Hydrangea macrophylla ‘Nikko Blue’ is an old favorite and still one of the best mophead hydrangeas in existence. It is deciduous and blooms on old wood. Nikko Blue hydrangeas bloom more profusely than almost any other hydrangea. I prefer lacecap and oakleaf hydrangeas to the big mopheads, but it is impossible to deny the charm of a properly grown Nikko Blue hydrangea.
Grown in gardens for decades Hydrangea ‘Nikko Blue’ is a classic. In soil with acidic pH, the flowers are blue whereas alkaline soil will produce pink blooms.
Nikko Blue hydrangeas were the first flowering plants I remember as a child. A hedge of them formed a blue wall during the summer months in the backyard of the house we lived in while my parents went to college. They tore down the house to build a new runway at Atlanta International Airport. The hydrangeas are gone and they never did get around to building the runway.
‘Nikko Blue’ hydrangea is a fast-growing macrophylla that blooms on old wood. Prune directly after flowering or leave it for another year. Old wood simply means a plant flowers on stems produced the preceding growing season. Thus, if you prune in fall or winter you are cutting off not just the stems but the flower buds that were waiting dormant for the following spring.
It grows six feet by six feet, and you simply must protect macrophylla hydrangeas from afternoon sun. The flowers and plant are more beautiful when shaded anyway. It is hardy in zones 6-9, but you can try it in zone 5. Give Nikko Blue winter protection in zone 5.
The perfect exposure for all macrophylla hydrangeas, not just Nikko Blue, is morning sun and afternoon shade.
Hydrangeas grown in full shade do not flower as profusely, if at all. Further, they can get lanky. The answer is morning sun. Err on the side of too much shade.
I recently read a complaint from a reader that their Nikko Blue had crispy, burnt leaves. I assure you this is no fault of the plant nor the nursery that sold it. Success with all plants comes through respecting the habitat in which they grow best and attempting to emulate it to the best of one’s ability. There are few things more disheartening in a garden than droopy hydrangea leaves in the harsh afternoon sun.
Give Hydrangea ‘Nikko Blue’ rich, well-drained soil that has been prepared with plenty of humous. Rotted pine bark and aged compost will loosen the soil and give your hydrangeas the consistency and nutrients needed to get them off to a good start. Do not plant too deeply or you place the hydrangea at risk of root rot during its winter dormancy. I tend to plant a half inch to an inch above the existing soil line. Gently grade the soil away from the stem of the plant and mulch with pinestraw or pine bark.
Even under the best growing circumstances, hydrangeas are heavy feeders and like lots of water. Hydrangea ‘Nikko Blue’ is definitely not a plant for xeric gardening, although once established, supplemental watering should not be required. Feed macrophylla hydrangeas with slow-release fertilizers a couple of times during the growing season.
Having grown up with the acidic soils typical of the Piedmont, I only know Hydrangea ‘Nikko Blue’ in its blue form. Having seen this cultivar bloom in blue waves for years, my preference is the blue color for which it is named. If you want to turn the flowers blue or pink, you will need to augment the soil.
How to turn a ‘Nikko Blue’ hydrangea blue (or pink)
Add aluminum sulfate, sulfur, or azalea/rhododendron mixtures to the soil in order to turn the flowers blue. You can get any of these items at most garden stores. For example, a half cup of sulfur spread around the roots and watered in during early spring will begin the process. It may take a season or three to get the flowers back to the desired color. A reportedly faster method is dissolving a tablespoon of aluminum sulfate per gallon of water and drenching the soil around the plant. Regardless of what method you use results are not instantaneous.
Add dolomitic lime, easily found at garden centers or Home Depot, to the soil for pink flowers. You should be consistent and patient. It is not unusual to see plants with a mix of pink and blue flowers on the same shrub. You may also see lavender flowers on macrophylla hydrangeas due to the roots reaching into soils with different pH levels. Keep at it and you will get the desired color.
I am often asked if it is possible to turn white hydrangeas either blue or pink. The answer is generally, no. I’ve read the white hydrangea ‘Fuji Waterfall’ may have light blue or pink petioles depending on the acidity or alkalinity of the soil, but I cannot confirm.
In a garden where I was working, I stumbled across a ‘‘Glowing Embers’ hydrangea whose flowers had turned as blue as the hydrangeas photographed on this page. ‘Glowing Embers’ is about as gaudy a pinkish-red hydrangea as you will ever see, but it still needs alkaline soil to remain the color the hybridizers envisioned.
Hydrangea ‘Nikko Blue’ and its ilk are truly the litmus strips of the garden world.
I’ve often thought it would be a cool learning experience for children, turning a hydrangea’s flowers from one color to another. Of course, youngsters are going to want to know not just how but why.
As the University of Georgia’s College of Agricultural & Environmental Sciences explains so well, hydrangeas have aluminum compounds in the flowers. All plants have preferred pH soil levels that allow them to make the most effective use of the nutrients available. When pH goes alkaline to 7.0 or above, macrophylla hydrangeas can no longer absorb the aluminum compounds naturally present within the soil, thus turning the flowers pink. As you lower the pH into the acidic range, the aluminum is more available to the hydrangea’s roots.
Why hydrangeas sometimes skip a year of bloom
One of the most asked questions with regards to macrophylla hydrangeas is regarding them not blooming. Macrophylla hydrangeas bloom on old wood, their stems produced the preceding year. There are new cultivars of macrophylla hydrangeas, such as ‘Twist-n-Shout’, that bloom on both old and new wood by the way.
During very cold winters or if there is a late hard freeze, dormant flower buds may be killed. It’s a fact of life and I personally wouldn’t worry about it too much unless you live at the northern limits of a particular cultivar’s winter hardiness.
This also happens with indica azaleas. The plant is fine but the flower buds are sometimes damaged or killed in winter. Orange growers in Florida experience late freezes that threaten their crops. Thankfully, for the vast majority of us, our careers do not depend upon whether our hydrangeas miss a year of flowering every once in a while.
|Genus/species||Hydrangea macrophylla ‘Nikko Blue’|
|Common name(s)||Nikko blue hyrangea, mophead hydrangea, french hydrangea|
|Of note||macrophylla hydrangeas flowers are blue in acidic soil and pink in alkaline soil – easy to grow – afternoon shade is a must|
|Water requirements||average, high|
|Soil quality||rich, well-drained|
|Suggested use(s)||cottage gardens, mixed borders/perennial beds, shade gardens, understory|
|Hardiness zone(s)||6a, 6b, 7a, 7b, 8a, 8b, 9a, 9b|
|Deciduous or evergreen||deciduous|
|Bloom period||mid to late spring, summer|
|Exposure||afternoon shade, filtered light|1 2