Cycas revoluta (king sago, sago cycad, Japanese sago palm), is a species of gymnosperm in the family Cycadaceae, native to southern Japan. It is one of several species used for the production of sago, as well as an ornamental plant. Cycads are not closely related to the true palms (Arecaceae). The Latin specific epithet revoluta means "curled back", in reference to the leaves. This very symmetrical plant supports a crown of shiny, dark green leaves on a thick shaggy trunk that is typically about 20 cm in diameter, sometimes wider. The trunk is very low to subterranean in young plants, but lengthens above ground with age. It can grow into very old specimens with 6–7 m of trunk, however, the plant is very slow-growing and requires about 50–100 years to achieve this height.
Propagation of Cycas revoluta is either by seed or by removal of basal offsets. It is one of the most widely cultivated cycads, grown outdoors in warm temperate and subtropical regions, or under glass in colder areas. It grows best in sandy, well-drained soil, preferably with some organic matter. It is fairly drought-tolerant and grows well in full sun or outdoor shade, but needs bright light when grown indoors. Of all the cycads, C. revoluta is the most popular in cultivation. It is seen in almost all botanical gardens, in both temperate and tropical locations. In many areas of the world, it is heavily promoted commercially as a landscape plant. It is also quite popular as a bonsai plant. It is tolerant of mild to somewhat cold temperatures, provided the ground is dry. Frost damage can occur at temperatures below -10°C, but require hot summers with mean temperatures of 30 to 35°C for successful growth, making outdoor cultivation impossible in colder temperate regions.
The pith contains edible starch, and is used for making sago. Before use, the starch must be carefully washed to leach out toxins contained in the pith. Cycad sago is extremely poisonous to animals (including humans) if ingested. Pets are at particular risk, since they seem to find the plant very palatable. Clinical symptoms of ingestion will develop within 12 hours, and may include vomiting, diarrhea, weakness, seizures, and liver failure or hepatotoxicity characterized by icterus, cirrhosis, and ascites.
All parts of the plant are toxic, however, the seeds contain the highest level of the toxin cycasin. Cycasin causes gastrointestinal irritation, and in high enough doses, leads to liver failure.
