Castor Oil for What Ails You?
Contact: K. L. Kyde, Maryland Department of Natural Resources
ANNAPOLIS, MD (September 1, 2011) - Castor aralia, Kalopanax septemlobus, is a tree from moist, deciduous forests in Asia that is planted as an ornamental in North America. It is the only large tree member of the Aralia family, and is related to our native plants, wild sasparilla (Aralia nudicaulis) and devil’s walkingstick (Aralia spinosa). Although it looks similar and is a strongly medicinal plant, castor aralia it is not the source of the old time medicine castor oil. That comes from seeds of the castorbean plant – in a completely different plant family. Because its fruit are visible now and its name comes from the same root word as the name of this month, MISC has designated castor aralia as the September Invader of the Month.
Castor aralia can grow 80 feet tall in the wild, but in cultivation is more often 40-60 feet tall. Mature trees have an upright, oval habit. The tree has alternate glossy green leaves that look somewhat like sugar maple or sweetgum leaves. They have 5-7 palmate lobes (the main leaf veins originate at the center base of the leaf), with small regular teeth and elongated lobe tips. The specific epithet for this plant comes from the presence of seven lobes: septem - lobus. The leaves can be as wide as 14” but are more frequently 8-10” wide. Leaves turn reddish or yellowish in the fall, but are not particularly flashy.
In late summer, castor aralia sports many tiny white flowers in large umbrella-like clusters. A whole inflorescence can be two feet across! The flowers produce tiny bluish-black fruits in the fall, which are avidly eaten by birds. The tree’s stout, coarse-textured branches and stems have many broad-based prickles or thorns, similar to its native cousin, devil’s walkingstick. These thorns sometimes become less prominent on older bark, which becomes dark, ridged and furrowed.
Castor aralia is native to the Russian island of Sakhalin, Korea, Japan and China. It was introduced to the U.S. in 1865, and is generally used ornamentally as a shade tree. It is not easily available in nurseries. It is not widely distributed in the U.S. – for example, the USDA Plants Database lists it as existing in the wild only in Maryland and Connecticut. There are specimen trees planted in arboteta across the country, including the Arnold Arboretum at Harvard and the Missouri Botanical Garden, and Baltimore’s own Cylburn Arboretum.
If castor aralia is hard to find in order to plant, and is not widespread, why does MISC consider it an invasive species risk? This tree is suspect because it shares many characteristics with known invasive plants. It is cold hardy, as it lives quite well at the University of Wisconsin Arboretum. It is adaptable to soil pH, and grows quickly in moist soil, especially with full sun. It is described as “long-lived and trouble free” by horticultural experts, with no pest problems. It produces great quantities of seed, which birds distribute, and produces many small seedlings. In short, it fits the description of a potentially invasive species well. It is possible that castor aralia is not an invasive plant; it is also plausible that castor aralia is a plant with invasive characteristics that is still within the “lag" period of its expansion, and that it has not become invasive yet.
So be on the lookout for castor aralia occurring in the wild, or for seedlings popping up near where a specimen tree has been planted. By many descriptions a lovely ornamental, castor aralia is not a species we want escaping in Maryland.
For more information about Invasive Species of Concern in Maryland, visit www.mdinvasivesp.org
photos available electronically on request.