A native to Northern China and Korea, hardy orange is recognizable by the large 3 to 5 cm thorns on the shoots and its deciduous leaves. It is also unique as a citrus plant because its fruit has a downy fuzz similar to a peach. The ripe fruit is very bitter but can be used to make marmalade.
| Common name | Hardy Orange, Trifoliate Orange |
| Botanical name | Citrus trifoliata |
| Family | Rutaceae |
| Species | trifoliata |
| Origin | Central and northern China |
| Life cycle | Woody |
| Plant type | Edible |
| Hardiness zone | 5, 6, 7, 8, 9 |
| Sunlight | Full Sun |
| Maintenance | High |
| Soil ph | Acid |
| Drainage | Well-Drained |
| Growth rate | Fast |
| Spacing | 6 – ft. – 12 ft. |
| Harvest time | Fall |
| Flowering period | Spring |
| Height | 8 ft. – 20 ft. |
| Width | 8 ft. – 20 ft. |
| Flower color | White |
| Leaf color | Gold, Yellow |
| Fruit color | Gold, Yellow |
| Stem color | Green |
| Fruit type | Berry |
| Fruit benefit | Edible |
| Flower benefit | Fragrant |
I. Appearance and Characteristics
The trifoliate orange, Citrus trifoliata (syn.Poncirus trifoliata), is a member of the family Rutaceae. Whether the trifoliate oranges should be considered to belong to their own genus, Poncirus, or be included in the genus Citrus is debated. The species is unusual among citrus for having deciduous, compound leaves and pubescent (downy) fruit.
It is native to northern China and Korea, and is also known as the Japanese bitter-orange (karatachi), hardy orange or Chinese bitter orange.
The trifoliate orange is recognizable by the large 3–5 cm (1.2–2.0 in) thorns on the shoots, and its deciduous leaves with three (or rarely, five) leaflets, typically with the middle leaflet 3–5 cm (1.2–2.0 in) long, and the two side leaflets 2–3 cm (0.79–1.18 in) long. The flowers are white, with pink stamens, 3–5 cm (1.2–2.0 in) in diameter, larger than those of true citrus but otherwise closely resembling them, except that the scent is much less pronounced than with true citrus. As with true citrus, the leaves give off a spicy smell when crushed.
The fruits are green, ripening to yellow, and 3–4 cm (1.2–1.6 in) in diameter similar in size to a lime and resembling a small orange, but with a finely downy surface and having a fuzzy texture similar to a peach. The fruits also have a distinctive smell from other citrus varieties and often contain a high concentration of seeds.
II. How to Grow and Care
Sunlight
Hardy orange prefers full sun, but can also grow in partial shade. It’s best to plant it in an open location where there is enough sunlight to grow well with abundant blooms; otherwise, it may make less fruit.
Temperature
Hardy orange has a high need for water. Especially in spring and fall, when it grows vigorously, regular and deep watering is needed to keep the soil moist. Hardy orange goes dormant in winter, so not much watering is needed then. Keep the soil semi-dry and well-drained. Be aware that overwatering may cause root rot, which can lead to death.
Watering
Hardy orange prefers moist soil and is somewhat drought tolerant, but it cannot grow well when waterlogged. Water deeply in spring before the leaves sprout, and then once per week during the budding season to keep the soil moist. During rainy periods, keep the soil well-drained without any standing water to prevent the buds from falling. Increase watering in dry weather, as the fruits may fall if the soil is too dry.
Soil
Loose, fertile, well-drained soil with a slightly acidic pH (5-6) is optimal for hardy orange. It does not tolerate poor or soggy soil, so if the soil in your garden is low in nutrients or poorly drained, add organic fertilizers, leaf litter, and river sand to improve it.
