In addition to attractive yellow flowers, the bark and branches are also interesting. The main branches on the double flowering type arch gracefully to a height of 8 to 10 feet (the width can be restricted to similar dimensions through general pruning and, specifically, removing suckers). Smaller branches radiate off the main branches in all directions. The branching pattern thus affords interest both vertically and horizontally; it is also relatively airy. The bark is a pleasing kelly green to greenish-yellow—a color retained throughout the winter. Japanese rose is generally planted as a nursery-grown potted specimen in spring or fall. It grows relatively fast, achieving its full size in the first year after planting a container-grown specimen. Japanese rose is largely a carefree shrub but requires you to frequently remove suckers if you want to prevent the shrub from spreading. Japanese rose does not make a good formal hedge because it requires extensive pruning, but it can work suitably as an informal screen. It also works very well for shady woodland gardens and shrub borders, where it can be allowed to sucker and spread.
Light
Grow this bush in partial to full shade for best flower production. It is one of the most shade-tolerant deciduous flowering shrubs. Too much bright sunlight causes the flower color to fade quickly.
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.
Water
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.
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.
Fertilizer
Fertilize Japanese rose shrubs lightly in spring with a slow-release fertilizer. Spreading a layer of compost or organic mulch around the plant will also provide nutrition to the plant.
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.
Pruning
This plant blooms on old wood (the previous season’s growth) in early-to-mid spring. If you must prune for shape or to control the height, do it just after it finishes flowering in spring . A second flowering later in the growing season is not unusual, but it is too late to prune at that point. Prune out dead branches as you find them. Older plants in need of rejuvenation can be cut down to ground level. Japanese rose spreads by suckering; remove these suckers as they appear if you wish to control their spread. The main problem with this plant is that it spreads vigorously, so stay ahead of it by regularly removing suckers.
Propagating Japanese Rose
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 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 Japanese Rose 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 Japanese Rose
When potting or repotting Japanese rose, choose a pot that is large enough to allow for growth. Most experts recommend a pot of at least 15 inches in diameter. When moving the rose, take care not to touch the roots, as this can damage them. Carefully use a large spoon or similar tool to scoop the rose bush up by the roots and transplant it to its new home. Japanese rose can be grown in pots for a few seasons until large enough to plant outside.
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.
Common Pests and Plant 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. 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 Japanese Rose 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.
