Native Plants of Hawai‘i

Cordia subcordata

Cordiaceae or Geiger-tree family

Synonyms: none

Kou

Cordia subcordata has numerous names outside Hawai‘i. See Hawaiian name below.

Natural Range & Environment

Cordia subcordata is found in Southeast Asia, Malesia, coastal regions of tropical East Africa, on Indian Ocean islands, in Australia (Queensland), throughout Micronesia, and on most Pacific archipelagoes [Flora of the Hawaiian Islands]. It's uncertain how much of this range is due to natural expansion and how much is the result of spread by humans, particularly through the Pacific region by the early migrations of Austronesian and Polynesian people.

Until recently, Cordia subcordata was considered by most scientists to be an early Polynesian introduction to Hawai‘i. However, Burney et al. [2001] discovered fossilized Cordia fruits inside Makauwahi Cave on Kaua‘i that predate human habitation of Hawai‘i. Therefore, kou is now considered to be indigenous to all the main Hawaiian Islands except Kaho‘olawe. However, since fossil evidence has only been discovered on Kaua‘i, it's possible that the spread to the other islands was human-assisted.

In Hawai‘i, Cordia subcordata is found sparingly naturalized as well as planted in mostly dry, coastal areas from near sea level to about 125 m (410 ft) [Plants of Hawai‘i]. Kou was once exceedingly common throughout the Hawaiian Islands, mostly due to plantings because of its numerous uses. However, the introduction of the kou leafworm (Ethmia nigroapicella), first recorded in 1883, changed all that. Reportedly, many trees died after the introduction, and, today, while most trees survive seasonal infestations, the species is far less common in Hawai‘i [Friday & Okano 2006].

The bright orange flowers of kou (photograph courtesy of Ken Chamberlain CC BY-NC 4.0), when pollinated, develop into hard, oval, marble-sized fruits that turn from green to dark brown or black when ripe (photograph courtesy of Steve Wells Public Domain). It's not unusual to see multiple seedlings, with their odd-looking seed leaves, sprout from a single fruit buried in the coastal sand (photograph courtesy of Fur, Fangs, Fins and Feathers CC BY-NC 4.0). Don't be fooled by the introduced Geiger tree (Cordia sebestena), today often planted as a kou replacement. Geiger trees have (1) redder flowers than kou, (2) leaves that are dull and sandpapery instead of smooth and shiny, and (3) white, fleshy fruits rather than hard, green to brown or black fruits.

Tapped into the water of this anchialine pool near the coastline on Hawai‘i Island, these wild kou trees are growing particularly well (photograph courtesy of J.B. Friday ©). While kou can be an attractive urban tree, its hard, oval, and slippery fruits can be a safety hazard on sidewalks (photograph courtesy of J.B. Friday ©). Rather, a safer planting site is in a grassy or sandy beach park (photograph courtesy of Forest & Kim Starr CC BY 3.0 US).

Description

Cordia subcordata is a small to medium-sized tree (up to 10 m [33 ft] tall) with rare trees (nearly always outside Hawai‘i) growing larger (up to 15 m [49 ft]) [Friday & Okano 2006]. It has a decades-long lifespan. Frequently, trees will grow to be as wide or wider than they are tall (see photographs above). The trunks of kou are often crooked and/or lowly divided and often tilted by wind. Trees typically have a shallow root system and often produce suckers near their base.

Kou has pale gray bark and light to moderately dense wood that is easily carved or otherwise worked. The sapwood is light tan, occasionally pinkish, while the heartwood is brown with darker brown to black streaks, occasionally with a purple tint. The wood is moderately durable and was favored in ancient times for cups, bowls, and calabashes because it did not impart a taste to the food [Abbott 1992; Krauss 1993].

Cordia subcordata has egg-shaped to elliptic, light green leaves, 3 to 8 inches [8 to 20 cm] long. The leaves' upper surface is shiny with a prominent midrib and major veins, while the lower surface is dull, usually with small hairs along the main veins. Kou with variegated leaves have been seen, but these are rare and not (currently) established as cultivars.

Kou produces clusters of pale to bright orange, funnel-shaped, perfect (bisexual) or staminate (male) flowers [Wagner et al. 1990] about 1 inch (2.5 cm) across with five to seven slightly wrinkled lobes. The flowers are short-lived and without scent. Flowering occurs continuously (or nearly so) year round. Pollinated flowers develop into hard, round to egg-shaped fruits about 1 inch (2.5 cm) in diameter. The fruits are initially green, turning dark brown to nearly black when ripe. The fruits float, which helps explain the species' wide geographic distribution. Each fruit contains one to four delicate, white seeds.

