change soil pH. Hydrated lime can be used, but it is fast acting and can burn or kill the plants. Sulphur Flour, High Yield Sulphur Flour and Aluminum Sulphate can all be used to lower soil pH. Sulphur Flour takes a fair amount of time to work and you may grow impatient with results, especially if your soil’s buffering capacity is high.
Raising or lowering soil pH should always be guided by a proper soil test and never by eye. The soil test will almost always tell you how many pounds of which product to apply in order to achieve your desired results. In addition, many soil test facilities ask about the plant group and leave room for comments. As a result, if you want your pink hydrangea to be blue, they may tailor their recommendations to your goals, while always keeping a watchful eye on proper soil science. A soil test can make your results both achievable and the process of knowing how much product to apply less stressful. All you have to do is plan ahead and have a soil test prepared.