A Detailed Guide to Rhododendron Cultivation Techniques and Pest and Disease Control Methods
Rhododendron, also known as yingshanhong, yangjiaohua, yangzhizhu, hongzhizhu, and hongzhizhu, is a semi-evergreen, evergreen deciduous shrub or evergreen small tree of the genus Rhododendron in the family Ericaceae. Flower types include single, semi-double, and double, with the corolla mostly red, in various shades such as bright red, pink, peach red, vermilion, purple red, and yellow; there are also pure white, golden yellow, pale yellow, and violet colors, or mixed with spots and stripes. There are about 800 species in the same genus, most of which are native to China. It is a traditional famous flower of our country and one of China's "Top Ten Famous Flowers." It is a precious tree for urban greening and garden courtyard decoration, and can be potted or made into stump bonsai. Rhododendron wood is hard and can be used for carving, and the roots, leaves, and flowers can all be used for medicinal purposes.
1. Propagation. Rhododendron propagation can be done by sowing seeds as soon as they mature; it can also be done by cutting propagation before germination in February, from early June to early July, and from late August to early September; by layering during the plum rain season; by division combined with transplanting; and by grafting in February in spring, during the plum rain season, and from August to September in autumn, selecting robustly growing hairy-leaf Rhododendron as rootstock, using cleft grafting, side grafting, and approach grafting methods.
2. Transplanting. Rhododendron prefers cool, humid, and semi-shaded environments, avoids scorching sun exposure, prefers acidic soil, and is relatively cold-tolerant. It can be cultivated in open fields south of the Huai River in China. When planting, choose a suitable location, dig a planting hole or trench at least 50 cm wide and 50 cm deep, and apply an appropriate amount of high-quality decomposed farmyard manure or urban waste as base fertilizer. Transplanting is often done in early spring or during the plum rain season, and it is best to transplant with a soil ball. Using nutrient pot seedling cultivation, transplantation can be done at any time. Within 1 month after planting, water 1-2 times daily, and spray leaf surface water (mist) several times a day. From after spring germination to early autumn each year, apply organic liquid fertilizer and phosphorus and potassium chemical fertilizer liquid 1-2 times per month, and spray foliar fertilizer at appropriate times. Water timely during the dry season, pay attention to drainage during the rainy season, and avoid waterlogging. During high temperatures in summer and early autumn, provide appropriate shade, and spray leaf surface water (mist) once each morning and evening every day. Prune appropriately once after flowering each year.
3. Pot cultivation. In addition to being planted in the ground for greening and ornamental purposes, Rhododendron can also be potted or made into tree or stump bonsai, and has long been known as one of the "Eighteen Scholars" of stump bonsai tree species. To make bonsai, in addition to artificial propagation and cultivation for shaping, you can also select and dig wild tree stumps from mountainous areas between February and March for cultivation and shaping. In spring or during the plum rain season, use the "combination of pruning and wiring" method for shaping. The potting soil requires a pH value of 4.5-6.5, loose and permeable, rich in humus. It can be prepared by mixing 5 parts of loose mountain soil, 2 parts of leaf mold or peat, 2 parts of decomposed high-quality farmyard manure or urban waste, and 1 part of fine river sand. Choose pots according to the shape. Transplanting into pots should be done in February-March in spring or September in autumn. After potting, place in a location without direct sunlight but with sufficient scattered light. Water and spray leaf surface water frequently to keep the potting soil and environment moist. When new branches grow vigorously, move to a semi-shaded, semi-sunny location for maintenance. After one year of survival in the pot, begin fertilizing and pruning.