The introduced and popular Geiger tree (Cordia sebestena) is sometimes mistaken for kou. However, Geiger trees have (1) redder flowers than kou, (2) leaves that are dull and sandpapery instead of smooth and shiny, and (3) white, fleshy fruits rather than hard, green to brown or black fruits (see photographs above).

Check out this short YouTube video by Wild Understories about Cordia subcordata.

Growth Requirements

General

Kou is an attractive shade tree or screen for hot, sunny, lowland locations — not so much for cool, cloudy, or rainy highlands. It does best in alkaline to neutral pH soils, such as sandy and calcareous soils, and struggles in acidic soils. However, it still does moderately well in clay-dominant (provided water drains away reasonably well) or lava/cinder-dominant soils.

When initially planted, water your kou regularly (i.e., once or twice a week) until it has established its root system and is visibly growing (typically after a couple of months). Then, decrease your watering to once a month or possibly not at all, depending on the rainfall and water table level at the site; your kou will "tell" you if it needs more watering with wilted or dropped leaves. Kou is adapted to dry climates, so avoid overwatering your plant.

Kou can tolerate extreme heat and moderate drought and winds. Several can be planted as a windbreak. However, because kou has relatively shallow roots, even mature trees can be uprooted during a windstorm. They also tend to grow tilted over time in locations with a persistent wind. Avoid digging or regularly disturbing the soil surrounding your tree because this will damage its shallow roots and potentially reduce the tree's health. Kou can survive in saline soils and tolerate some salt spray, but heavy or persistent salt spray will damage its leaves.

Young kou seedlings grow quickly under optimum conditions (e.g., a warm, sunny site with sandy soil and moderate soil moisture) and can reach 3 to 5 feet (1 to 1.5 m) tall in about a year. After four years, a tree can be as much as 23 feet (7 m) in height. However, growth can be considerably slower in less favorable locations or if the tree is regularly attacked by the kou leafworm (see Pests and Diseases below). Additionally, if you are growing kou to eventually harvest the wood, you will be waiting decades. Expect to see your first flowers after three to five years [Friday & Okano 2006]. Fortunately, once kou begin flowering, they almost never stop.

Kou trees rarely grow very straight, and they branch frequently. Therefore, plan on regularly pruning your tree to establish and maintain a desired shape. They also regularly produce shoots near their base, which need to be removed to prevent a "bushy" tree.

Mature kou produce lots and lots of fruits that can potentially become a problem. In the middle of a sandy or grassy park or lawn, these fruits tend to disappear into the soil but may also begin sprouting, requiring regular removal if you don't want to create a forest of kou seedlings and saplings. Kou planted near hard surfaces such as sidewalks and roads create a different problem. The hard, nearly-round, and sometimes slippery fruits can become a real hazard for anyone stepping on them without caution. For this reason, kou are not always the perfect choice for urban areas.

NTBGSavePlants has a great YouTube video restating much of the above-described requirements and recommendations. Check it out.

Pests and Diseases

The most serious threat to kou is the larvae of an introduced moth called the kou leafworm (Ethmia nigroapicella) (see photographs below) that can completely defoliate and, in severe cases, kill a kou tree. Today, most healthy trees survive the seasonal attacks by this pest, going without leaves for a few weeks or months (see photograph below). However, if you want to help your tree, you can (1) remove the caterpillars by hand and also remove the damaged and fallen leaves, which may be hiding cocoons; (2) spray your tree with Bacillus thuringiensis (Bt), which will kill the leafworms; or (3) apply a systemic insecticide that will kill the leafworm but can also kill other insects such as bees visiting the flowers.

Because of the kou leafworm, many local landscapers today plant the introduced Geiger tree (Cordia sebestena), native to the American tropics, which is not attacked by the kou leafworm. While we understand the reasoning behind this switch, we still feel that, because of its native status and cultural importance, kou should always be the preferred choice for local landscapes.

Seedlings and saplings sometimes die from overwatering, presumably because of a soil-borne fungus or bacteria.

Kou leafworm (Ethmia nigroapicella) larva (photograph courtesy of Forest & Kim Starr CC BY 3.0 US) and adult (photograph courtesy of Rosario Douglas CC BY 4.0). Kou trees defoliated by the kou leafworm (photograph courtesy of J.B. Friday ©).

