Last week’s beautiful and unexpected plant treat was a single-flowered Kerria japonica (Japanese Kerria) in a front garden on Earlham Road in Norwich.

Too many gardens have become barren car parks and it is always a pleasure to see one full of plants and even better when it’s full of unusual and interesting plants. Floating above a carpet of tiny Lesser Celandine flowers, the Kerria japonica was stunning, a cheery yellow on a mostly grey spring day.

Kerria japonica (Japanese Kerria). The single-flowered form. 21st March 2024.

Kerria japonica (single flowered)

Like the plant I saw on Earlham Road, the wild form of of Kerria japonica has open, single yellow flowers with five petals (note 1). The flowers remind me of the beautiful yellow rose Rosa xanthina ‘Canary Bird’ which we grow in our back garden. This is not surprising, as roses and Kerria are both members of the Rose family, Rosaceae.

But I’ve only seen single-flowered form twice. I usually see the double-flowered form in gardens, Kerria japonica ‘Pleniflora’.

Kerria japonica ‘Pleniflora’

I think Kerria japonica ‘Pleniflora’ lacks the charm of the single-flowered form but it’s still a cheery plant and it was always easier to find in garden centres. I first grew it as a ten year old child in my garden in Scotland and it brings back happy memories.

Kerria japonica spreads by suckering shoots to form a thicket of stems and one of these shoots spread from a neighbouring allotment through a Lawson’s Cypress hedge into the edge of my plot, where it was a welcome visitor.

Kerria japonica ‘Pleniflora’ in our back garden. Early April 2017.

I dug up some of the suckers and introduced them into our garden at home. We were planting up Grapes Hill Community Garden in Norwich at the time (spring 2011) and I used another sucker to introduce Kerria japonica ‘Pleniflora’ there too.

An Adaptable Plant

In the wild, Kerria japonica grows in in thickets on mountain slopes in China, Japan and Korea. It is the only species of Kerria.

Both forms of Kerria japonica are cheerful, adaptable and long flowering shrubs. The main flowering period is spring, from March until early May, but there can be a smaller flush of flowers in August.  Kerria japonica has alternate, simple “birch-like” leaves with a doubly serrated margin.

Kerria japonica grows happily in many different aspects, from full shade to full sun. I think it looks at its best in dappled shade; flowers soon bleach in full sun. It tolerates most soil types but dense soils can sometimes cause the roots to rot. I have grown it in loam and in rather sandy soil.

With its suckering habit Kerria japonica soon develops into a thicket of tall, graceful stems, which can cover up a wall or fence, even in dark corners or against north-facing walls.

Stems grow to six to ten feet (1.8 – three metres) tall and the plant can spread to a similar width. Any spare suckers are easy to chop off with a spade and can be used to make new plants and, perhaps, new gardening friends. But this is the way the plant renews itself, so leave some to grow. Old, dead stems can be cut back to allow air into the centre of the shrub and to keep it tidy. The time to prune is after flowering, in June and July. It’s best to take cuttings from August to October, though I’ve dug up roots in early spring and, with enough watering, they’ve taken well. (But please read below about Kerria Twig and Leaf Blight below before you give away cuttings.)

Kerria and Wildlife

The Gardeners’ World website says Kerria ‘Pleniflora’ has no particular known value to wildlife in the UK. Double-flowered plants tend to lack pollen and nectar, so are usually pretty hopeless for insects. However, on one sunny day in early April 2017 we saw two species of hoverfly and a Conopid fly visiting the flowers in our back garden (note 2). The hoverflies may have been attracted by the yellow flowers rather than any reward; the Conopid fly would have been hanging around waiting for a solitary bee to parasitise.

I haven’t seen the single-flowered forms of Kerria japonica often enough to see any insects on the flowers. The Wildflower Web website says that “the plant is also a favorite of pollinators, including bees and butterflies, making it a great choice for those looking to attract wildlife to their garden”. Please let me know if you have any observations.

