A Detailed Guide to Rhododendron Planting Techniques and Care Methods
Rhododendrons are distributed across all provinces and regions of our country north and south except for **, with the greatest number of species found in Yunnan, Xizang and Sichuan, which is the world distribution center of the genus Rhododendron. The genus Rhododendron has many species with greatly differing habits, but most species originate from high-altitude regions and prefer cool, humid climates, and dislike intense heat and dryness.
Site Selection and Land Preparation
For site selection, choose a warm and humid location with not-too-strong light, a cool climate, fertile soil, and humus-rich acidic soil with a pH below 7, or neutral soil with a pH between 5-6, with good drainage and aeration.
Propagation Methods
1. Seed propagation method: The vast majority of rhododendrons can set seed and be collected for seed, only double-flowered ones do not set seed. Generally, the seed maturity period runs from October to January each year; when the fruit skin turns from green to yellow and then brown, the top of the fruit splits open and the seeds begin to scatter, and at this point they must be collected at any time. Those that have not split but have turned brown are all collected, spread out to cool in a well-ventilated indoor place, allowed to split naturally, then the fruit shells and other impurities are removed, put into paper bags or cloth bags, and stored in a cool, ventilated place. If greenhouse conditions are available, sowing immediately after collection gives a high germination rate. Generally, the sowing time is March-April, using pot sowing; because the seeds are small, wash the pot inside and out, dry it in the sun, sterilize and disinfect it, and also sterilize and disinfect the soil; when filling the pot ± use acidic soil with good permeability, moist and fertile, and rich in organic matter. To ensure uniform emergence, mix the seeds with some fine soil, scatter them into the pot, then cover with a thin layer of fine soil, water by the seepage method to let water seep into the pot, place the pot on the front windowsill, and cover with a layer of glass or plastic film in order to raise the temperature inside the pot. After the seedlings emerge, gradually reduce the covering time; because the seedlings are tender and small, pay attention to temperature changes, sudden high and low changes, and strong light exposure. The seedlings grow very slowly, and only grow 2-3 true leaves by May-June; at this time, do the first transplanting indoors, with a plant and row spacing of 2-3 cm, and when the seedlings are 2-3 cm tall (around November), large seedlings are transplanted one plant per 10 cm pot, and small seedlings three plants per pot. Use a fine watering can to water and apply dilute fertilizer water. After sowing, in the second spring, move them out of the greenhouse and place them under a shade shed for care. In June, move them into 13.3 cm pots; in the third year, when the plants are 20 cm tall and already have several branches and some flower buds appearing, move them into 16.7 cm pots; afterward, according to the size of the plants, repot year by year.
2. Cutting propagation: The cutting method is the most commonly used propagation method for rhododendrons; apart from very few that cannot root, all can survive by cutting. It is easy to collect material, simple to operate, and can preserve the excellent characteristics of the variety. Cuttings can be taken throughout the year, with the best time being May-June, followed by August-September. However, because varieties differ, the times vary: Western azaleas from late May to early June, hairy azaleas in early June, and Eastern azaleas and summer azaleas from mid-June to late June.
① Selection of cuttings: For cuttings, choose robust growth, free of pests and diseases, current-year new shoots, semi-mature branches, already lignified; terminal shoots on the outer part of the crown are best. In principle, if the mother plant has 3 new shoots, take 1 as a cutting; if it has 5, take 2 as cuttings. The lengths are respectively: Western azalea 5-7 cm, Eastern azalea and summer azalea 6-8 cm, hairy azalea 8-10 cm. For overly long branches, the tip can be cut off, the lower leaves removed, and 4-5 small leaves kept at the upper end; take and insert immediately.
② Pot cutting method: Wash and disinfect the pot, fill with potting soil, place coarse-grained soil in the bottom 1/3 as a base layer; fill 2/3 with fine soil, make holes, insert the cuttings to 1/2-1/3 of their length, with a spacing of 3 cm, and water by the seepage method with a fine-hole watering can.
