Citron, commonly known as Torreya grandis, is a species of eucalyptus belonging to the genus Polygonaceae. It is an evergreen tree that can grow up to 30 meters tall with a trunk diameter of around 5 meters at breast height. The bark features irregular longitudinal cracks, and all parts—branches, leaves, and wood—emit a distinct camphor-like aroma. The leaves are alternate, thin-leathery, ovate or ovate-elliptic in shape, measuring 5–10 cm in length and 2.5–5.5 cm in width. They have an acute or subapical tip, a broadly cuneate to suborbicular base, and entire margins with slight undulations. The leaves are hairless on both sides, with three prominent veins emerging from the base and pinnate venation, along with noticeable glandular pits. The petiole is 2–3 cm long.
The flowers are bisexual and arranged in axillary panicles. The fruit is nearly spherical, with a diameter of 4.5–6 mm, initially green, turning red as it matures, and finally developing a shiny purple color. The fruit has a truncated top. Flowering occurs between April and May, while the fruits mature from October to November. Citron typically produces fruit every one or two years. This species thrives in warm and humid climates, where the average annual temperature exceeds 16°C, and the minimum temperature does not drop below -7°C. It prefers deep, fertile, moist, neutral or acidic soils. Young trees are shade-tolerant, but they grow vigorously in their prime and can live for many decades. Old trees are often found near villages.
Propagation of Citron primarily involves seed sowing, though hardwood cuttings, root cuttings, and tiller propagation are also possible, albeit with lower survival rates. Seed collection should be done from well-developed, mature Torreya grandis trees between October and November. The fruits turn from green to red and then to purple when ripe, and they fall naturally. Harvesting should be timely to prevent bird damage. Early collection may result in lower germination rates. After collection, seeds must be processed quickly to avoid heat buildup and embryo damage. The pulp should be removed after 2–3 days, followed by washing, soaking in ash for 12–14 hours, and drying. The seeding rate is 25–30%, with 1000 seeds weighing 120–130 grams, totaling 7,600–8,000 seeds per kilogram. Germination rates are usually 80–90%. Seeds can be sown in autumn or mixed with wet sand for spring planting.
In nurseries, shaded and wind-protected areas are ideal. Eucalyptus seeds germinate slowly, so pre-treatment is essential. Dry seeds can be soaked in warm water, and those stored with sand should be treated with 0.5% potassium permanganate for two hours to enhance germination. Sowing generally takes place in mid-February, though it can be delayed until mid-March if frost is expected. In frost-free regions, sowing can be done earlier. Seeds are sown in trenches about 3 cm deep, with 40–50 seeds per meter, covering them with humus or fire soil and grass. After germination, the soil should be kept about 2–3 cm deep, with spacing of 20–25 cm between seedlings. Germination can take 40–50 days, lasting over 50–60 days. Seedlings are irregularly spaced, and those in wet conditions may germinate faster but less uniformly. Soaking seeds before sowing helps improve uniformity. No shading is needed once seedlings emerge. Seedlings with several true leaves are ready for transplanting. By July–September, they grow rapidly, requiring proper fertilization and drought protection. Annual seedlings reach 30–50 cm in height and are suitable for afforestation. Winter protection is necessary, especially in northern areas, using wind barriers or straw to protect young roots. When lifting seedlings, care must be taken to avoid root damage. Pruning branches and leaves can increase survival rates. To promote lateral root development, cutting 5–6 cm deep into the roots when the seedling has 3–5 true leaves can help. Watering after cutting is essential. Some seedlings can be directly planted during the rainy season, which is more efficient than waiting for the next year.
Transplanting large trees of Torreya grandis is common for trees with a trunk diameter of 10 cm or more. The best time is early spring or late autumn, before the ground freezes, when the tree is dormant. Crown pruning is used for specific landscape needs, reducing leaf retention to minimize moisture loss. Root reduction and crown reduction techniques are widely applied, maintaining the tree's structure while cutting roots within 8 times the trunk diameter. For large trees that are difficult to move, the cut-dry method is used, where the trunk is cut at 3–3.5 meters. Precautions include applying preservatives to branch wounds, wrapping roots with straw, and watering before excavation. The soil ball should be 6–8 times the trunk diameter, wrapped tightly with straw, and the earth ball thickness should be 3–6 times the trunk diameter. Care must be taken not to damage the bark during lifting. The tree is transported with the canopy facing backward and the trunk wrapped in soft material. Upon planting, support structures are used to prevent wind damage, and regular watering, fertilization, and pruning are required. Shading is provided in summer, and winter protection is given to prevent cold damage. Following the "three-point cultivation and seven-point care" principle ensures successful transplantation.
ZHONGSHAN G.H.L. TRADING CO., LTD. , https://www.ghltrade.com