4. Maintenance. Rhododendron bonsai should usually be maintained in a place with sufficient light and humidity. Water frequently to keep the potting soil moist, and avoid water accumulation in the pot. In summer and early autumn, use shade nets, bamboo curtains, etc. to block 50-60% of sunlight, avoid strong direct sunlight, and water and spray leaf surface water once each morning and evening every day. After early autumn and in winter, appropriately reduce watering and keep the potting soil moist but slightly on the dry side. In the south, it can overwinter outdoors in a wind-protected, sunny location; in the north, it should be moved indoors for winter, keeping the room temperature at around 10°C. During the growing season, apply dilute organic fertilizer liquid and phosphorus and potassium chemical fertilizer liquid once every 7-10 days; during the plum rain season, do not apply liquid fertilizer, but instead apply dry cake fertilizer or flowering-type compound flower fertilizer on the pot surface. Stop fertilizing during the flowering period, high-temperature season, and winter dormancy. During the growing period, appropriately pinch and remove buds at appropriate times. When flower buds grow to a certain size, moderately thin the buds. After flowering and during winter dormancy, appropriately prune the branches once. Every 2-3 years (medium and large bonsai can be over 4 years), repot and change soil once after flowering, replacing about 1/2 of the old soil. Combined with repotting, moderately raise the roots to expose them, so that the roots hang and claws are revealed, to improve the ornamental effect.
5. Control of Major Pests
Red spider mite. Can be sprayed and killed with 40% dimethoate 1500-2000 times liquid, 58% Fengleiji emulsifiable concentrate 1500-2500 times liquid, etc.
Lace bug (pear lace bug). Can be sprayed and killed with 50% fenitrothion 1000 times liquid.
Tea bagworm (bagworm, bag moth). Can be sprayed and killed with 90% trichlorfon 2000 times liquid.
Tea tortrix (leaf-rolling moth). Can use black light traps to kill adults, and use 20% diflubenzuron suspension concentrate 8000 times liquid, 20% Duochongwei 2000 times liquid, etc. to spray and kill larvae.
Inchworm (looper). Can use black light traps to kill adults, and use 20% Jusha emulsifiable concentrate 2000 times liquid, 25% Fengmang emulsifiable concentrate 800-1000 times liquid, etc. to spray and kill larvae.
6. Control of Major Diseases
Brown spot disease. Can be sprayed with 70% thiophanate-methyl 1000 times liquid, 65% zineb 600-800 times liquid, etc. Spray once every 5-7 days, and spray 3-5 times continuously depending on the severity of the disease.
Root rot. Strengthen cultivation management, keep the soil moist and loose, and avoid waterlogging. In the early stage of the disease, drench the roots with 12% copper rosinate emulsifiable concentrate 800-1000 times liquid or Lüheng No. 1 3000 times liquid, once a week, continuously for 2-3 times. When the disease is severe, promptly dig the plant out of the soil (potted or bonsai plants should be repotted and soil changed in time), wash off the soil from the roots, cut off the diseased root system, then soak the plant roots in 12% copper abietate emulsifiable concentrate 600 times liquid, or Lüheng No. 2 800 times liquid, or 50% Genfuling wettable powder 500 times liquid for 30 minutes, then rinse the roots with clean water, then use 75% chlorothalonil wettable powder 1000 times liquid, or 50% carbendazim wettable powder 800 times liquid, add an appropriate amount of fine red (yellow) soil, stir into a thick mud slurry, dip the roots and wait to plant. For ground planting, re-dig holes and loosen the soil (for potted and bonsai plants, replace with loose new culture soil), and use 50% Genfuling 600 times liquid or 50% Shajunwang water-soluble powder 1000 times liquid, Lüheng No. 1 3000 times liquid, 12% copper rosinate emulsifiable concentrate 600 times liquid, etc. to drench the soil (culture soil) for disinfection, then plant the plant again. After planting, spray the leaf surface once a week with 12% copper abietate emulsifiable concentrate 1000 times liquid or 50% Genfuling wettable powder 800 times liquid, Lüheng No. 2 600-800 times liquid, 70% manganese fungicide wettable powder 600-800 times liquid, and other fungicides. It is best to alternate two or more of them, spraying continuously 3-5 times.
Leaf gall disease (blister disease). Strengthen cultivation management, pay attention to environmental ventilation and light transmission. When diseased leaves and diseased shoots are found, promptly and thoroughly remove them and destroy them in time. Before germination in spring, spray the plant once with 1 degree Baumé lime sulfur mixture. During the new leaf expansion period, spray the leaf surface 2-3 times with 0.5 degree Baumé lime sulfur mixture or 1% Bordeaux mixture, spraying once every 7-10 days.