Citrus trifoliata (trifoliate orange, "Karatachi")

Japanese name: カラタチ （枳殻、枸橘）
Scientific name: Citrus trifoliata

[Overview]
A deciduous shrub 2–3 m tall.
Young branches are green, and thick, sharp thorns 1–3.5 cm long are borne alternately at the leaf axils.
The leaves are alternate and trifoliate, 3–5 cm long; the leaflets are elliptic to obovate, 1.5–3.5 cm long and 0.8–2 cm wide.
The leaf margins have fine serrations.
The petiole is winged.
Flowers are borne singly or in pairs in the axils of the thorns on the previous year's branches, and open before the leaves appear.
The flowers are white, 5-petaled and fragrant.
The petals are 1.5–2 cm long.
There are 20 stamens, each separate from the others.
The flowers have no flower stalks.
The fruit is spherical, 3–4 cm, and ripens yellow.
The surface of the fruit is densely covered with short hairs.
Each fruit contains about 30 seeds.
The juice is strongly sour and also bitter, so the fruit is not suited to eating.

[History]
It was introduced to Japan more than 1000 years ago via the Korean Peninsula, and has become naturalized in many regions.

[Uses]
It is used as a rootstock for other citrus plants.
The immature fruit is used as a crude drug.

[Cultivation]
Propagate from seed, cuttings or grafting.
The germination rate drops when seeds dry out, so sow them immediately after harvest, or store them kept moist so they do not dry out and sow them the following spring.
It is highly cold-hardy and can be grown outdoors in southern Hokkaido and areas to the south.
It prefers a sunny site with good drainage and good water retention.
For plants in the ground, watering is unnecessary unless sunny weather continues and the soil dries out.
For potted plants, water thoroughly when the soil surface dries.
In spring and autumn, apply organic fertilizer or a fast-acting chemical fertilizer.
It sprouts vigorously and rarely dies even when pruned hard.
Aphids can occur.
It is a food plant of the swallowtail butterfly.

[Notes]
The name is an abbreviation of karatachibana.

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