Foamflower (Tiarella) in Japan: A Wild Cousin in the Mountains, a Shade Plant in the Garden
Foamflower is a small woodland perennial. In spring it sends up spikes of tiny white or pale pink flowers, and up close each flower looks like a little burst of fireworks. In Japan we call the garden plant tiarella, from its scientific name, and it has become a favourite for the shady corners of gardens.
We are a small tree care and landscape business in Mie Prefecture, central Japan. In the gardens we look after, we sometimes find foamflower growing in a quiet clump under the trees. This page explains where the plant comes from, why Japan has a foamflower of its own, and how we use it in shade.
A small genus split between two continents
Tiarella belongs to the saxifrage family (Saxifragaceae), the same family as heuchera, or coral bells. The genus is small. It has one species in Asia, from the Himalayas and China to Japan, and a few in North America. Botanists do not agree on how many of the North American plants are separate species, so the counts differ between references. The Japanese wild plant reference Mikawa no Yasou counts four species in all.
The foamflower planted in gardens is mostly the North American species and hybrids bred from it. The Heibonsha World Encyclopedia, a standard Japanese reference, notes that Tiarella cordifolia from eastern North America is the species used in woodland and rock garden plantings. NC State Extension describes Tiarella cordifolia as native to eastern North America, from Nova Scotia along the Appalachian Mountains to Alabama, and west to Minnesota, where it grows in moist forests. It spreads by slender runners into low colonies, and its leaves are shaped a little like maple leaves.
So for a reader in North America, the tiarella in a Japanese shade garden is very likely your own native plant, or a selection made from it, that has travelled across the Pacific.
Japan's own foamflower: zudayakushu
Japan has a foamflower of its own, Tiarella polyphylla. Its Japanese name is zudayakushu. It grows wild in Hokkaido, Honshu and Shikoku, and some sources add Kyushu. The Heibonsha World Encyclopedia describes it on damp forest floors deep in the mountains, in beech forests and under subalpine conifers. It flowers from June to August, and its small white flowers hang slightly downward.
The name is the interesting part. Zudayakushu can be written with characters meaning "asthma medicine". Zuda is said to be a word for asthma in the dialect of Shinshu, today's Nagano Prefecture, and the plant was used as a folk remedy for coughs. Not every source agrees on where the dialect word comes from, so we treat this as the traditional explanation rather than settled fact. Please do not use the plant as medicine.
Zudayakushu is a plant you meet on mountain walks rather than in gardens. It is easy to walk past. The garden tiarella and the mountain zudayakushu are close relatives, one from each side of the Pacific.
How we use it in a Japanese shade garden
Many gardens in Japan have a shaded strip on the north side of the house, or a corner under a tree that has grown large. Many people feel that nothing will grow there. Foamflower is one of the plants that can change that.
It does best in dappled light, where the sun reaches it only in the morning or through the leaves of trees. Strong western sun in our hot, humid summer is too much for it. It likes soil that stays moist but drains well, and NC State Extension adds that it is not drought tolerant and prefers humus-rich, neutral to slightly acidic soil. Under deciduous trees, the fallen leaves of each autumn slowly help to build that kind of soil.
It combines well with hosta (giboshi in Japanese), astilbe, heuchera and ferns. The different leaf shapes give depth to a shaded space, and in spring the pale flower spikes move in the wind among them. Garden foamflower flowers in spring, from about April into early summer.
In November: leave the leaves
Now, in November, there are no flowers, and this is when the leaves matter. Some selections turn bronze or red in the cold and keep their leaves through winter. NC State Extension notes this for the cultivar 'Running Tapestry'. In a shaded garden where little else has colour in winter, this is a quiet pleasure.
Autumn is also a good time to divide clumps that have spread too far. You can lift a clump and separate it, or cut away the young plants that form at the ends of the runners. NC State Extension advises giving those young plants at least two growing seasons to root before separating them from the parent. If the leaves look tired after winter, they can be cut back at the start of the growing season, and fresh leaves will replace them.
Foamflower and heuchera
Out of flower, foamflower is easily mistaken for heuchera, which has many coloured-leaf selections. The two have been crossed, and the hybrids are sold as ×Heucherella. In Japan they are popular in shade gardens too, combining the leaf colours of heuchera with the airy flowers of foamflower.
