Japanese holly

Identification and origin

Ilex crenata belongs to the Aquifoliaceae, the holly family. It is native to Japan, Korea, China (including Hainan), Taiwan, Sakhalin and the Kuril Islands, as well as the eastern Himalaya (Nepal, Tibet, Assam), Myanmar and Vietnam. It has naturalized locally in Europe and the United States. In Japan it is called inu tsuge, “false boxwood.”

It is a dense evergreen shrub with many stiff stems and a rounded habit, or more rarely a small tree. Growth is slow. In cultivation it usually reaches 5–10 ft tall and 5–8 ft wide; many cultivars stay more compact, around 3–4 ft. Over time, though, it can reach 13–26 ft in ten to twenty years.

The alternate, elliptic to obovate leaves are 1–1 in long and up to nearly 1 in wide. They are glossy dark green above and dull pale green below, where small dark or translucent dots can be seen. The margins are finely scalloped, which gives the species its name, and have no spines. Young stems are green and downy and turn grayish within two or three years; the bark stays smooth.

The plant is dioecious: each individual is either male or female. In late spring or early summer, small greenish-white four-petaled flowers, fragrant and inconspicuous, attract bees. Only pollinated female plants then bear small round black drupes, 0–0 in across, which ripen in early fall and stay on the shrub all winter, although they are often hidden under the foliage.

Nearly all Japanese hollies in the trade are cultivars, and there are several hundred of them:

• ‘Convexa’: cupped, convex leaves; female, fruits heavily and is fairly cold hardy.
• ‘Helleri’: dwarf, spreading form with small leaves, about 4 ft tall and 5 ft wide; female, fruits only occasionally.
• ‘Hetzii’: a dwarf version of ‘Convexa’, 2–3 ft.
• ‘Compacta’ and ‘Stokes’: dense dwarf forms, male.
• ‘Sky Pencil’: narrow, columnar outline without pruning; female.

It is often mistaken for boxwood (Buxus sempervirens), which it frequently replaces in hedges. Its foliage, however, is darker and glossier, and it suffers less from insects and diseases.

Light

Japanese holly grows in full sun or part shade, with anywhere from two to more than six hours of direct sun a day. A sunny spot gives more compact growth and more fruit; in shade the plant stays looser. A site sheltered from cold winds spares the foliage from winter damage.

Watering

It likes soil that stays evenly moist but never soggy: it is very intolerant of poorly drained soil, and excess water encourages root rot. Once well established it copes better with drought, but it does not like very dry ground.

Rather than following a schedule, check the soil: water deeply when it starts to dry out below the surface, especially during the first year and in dry summers, then let it drain. An organic mulch holds moisture and keeps the roots cool. In a container, check the root ball more often, since it dries out much faster than garden soil.

Potting mix

It needs well-drained soil that is rich in organic matter and slightly acid. It accepts loam, sand and even clay, from acid to neutral pH, as well as poor soils and urban air pollution. Alkaline soil, on the other hand, turns the leaves yellow. On heavy or wet ground, plant on a mound or in a raised bed so water can drain away.

Temperature

The species is hardy in USDA zones 5a to 8b, which means winter lows down to about -20 °F. It easily withstands winters throughout northern Europe, in the range of -4–5 °F.

Heat is the bigger limit: it does best with cool, moist summers and grows poorly where it is both very hot and very humid. Winter winds damage the foliage, so a sheltered site suits it best.

Repotting

In the ground, it transplants easily. Container-grown plants are best planted in early fall, in prepared, well-drained soil, and then mulched.

In a pot, choose a container with drainage holes and a free-draining, slightly acid mix. Repot when roots fill the pot, in spring, just before growth starts. Growing it as bonsai follows its own repotting rules, covered in the dedicated guide.

Pruning

Japanese holly tolerates repeated and even severe pruning, which makes it an excellent choice for hedges, edgings and topiary.

• Maintenance: light pruning is enough to keep a natural shape. Trim hedges in early spring.
• Architectural shapes: in Japanese gardens it is either allowed a natural form or pruned to reveal the structure of its branches, as low, wide mounds (tamamono) or large, connected wave-like masses (o-karikomi).

Propagation

• Cuttings: the most common method. Take semi-ripe cuttings in late summer or early fall, or hardwood cuttings in midwinter with bottom heat.
• Seed: possible but slow, since germination can take two years. Seedlings do not come true to a cultivar.

Common problems

The same symptom can have several causes: yellowing can come from alkaline soil, excess water, a root disease or a pest. Check drainage and soil before treating anything.

• Black root rot (Thielaviopsis basicola): favored by wet, cold soil. The plant is stunted, leaves yellow between the veins, roots darken and the shrub declines during dry spells. Improve drainage or plant on a mound.
• Phytophthora: Japanese holly is very susceptible in poorly drained soil. Honey fungus can also attack it.
• Nematodes: several kinds damage the roots; there is no effective treatment for an established plant.
• Mites: fine yellow stippling followed by a bronze cast points to southern red mite, most active in spring and fall.
• Scale, aphids and leafminers: scale causes yellowing and leaf drop, aphids target young shoots, and holly leafminer makes winding mines in the leaves.
• Holly leaf blight: an occasional disease.

Overall, the species is not prone to serious problems.

Toxicity

Japanese holly is toxic. Its fruits contain saponins: if swallowed, they can cause vomiting and diarrhea in people, with low toxicity. Hollies are also toxic to dogs, cats and horses: eating the leaves or berries causes nausea, vomiting, diarrhea and lethargy. The plant does not irritate the skin. Keep the fruits out of reach of children and pets.