Fertilizing
If you want tasty fruit, add nutrients, apply fertilizer every 1-2 months in spring and summer and every 2-3 months in fall and winter. Use phosphorus-rich fertilizer (N-P-K = 10-30-20) during the blooming season, and a nitrogen- and potassium-rich fertilizer when fruiting (N-P-K = 18-10-20). Before winter, dig a shallow trench around the trunk and apply organic fertilizer once to provide sufficient nutrients for the plant to withstand the winter. This also promotes root growth so your hardy orange accumulates nutrients well.
Planting Instructions
When planting the trifoliate orange, select a site with full sun exposure and dig a hole twice as wide as the root ball. Amend the soil with organic matter to improve drainage if necessary. Place the plant in the hole, ensuring that the top of the root ball is level with the surrounding soil, and backfill with the amended soil. Water thoroughly after planting to settle the soil.
Pruning
Trifoliate orange grows quickly and benefit from annual pruning. When pruning, wear heavy leather gloves to protect your hands from the sharp thorns. Prune in winter when the plant’s structure is clearly visible. Remove dead, broken, and crossing branches. Trifoliate orange spreads slowly; keep it in bounds by digging up stems that emerge beyond its allotted areas in the landscape as soon as they are noticed.
Propagation
You can propagate hardy orange through grafting or layering. Perform grafting in spring by choosing a vigorous branch as rootstock. Saw the rootstock 10 to 15 cm from the ground with a flat surface and make a 2 cm cut on one side of the trunk. Then pick a 5 cm-long twig with leaf buds, and cut a 2 cm-long bevel at the bottom. Finally, insert the twig into the incision on the rootstock, make the cambium (the green layer between the bark and wood) of the twig touch that of the rootstock, and tie them together with plastic film.
If you want to do (aerial) layering, do it also in the spring. Select a 1-to-2-year-old vigorous branch and peel a 2 to 3 cm-wide strip of bark off at the bottom. Cover the wound with a ball of loose, fertile soil and wrap the wound and soil with plastic film. It will root in 40-60 days, and after 4 months you can cut the branch with the soil ball, tear the film off, and plant it in partial shade. Remember to keep the soil covering the wound moist. Once the cuttings are established, transplant them into the garden the following spring.
Pests and Diseases
The trifoliate orange is relatively pest-resistant but can occasionally be affected by common citrus pests such as aphids (Aphidoidea), spider mites (Tetranychidae), and scale insects (Coccoidea). These pests typically feed on the sap of the plant, which can lead to reduced vigor and, in severe cases, defoliation. Treatment includes the use of horticultural oils or insecticidal soaps. It is also important to maintain good cultural practices, such as proper watering and pruning, to reduce the likelihood of pest problems.
III. Uses and Benefits
- Cultivation
The cultivar ‘Flying Dragon’ is dwarfed in size, has highly twisted, contorted stems, and has even stronger thorns than the type. It makes an excellent barrier hedge due to its density and strong curved thorns. Such hedges have been grown for over 50 years at Oklahoma State University in Stillwater, and are highly student-proof. The plant is also highly deer-resistant. In central London, mature specimens of the trifoliate orange can be seen in the gardens of St Paul’s Cathedral.
Trifoliate orange and various hybrids of this plant are widely used as citrus rootstock, and valued for their resistance to cold, the tristeza virus, and the fungus Phytophthora parasitica (root rot).
Recent studies have revealed that the trifoliate orange contains auraptene at a high concentration, which is one of the functional components having immunity against citrus tristeza virus (CTV).
- Culinary uses
The fruits are very bitter, due in part to their poncirin content. Most people consider them inedible fresh, but they can be made into marmalade. When dried and powdered, they can be used as a condiment.
- Medical uses
The fruits of the trifoliate orange are widely used in medical traditions of East Asia as a treatment for allergic inflammation.
IV. Harvesting and Storage
Harvest trifoliate orange fruits in fall when they turn deep yellow and have a light citrus fragrance. The fuzzy fruit has a thick rind and is extremely sour. Dispose of fallen fruit to prevent rampant reseeding.
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