How to Grow Nikko Blue Hydrangea

nikko blue hydrangea

The Spruce / Evgeniya Vlasova

While there are many varieties of hydrangea—all beautiful and bountiful with blooms—there are some classics that come to mind when you speak of the plant, and Nikko blue hydrangea is one of those. Considered part of the Hydrangea macrophylla species, the Nikko blue hydrangea is classified as a bigleaf hydrangea

These bushes are deciduous flowering shrubs and are classified as being in the mophead, or hortensia group. The mopheads are so-called because they produce large balls of sepals. By contrast, the "lacecap" type of hydrangea displays a flatter flower head, with the sepals aligned more around the perimeter, surrounding the tiny, fertile blooms in the center.

Members of the hydrangea macrophylla species, including the Nikko blue hyndragea, are native to Japan and best planted in fall or early spring. They'll grow rapidly and can add up to 24 inches of height a year.

Botanical Name Hydrangea macrophylla
Common Name Bigleaf hydrangea
Plant Type Deciduous shrub 
Mature Size 6-10 ft. tall, 6-10 ft. wide
Sun Exposure Partial shade, full shade
Soil Type Moist but well-drained
Soil pH Acidic, alkaline
Bloom Time Summer, fall
Flower Color Blue, pink, purple, white
Hardiness Zones 5-11 (USDA)
Native Area Asia
Toxicity Toxic to dogs and cats
Nikko hydrangeas

The Spruce / Evgeniya Vlasova

Nikko blue hydrangea shrub

The Spruce / Evgeniya Vlasova

closeup of Nikko hydrangeas

The Spruce / Evgeniya Vlasova

Nikko Blue Hydrangea Care

Nikko blue hydrangeas grow to be around 6 feet tall, with a similar spread. The more shade the bush receives, the shorter it may remain—in extreme cases, the plant may stay short enough to resemble a large perennial in size. The shrubs are multi-stemmed and grow in an upright habit.

When referring to the "flowers" of this shrub, we're actually referencing the sepals, which first appear in July and are tough, bract-like structures that persist for months. Despite bearing a cultivar name indicating that they are blue hydrangeas, the flower color on the plant really depends heavily on soil pH.

Light

In general, hydrangeas should be grown in partial shade, though they do appreciate some morning sunlight. Gardeners in the south may wish to supply a bit more shade, while those who live in cooler climates can get away with a location in full sun. All in all, the plants will need six to eight hours of good sunlight to experience full bloom.

Soil

Hydrangeas like soil that is rich and moist, but well-draining. The biggest consideration when it comes to the soil you plant your hydrangeas in is the mixture's pH level. Happily, it is very easy to change a bigleaf hydrangea's color to either pink or blue, depending upon what you want it to be.

These bushes will bear blue flowers in acidic soils, but pink blooms in more alkaline blends. If a soil test reveals insufficient acidity and you desire a blue color, just raise the acidity by amending it with sphagnum peat moss or nitrogen fertilizer.

Water

As your hydrangea plants are getting established, water them deeply several times a week to promote strong roots. Beyond that, the plant will appreciate consistent moisture and you should aim for at least 1 inch of water per week (or more, if you are experiencing especially warm weather).

When watering, aim your hose towards the base of the plant to avoid soaking the delicate flowers and leaves—this will keep from damaging the plant, and also help you avoid fungal diseases.

Temperature and Humidity

Hydrangea can be grown in a variety of hardiness zones and can even be overwintered in some areas. Mature hydrangea can handle temperatures as low as 32 degrees Fahrenheit, but younger plants should be kept at temperatures no lower than 35 to 40 degrees Fahrenheit.

Additionally, hydrangeas like average to high humidity and will often thrive in the moist heat of summer. However, if they experience too much dry air, like in an arid, desert climate, the plant's leaves can wilt or droop.

Fertilizer

Unless you're looking to amend your soil to change the hue of your hydrangea, you only need to feed your plant once a year. At the beginning of spring, apply a slow-release fertilizer blend formulated for shrubs and trees to your plant—you can also use organic matter if you prefer.

Are Hydrangeas Toxic?

Unfortunately, hydrangeas can be considered toxic to small pets like dogs and cats—but it's not as scary as it sounds. Your animal would have to eat a seriously large amount of the plant in order to feel the impact of their toxicity, so in most cases, pets either don't feel ill, or their mild symptoms go unnoticed. If you do witness your pet exhibiting any of the below symptoms, contact your vet.

Symptoms of Poisoning

  • Vomiting
  • Diarrhea
  • Lethargy
  • Depression
  • Confusion

Bigleaf Hydrangea Varieties

Bigleaf hydrangea is said to be the most popular hydrangea shrub, and the Nikko Blue varietal is one of the most prized of the group. In addition, other popular varieties include:

  • H. macrophylla 'Endless Summer': This varietal is aptly-name, as it's a repeat bloomer throughout the season. It's also valued for its hardiness and ability to grow in USDA hardiness zones 4 through 9. 
  • H. macrophylla 'Variegata': This varietal boasts variegated leaves and lacecap flowers.
  • H. macrophylla 'Mariesii Perfecta': Also known as "Blue Wave," this varietal is one of the better-known lacecap varieties.

Pruning

A popular shrub for use in the cottage garden style, Nikko blue hydrangea bushes are attractive enough to use as specimen plants when in full color—they are also sometimes planted along property borders. Because they consist mainly of sepals, the flower heads persist through fall (though the color will fade), adding interest to the autumn landscape, and they can be harvested for dried arrangements. Some growers prize the foliage, but others are indifferent to it.

Nikko blue hydrangea bushes do not require much (if any) pruning, but the main thing that you need to know if you do feel compelled to prune them is that they bloom on old wood. Buds are set in late summer to early fall, so you should prune prior to this. If you prune after the buds have been set, you risk losing flowers for next year (but, as always, dead branches can be pruned out at any time). Prune back to where you see healthy buds growing.

Common Pests and Diseases

Hydrangeas must contend with a variety of common diseases. One common issue is botrytis blight, which is a fungal infection that can kill off flower buds before they get the chance to open. Similarly, leaf spot can show itself as purple or brown spots on the leaves of the plant. Both problems can be caused by excess moisture and can be mitigated by watering the plant at its base instead of throughout the dense bush.

Scale and aphids can also be an issue for the plant, especially if they're already present elsewhere in your garden or landscape. Treat the plant at the first sign of infestation using a mild insecticide or horticultural oil like neem oil.