Uses

Ancient Hawaiian

While we now know that Cordia subcordata is indigenous to Hawai‘i [Burney et al. 2001], most ethnobotanists think that ancient Polynesian voyagers also very likely brought kou (fruits or seeds) with them to Hawai‘i because of its useful wood and flowers [Abbott 1992].

Krauss [197-; 1993] provides the most comprehensive list of Hawaiian uses for kou, many of which are also reported in the accounts of other ethnobotanists and botanists such as Abbott [1992], Buck [2024 reprint of 1957 book], Handy & Handy [1972], Malo [1898/1903], and Neal [1965]. According to Krauss [197-], kou was planted in hot, southern, and leeward locations in and near settlements around houses and by the seashore. There, under the trees, Hawaiian women would beat their kapa.

Nearly all the above-mentioned authorities note that, along with koa (Acacia koa), milo (Thespesia populnea), and kamani (Calophyllum inophyllum), Hawaiians favored kou wood for making containers such as pola (cups), ʻumeke (bowls), and pā (platters). (All four types of wood were easy to work/carve, durable, and attractive.) Additionally, unlike the native koa, kou wood did not impart a (disagreeable) flavor to the food held in the container. Other types of containers, such as those for holding fishing hooks or feathers, were also made of kou wood. A special type of container called an ʻumeke māna ‘ai, made to be used only by a favorite child (e.g., firstborn), was made from the wood of a kou tree that had been planted over the afterbirths of the grandparents. Kou wood was also used to fashion idols.

Krauss [197-; 1993] and others also note that kou leaves were used to make a tan or brown dye that was used to stain fishing lines and kapa, respectively, and that the flowers were used to treat ‘ea (thrush).

Of course, the most common use of kou flowers was for lei. According to Abbott [1992], these lei kou were always made in the kui style (strung through the center one after another), a style that required greater skill back then than today because a non-metallic needle needed to be fashioned beforehand.

Because Cordia subcordata has such a wide geographic distribution, it has been extensively used by other non-Hawaiian indigenous people. Unfortunately, sometimes, these uses are incorrectly attributed to ancient Hawaiians without supportive documentation. For a partial list of these uses, please refer to Useful Tropical Plants.

Modern

Today, lei kou are still commonly made in Hawai‘i [McDonald & Weissich 2003], and local woodworkers and artisans sometimes use kou wood to make traditional Hawaiian items such as ʻumeke as well as jewelry and furniture [Friday & Okano 2006]. Check out this short Instagram video by pala.hi on how to make a lei kou.

Special Features and Information

General

Cordiaceae is a new plant family recognized by some international groups but not by others (which maintain its genera in Boraginaceae). Currently, it consists of two genera of tropical and subtropical shrubs and trees, Cordia with about 228 species and Varronia with about 141 species [WikipediA; Cordiaceae]. In Hawai‘i, Cordia subcordata is the only native (indigenous) species of Cordiaceae, along with two or three naturalized species and several species found only in cultivation [Flora of the Hawaiian Islands; Plants of Hawai‘i].

In Hawai‘i, there is an old legend or proverb about a demanding young chiefess who was consumed by sharks because she repeatedly taunted an old woman and sorceress in ‘Ewa, O‘ahu, demanding the woman give her the lei kou she was making. Out of respect and caution, people living in ‘Ewa who know the story avoid wearing lei kou. Additionally, in Tonga, it's believed that the god Maui, who brought fire from the underworld, when a downpour of rain came that might extinguish the fire, told the fire to hide within the kou, coconut, and breadfruit [Neal 1965].

The wood of Cordia subcordata contains flammable resins and other chemicals that make it burn easily and rapidly. Because of this, the name "kerosene wood" is commonly used for this species in places like Papua New Guinea, where the species is indigenous, as well as in other places [Friday & Okano 2006].

Hawaiian Name

While nearly everyone in Hawai‘i refers to Cordia subcordata as Kou, outside Hawai‘i, the species has a multitude of names. English names include beach cordia, ironwood, island walnut, kerosene wood, and sea trumpet. Refer to Friday & Okano [2006] for a rather comprehensive list.

Etymology

The genus name, Cordia, honors Euricius Cordus (1485–1535) and his son Valerius (1515–1544), both German botanists and pharmacists [Wagner et al. 1990]. The species name, subcordata, is from the Latin words, sub, meaning "below," "approaching," or "nearly," and cordata, meaning "heart-shaped" [Gledhill 2008], referring to the nearly heart-shaped leaves of this species.

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