This dense shade tree is particularly useful for planting near streets and in parking lots and lawns. It is also popularly used as a bonsai tree. Japanese zelkova is resistant to Dutch elm disease and has therefore been increasingly used in place of American elms.
I. Appearance and Characteristics
Zelkova serrata (Japanese zelkova, Japanese elm, keyaki, or keaki; Japanese: 欅 (ケヤキ) keyaki /槻 (ツキ) tsuki; Chinese: 榉树/櫸樹 jǔshù; Korean: 느티나무 neutinamu) is a species of the genus Zelkova native to Japan, Korea, eastern China and Taiwan. It is often grown as an ornamental tree, and used in bonsai. There are two varieties, Zelkova serrata var. serrata in Japan and mainland eastern Asia, and Zelkova serrata var. tarokoensis (Hayata) Li on Taiwan which differs from the type in its smaller leaves with less deeply cut serration on the margins.
Zelkova serrata is a medium-sized deciduous tree usually growing to 30 m (98 ft) tall. It is characterized by a short trunk dividing into many upright and erect spreading stems forming a broad, round-topped head. The tree grows rapidly when young though the growth rate slows to medium upon middle age and maturity.
It has alternately arranged leaves growing to 5 cm long and broad. The leaves themselves are simple and ovate to oblong-ovate with serrated or crenate margins, to which the tree owes its specific epithet serrata. The leaves are acuminate or apiculate, rounded or subcordate at the base, and contain 8–14 pairs of veins. The leaves are rough on top and glabrous or nearly glabrous on the underside. They are green to dark green in spring and throughout the summer, changing to yellows, oranges and reds in autumn. The petioles are 2–5 mm (1⁄16–3⁄16 in) long.
Zelkova serrata is monoecious. It develops flowers in spring with the leaves. Buds are ovoid, acutish, with many imbricate, dark brown scales. They diverge at a 45 degree angle from the stem. The staminate flowers are shortly pedicellate and approximately 3mm in diameter, clustered in the axils of the lower leaves. The pistillate flowers are solitary or few in axils of the upper leaves, sessile and usually about 1.5 mm in diameter. The flowers are yellow-green, not showy, and occur in tight groups along new stems. They give rise to small, ovate, wingless drupes that ripen in late summer to autumn. The drupe is green maturing to brown, subsessile and 2.5 to 3.5 mm (3⁄32 to 1⁄8 in) in diameter.
To identify Zelkova serrata, one would look for a short main trunk, low branching and a vase-shaped habit. The twigs are slender with small, dark conical buds in a zigzag pattern. The branches are usually glabrous. The bark is grayish white to grayish brown and either smooth with lenticels or exfoliating in patches to reveal orange inner bark. The branchlets are brownish-purple to brown.
II. How to Grow and Care
Sunlight
Japanese zelkova needs about 6 hours of direct sunlight to thrive. While the tree can tolerate partial shade to an extent, this will restrict its growth and health.
Temperature and Humidity
The Japanese zelkova is relatively cold hardy and can thrive in most temperate regions of the United States. The ideal USDA zones for the zelkova are 5 through 8.
Watering
When grown in the garden, your japanese zelkova will only require watering while establishing, meaning during the first year or two after planting. Water deeply every 1-2 weeks during the growing season to keep the soil moist but not soggy. Be careful to avoid overwatering, especially if you live in a rainy climate.
If you are growing your Japanese zelkova as a bonsai, the soil should be kept slightly moist year-round. You don’t want your Japanese zelkova to dry out, but you should also be careful to avoid overwatering – soil that is too wet can cause root rot.
Soil
Japanese zelkova thrives in fertile, well-drained soil. It tolerates a variety of soil types, as well as acidic, neutral, and alkaline soils, with an ideal pH range of 5-7.5. Wet, poorly-drained soil should be avoided. If growing in a container, use a rich, well-draining potting mix.
Fertilizing
Japanese zelkovas require no regular supplemental fertilizer. If the tree is not performing well, it might be wise to amend the soil to provide a bit of a boost. Test your soil to establish the best type of amendments to handle any soil deficiencies that may be occurring.
