In spite of its graceful appearance, Kerria Japanese rose, also known as Japanese rose plant, is as tough as nails, growing in USDA plant hardiness zones 4 through 9. Kerria Japanese rose is rarely bothered by pests and tend to be deer resistant. Read on for tips on growing a Japanese Kerria in your own garden.
I. Appearance and Characteristics
Kerria japonica, commonly known as Japanese kerria or Japanese rose, is a deciduous, yellow-flowering shrub in the rose family (Rosaceae), native to China, Japan and Korea. It is the only species in the genus Kerria. In the wild, it grows in thickets on mountain slopes. Japanese kerria has been used for medicine and is also planted in gardens. A double-flowered cultivar, K. japonica ‘Pleniflora’, is commonly called bachelor’s buttons.
The genus name Kerria is also a common name for the species (Kerria). It is named after Scottish gardener William Kerr, who introduced the Japanese kerria cultivar ‘Pleniflora’.
In addition to the common names mentioned above, i is also known as Japanese marigold bush or miracle marigold bush in northern New England, although K. japonica is not closely related to the true marigolds (Calendula spp.), which are asterids.
Kerria japonica grows to 1–3 m (3.3–9.8 ft) tall, with weak arching stems often scrambling over other vegetation or rocks. In the wild it grows in thickets on mountain slopes. The leaves are alternate, simple, 3–10 cm long, with a doubly serrated margin. The flowers are golden yellow, with five petals, and evenly-spaced along branches of new green growth. The fruit is a dry single-seeded achene 4–4.5 mm long.
Kerria is valued in gardens for its golden yellow flowers, which appear in the spring. It is best grown with some shade from full sunlight to avoid blanching the flowers, and needs to be pruned after flowering to maintain health and vigour. Two cultivars have gained the Royal Horticultural Society’s Award of Garden Merit, K. japonica ‘Golden guinea’ and the double-flowered K. japonica ‘Pleniflora’, commonly known as bachelor’s buttons.
II. How to Grow and Care
Sunlight
While kerria is not particular about lighting and grows well in full sun or full shade, partial shade is preferable because the blooms last longer. In full sun, the flowers fade.
Temperature and Humidity
Japanese rose shrubs thrive in all climate conditions over its hardiness range, USDA hardiness zones 4 to 9. In the southern part of the range, it will appreciate deeper shade.
Watering
Japanese rose shrubs need regular water, but take care not to overwater, especially in denser soils. This shrub has a good tolerance for short periods of drought.
Soil
Japanese rose prefers loamy soil and is not overly fussy about soil pH. Avoid planting it in heavy clay soil. It will tolerate poor soils but will perform better in soils enriched with organic material. The soil should be kept evenly moist, but it doesn’t like soggy conditions. Lightly mulching will help conserve soil moisture.
Planting Instructions
Plant kerria in the spring. Dig a hole that is twice as wide as the root ball and has the same depth. Place the shrub in the center. Backfill with original soil and tamp it down. Water immediately and deeply, then spread a 2-inch-thick layer of mulch around the base of the shrub to preserve soil moisture. Water kerria regularly during the first year after planting to establish a strong root system.
Space shrubs 4 to 8 feet apart, depending on the variety and its mature size.
Pruning
Mature kerria shrubs occasionally experience winter dieback. Older, central stems often die. These white-brown dead stems are noticeable in early spring among the live stems, which are bright green and beginning to unfurl leaves. Remove the dead stems as soon as they are noticed in early spring. Old, ragged kerria shrubs can be rejuvenated by renewal pruning in early spring. Begin renewal pruning by removing all the dead stems and then cutting the remaining live stems back to about 12 inches above the ground. Maintain the desired size and shape by pruning kerria moderately in spring just after flowering.
Propagation
This shrub is best propagated by rooting softwood cuttings. Here’s how:
- At the beginning of summer after flowering has finished, snip a four- to five-inch cutting from the terminal end of a branch. Make the cut just below a leaf node. Trim away all the leaves from the bottom half of the cutting. Dip the end of the cutting in the rooting hormone.
