Ajisai: flowerheads, colour and horticultural change

The broad coloured parts that resemble petals are enlarged sepals of decorative flowers. Tiny petals, stamens and a pistil may still be present at their centre. The less conspicuous bisexual flowers have smaller sepals and reproductive organs. Showy flowers help attract pollinators, while the small flowers are important for seed production. Decorative flowers are not universally sterile imitations: a University of Shiga Prefecture study notes viable pollen and functional ovules in both flower types of H. macrophylla. The development of these organs and reproductive capacity vary among cultivars.

A lacecap head has large decorative flowers around a central cluster of small flowers, rather like a frame. Mopheads have many more decorative flowers and form a dome or ball. Both occur among cultivated bigleaf hydrangeas, while mountain hydrangeas also include lacecap forms. Flowerhead shape and botanical species describe different aspects of the plant. A 2006 study compared the lacecap cultivar Blue Sky with a mophead arising from a mutated branch. Some upper branches of the flower cluster were replaced by decorative flowers, producing a fuller surface; small non-decorative flowers were still present within it. The mophead is therefore a cluster composed of many flowers.

Blue colour involves anthocyanin pigment, aluminium ions and copigments. In acidic soil, aluminium can become more soluble and be absorbed by roots; in sepal cells it interacts with pigments to produce blue. Cell analysis by Nagoya University researchers also showed that vacuolar pH, aluminium content and copigment composition jointly affect the range from red through purple to blue. Soil pH and the pH inside a cell’s vacuole are different things.

A cultivar’s pigments and copigments influence how its colour responds to the environment, so acidic soil does not make every plant blue. White-flowered types lacking the relevant pigment do not turn blue simply through a change in soil pH. Colour variation within a plant, together with ageing after flowering, also makes colour alone an unreliable way to identify a cultivar.

Many familiar bigleaf hydrangeas prepare next year’s flower buds on shoots grown during the current season. Inflorescences and floral organs differentiate through autumn, development pauses in winter, and growth resumes as spring warms after chilling has released dormancy. Flowers then open in early summer. Late or deep pruning can therefore remove the future flowering parts, as Fukuoka Botanical Garden’s cultivation advice explains. Timing is not identical across Japan or across cultivars: a 2024 Aichi study of 25 cultivars found substantial differences in the autumn start of bud development. Cuttings root pieces of a selected plant to preserve its characteristics; this is how the Rokko Shichidanka was multiplied.

Growers use the bud-and-dormancy cycle to schedule potted flowers. The Aichi report distinguishes March shipments produced with refrigerated chilling followed by heating; April–May shipments using outdoor winter chilling followed by heating; and June–July shipments kept under natural outdoor conditions. Cultivar-specific bud development and chilling requirements matter: otherwise flowering and non-flowering shoots can occur in the same crop. A flowering pot sold in spring does not necessarily mark the flowering season of local outdoor plants.

At the Buddhist flower festival, amacha is poured over an image of the infant Buddha and also served as a drink. It uses processed leaves from particular amacha plants related to mountain hydrangeas. Ornamental bigleaf hydrangea leaves are not interchangeable with them: Japan’s health ministry records poisoning after decorative leaves served with food were eaten, and notes that the toxic constituent has not been fully established.

The Man’yōshū already contains the plant name read as ajisawi, written in several forms. The characters 紫陽花 later became a familiar way to write ajisai. Variation and selection in cultivation produced forms with more decorative flowers, including mopheads derived from lacecap plants. The precise time and place of their first development remain uncertain.

Breeders cross species to widen the genetic background available for selection. Gunma’s Agricultural Technology Center developed the winter-flowering Spring Angel series from a garden bigleaf hydrangea and an evergreen relative. Mature seeds were difficult to obtain, so immature ovules were cultured to raise hybrid plants. Managed indoors, they can flower in January–February and again in early summer; the series includes doubled pink, blue and white forms. Another garden example, Bridal Shower, produces flowers on lateral branches, changing their distribution along a shoot. Season, flower-head form and flowering position are distinct traits that breeding can alter.