Planting Instructions
A newly-planted japanese zelkova will be susceptible to frost, so plant your tree in the spring as soon as the danger of frost has passed. Dig a hole that’s approximately twice the size of your tree’s root ball or pot. Adding compost to your planting hole will increase fertility and improve drainage. plant your japanese zelkova so that the soil is at the same level on the tree as it was previously, before packing the soil in well around the roots to remove air pockets. Water well after planting.
Pruning
Prune your Japanese zelkova in the fall after the leaves have fallen. In addition to shaping the tree, you should also remove any dead or damaged branches. The tree may sometimes put out multiple leader branches – these should always be pruned so that there is only one main leader or trunk. Never prune more than a third of the branches in a year.
If you choose to cultivate your japanese zelkova as a bonsai, it will need much more attention and pruning. Wiring and pruning are best done in the fall to correct the shape of the branches and to achieve a broom-style silhouette.
Propagation
Propagate a Japanese zelkova by seeds or plant cuttings. Seeds can produce variations of zelkova but cuttings taken from a specific tree will give you uniform seedlings if you prefer to propagate a group of similar zelkovas. Here’s what to do when you take a cutting in the spring or early summer:
- With a sterile cutting tool, take a new branch that is 4 to 6 inches long.
- Remove the lower leaves but leave a couple on top of the cutting.
- Dip the bottom of the cutting into the rooting hormone.
- Place the cutting into a pot with moist potting soil. If you are putting a few cuttings into one pot, space them about 1 inch apart.
- Expect cuttings to root in about 10 weeks. Repot to larger pots until they can be planted in a permanent spot.
How to Grow from Seed
Growing this tree from seed can be achieved, but it requires a lot of patience. You will need to keep the seedling in a pot for a couple of years until it is strong enough to be planted outdoors in its permanent place. If you want to grow Japanese zelkova from seed, harvest the seeds in the fall when the tree’s flowers have faded and the pods have dried. Take these steps:
- Put seeds into a bag of moist sharp (or builder’s) sand and refrigerate for six to eight weeks for a cold period that will break dormancy. Alternatively, you can keep them in a bag stored in a cold outdoor shed or outbuilding.
- Then sow the seeds (and any that have already germinated) into pots filled with garden soil about 1/4 inch deep.
- Keep the pots out of direct light and keep soil moistened.
- Seeds should take a couple of months to germinate.
Pests and Diseases
Common Pests & Plant Diseases
Zelkova trees are susceptible to aphids that will attack the tree in the spring. Aphids will cause leaf and twig damage and ultimately death. Scale insects are another issue for zelkovas. These insects suck the juices from leaves and twigs of the tree, leaving behind waste, called “honeydew.”
The tree can experience Phytophthora stem and collar (the swollen area around the base of a branch and trunk) rot disease. This soil-borne organism forms cankers on the tree’s twigs, but not on the leaves.
Common Problems
The primary concern that Japanese zelkova presents is how easily it breaks. A habit the species has is the tendency to grow multiple leaders while having very fragile wood. Having multiple leaders is a genetic weakness in the species that is somewhat solved in some available cultivars but not in the wild type. If searching for a Japanese zelkova, it is crucial to buy a tree with a single trunk or leader, or the tree will eventually experience breakage at the tree’s crotch later in its lifespan or if it experiences ice.
III. Uses and Benefits
Zelkova serrata is planted as a lawn or park tree for its attractive bark, leaf color and vase shape. It provides good shade and has an easy fall cleanup. It is easy to transport, and often available in burlap form. It is also commonly used for bonsai; its attractive shape and colors make it a popular choice for the art. It is often grown as an ornamental tree, both in its native area and in Europe and North America. The first cultivation outside of Asia was by Philipp Franz von Siebold, who introduced it to the Netherlands in 1830. Recently, it has been planted as a “street tree” in New York City. In the UK it has gained the Royal Horticultural Society’s Award of Garden Merit.
Find Where to Buy the Best Japanese Zelkova (Zelkova serrata)
[content-egg-block template=offers_logo_shipping cols_order=3,2,1,5,4 hide=badge visible=number border_color=info limit=8]