- Plant the cutting in a small pot filled with an equal mixture of perlite and sand, or perlite and peat moss. Place the potted cutting in a large plastic bag or under a plastic dome and place it in a location that receives bright indirect light. Mist the cutting daily.
- Roots should begin to develop in four to six weeks. Once you detect roots, allow the cutting to grow for another month, then transplant into a larger pot, if necessary. Keep the plant indoors until the following spring, at which time you can plant it outdoors.
How to Grow from Seed
Growing Japanese rose from seed works the same as any other rose plant. You can buy commercial seeds or find your own by leaving some roses on the vine after they die off and allowing them to develop rose hips; these hips contain the seeds. Once you have acquired the seeds, sow them about 1/4 inch deep in potting soil, water them thoroughly until moist but not soaking wet, cover the container with plastic and place them in the refrigerator for three months to cold-stratify them.
In the early spring, remove the seeds from the refrigerator and move them to an area of warmth; about 70 degrees Fahrenheit is ideal. Expect only about 20 to 30% of the seeds to sprout. Those that do will sprout in two to three weeks. When the seedlings are strong enough, transplant them into a larger container, provide them with half-strength diluted fertilizer designed for roses, and give them plenty of sunlight.
Potting and Repotting
To plant kerria in a container, select a dwarf variety such as ‘Picta’ because the other shrubs get quite large and are not suitable for growing in containers. Choose a container with large drainage holes. Place a layer of rocks or pebbles in the bottom of the container for stabilization. Fill it with potting mix and a few handfuls of compost and plant the kerria in the container. Keep it well watered at all times, as container plants need more frequent watering and fertilization than plants in the landscape.
‘Picta’ is a slow grower that only reaches its mature size in five years, so repotting to a larger pot with fresh soil won’t be necessary unless the roots outgrow the container.
Overwintering
To help protect the plant during the coldest winter months, provide it with a healthy layer of mulch. Dried tree leaves about 1 foot deep are also a great option. Straw will work as well.
Pests and Diseases
Although it is an overall tough specimen, Japanese rose can become infected with twig and leaf blight, a disease caused by the Blumeriella kerriae fungus.1 The symptoms are small red-brown spots that gradually coalesce and cause leaves to turn yellow and brown before falling. Bi-weekly sprays of chlorothalonil fungicide will reduce the spread of the disease, but badly affected stems should be pruned away. This plant has no serious pest enemies.
How to Get to Bloom
Two common problems can lead to fewer blooms on a Japanese rose. One of these is too much nitrogen in the soil, which encourages great foliage but tends to inhibit blooming. Make sure your soil has a balanced nutrient content that isn’t too high in nitrogen. The second reason could be caused by pruning at the wrong time. Make sure you prune right after spring flowering, and you might get a second round of roses before the end of the growing season.
III. Types of Japanese Rose
Here are several popular named cultivars of Japanese rose:
- Kerria japonica ‘Pleniflora’: This is a popular double-flowered cultivar with two-inch pom-pom flowers. It grows 8 to 10 feet tall.
- K. japonica ‘Picta’: ‘This variety is a low-growing, spreading cultivar that grows only two feet tall but has the potential to spread. It has wide, rose-shaped flowers and grayish-green leaves with white edges.
- K. japonica ‘Albescens’: This cultivar has creamy white 2-inch flowers and grows 4 to 5 feet tall. The foliage turns pale yellow in fall.
- K. japonica ‘Golden Guinea’: This single-flowered cultivar has yellow flowers roughly the size of a gold coin (2 1/2 inches). It grows 6 to 8 feet tall.
- Kerria japonica ‘Simplex’: ‘This cultivar is known to rebloom, and it has bright green stems that provide winter interest to the landscape. It grows 6 to 8 feet tall.
IV. Uses and Benefits
- Ornamental uses
Plant a single kerria in a shrub border or in the back of a perennial border where it will stand out in spring and blend in with other plants the rest of the season. It also makes a great foundation plant, grows well along property lines or fences, and reliably adds seasonal interest in spring and again in winter. Or you can plant several kerria together for a living garden screen.
- Medicinal uses
The plant has anti-inflammatory properties and a decoction of the flowering shoots with honey has been used in China in the treatment of coughs and gynecological disorders.