Seeing double: Kerria japonica ‘Pleniflora’ with Syrphus sp. hoverfly. 2nd April 2017.

Kerria japonica and Conopid fly (Myopa sp.). 2nd April 2017.

Other Names

Kerria is named after the Scottish plant collector William Kerr (note 3). The specific name “japonica” refers to Japan.

As well as Japanese Kerria, Kerria japonica is known as Jew’s Mallow and Kerria japonica ‘Pleniflora’ is sometimes known as Bachelor’s Buttons. The Wildflower Web website also gives the names Easter Rose, Japanese Marigold Bush, Japanese Rose, Miracle Marigold Bush, Yamabuki and Yellow Rose (note 4).

Kerria Twig and Leaf Blight

All is not well with Kerria japonica.

In 2018 we noticed that our Kerria japonica was dying back. At first I thought this was because of the long summer drought but more recently I realised the plant had Kerria Twig and Leaf Blight, caused by the ascomycete fungus Blumeriella kerriae. 

The RHS website gives the symptoms:

• On the leaves the infection starts as small red-brown spots (1 – 5 mm diameter) with dark purple borders and yellow haloes. Spots are visible on both leaf surfaces and sometimes number in the hundreds on a single leaf
• In wet conditions the spores of the fungus may be visible as white clusters in the centre of the spots
• As the infection progresses the spots coalesce and the leaves turn yellow through to brown and fall from the stems
• Stem lesions appear as purple-brown, slightly-sunken elliptical cankers which remain visible on the stems throughout the year. Cankers which girdle the stem result in extensive stem die-back.

The Fungi of Great Britain and Ireland website has descriptions and photographs of leaf lesions and microscopic details, including conidia (the asexual spores; the sexual, teleomorph stage of the fungus is not known in the British Isles).

Stem lesions on Kerria japonica caused by Blumeria kerriae.

Since the first infection I’ve cut off and binned the diseased shoots but the fungus has taken hold and the few shoots that our shrub produces no longer grow as tall or as strong as they did in 2017. I know of other people who have dug out their Kerria plants.

Blumeria kerriae spores are thought to be transferred between plants via rain splash, wind, and transfer on contaminated tools.

Blumeria kerriae overwinters on fallen leaves and lesions on the stems then releases spores to reinfect new growth in spring. Spore production has been observed throughout the year on Kerria plants in the UK.

Blumeria kerriae was known in the United States but the first observations in the British Isles date from 2014. Presumably it came here on diseased plant material.

If you grow Kerria japonica and intend to pass on cuttings from your plant, please make sure it has no signs of disease.

Sadly, the occurence of Blumeria kerriae in the British Isles means that Kerria japonica is no longer the tough, disease-free plant it once was.

Happily, the Earlham Road plant looked healthy. I’ll enjoy it while I can.

Kerria japonica (Japanese Kerria). The single-flowered form. 21st March 2024.

Photographs by Vanna Bartlett, except for stem with Blumeria kerriae.

Notes

Note 1 – ‘Simplex’ and ‘Golden Guinea’ are named, single-flowered forms of Kerria japonica.  The latter has been given an Award of Garden Merit (AGM) by the Royal Horticultural Society, as has the double-flowered ‘Pleniflora’. There is a variegated form too, if you like that sort of thing.

Note 2 – The hoverflies were a male Eupeodes luniger and a species of Syrphus.

Note 3 – William Kerr was born in Hawick in the Scottish borders. He sent back to Britain examples of 238 plants new to European gardeners and to science. He died in 1814.

Other plants William Kerr brought back include Euonymus japonicus, Pieris japonica, Nandina domestica and Rosa banksiae.

Note 4 – Yamabuki is the Japanese name for the plant. According to the Yamabuki Japan website, he word was first used in the description of spring scenery in the oldest Japanese poem, “Manyo-shu”.