③ Ground cutting method: Use bricks to enclose the surroundings to a height of 15-20 cm and a width of 80-100 cm, and fill the inside with coarse-grained soil and fine soil, in the same way as pot cutting.
The above two methods should both be under semi-shaded conditions. Build a shade shed to prevent side sunlight exposure, hang curtains, keep air humidity at 85 percent, pay attention to water management, and always keep the cuttings fresh without dropping leaves; in this way they will root in one month.
3. Grafting propagation
① Approach grafting: Approach grafting is rather troublesome, and after surviving the shape is not very attractive, so it is not very suitable for large-scale production. The method is: in the growing season, select the scion and rootstock for approach grafting, at a suitable position, smooth and without nodes, make one cut on each, with the cut surface 3-4 cm long, reaching the xylem. Align the cambium layers of the two, tie tightly with a plastic bag, and after 4-5 months the wound heals; cut off the scion to separate it from the mother plant, cut off the rootstock above the grafting point, and it becomes an independent plant. This method is reliable for survival, quickly forms a tree, and gives quick results. If it fails, the loss to the plant is not great.
② Green-twig cleft grafting: Cleft grafting is mostly carried out in May-June, when the plants grow vigorously, scion resources are abundant, the material is moderately old and tender, healing is fast, and the survival rate is as high as over 90 percent. For the rootstock, choose hairy azalea cuttings grown for 2 years, without forking; for the scion, choose one with high ornamental value, free of pests and diseases, 3 cm long, retain 3-4 small leaves at the upper end, remove all lower leaves, and cut it into a wedge shape 0.5-1 cm long; cut off the new shoot of the rootstock at 2-3 cm from its base, split it vertically in the center with one cut, slightly deeper than the scion cut surface. Insert the scion so that one side or both sides have the cambium layers matching. Tie tightly with a plastic bag, cover the scion and grafting point with a plastic bag, tie the transparent bag well below the grafting point to prevent rain from entering and to maintain temperature. Usually keep the temperature inside the bag, prevent direct sunlight, give a little fertilizer water, promptly cut off sprouts from the rootstock, and after 2 months when it has survived remove the bag.
③ Green-twig side grafting: Side grafting is done from mid-May to late May, still using hairy azalea as the rootstock, making a slanting cut at 6-7 cm from the base down to the xylem, 0.5-1 cm long; the requirements for the scion are the same as for cleft grafting, but the front is one long and one short side; insert the long side facing inward and the short side facing outward, cover and cut well, align the cambium layers, tie well and put on a bag, and place in a shaded place.
4. Layering propagation: Layering propagation is mostly carried out between May and June, and is divided into two methods: one is air layering, and the other is ground layering.
① Air layering: Choose 2-3-year-old branches, at a suitable position, cut a ring around the branch down to the xylem, with a width of 0.5 cm, remove the bark, then take a prepared bamboo tube, 8 cm long and 2-3 cm in diameter, with a node on one side, split it open from the middle, and at the node carve a semicircular small hole according to the thickness of the plant stem; fit the two semicircular bamboo tubes around the cut area. Fill the tube with acidic soil, water it, and keep the soil in the tube moist; after 3 months many fibrous roots will grow inside, and it can then be cut off from the mother plant and planted in a pot for cultivation.
② Ground layering: Ground layering uses shoots sprouting from field-grown rhododendrons; at a suitable position, cut a ring, remove the bark, bury it in the soil, water after 3 days, and keep the soil moist; after 3 months, when fibrous roots are found to have grown, separate it from the mother plant and pot it for cultivation.
Field Management
1. Soil treatment: At present, rhododendrons for garden ornament are still mainly potted, and this method helps meet the ecological conditions of different varieties. In cultivation, the soil requirement is mainly acidic soil, and it is best if the pH must be below 6. Whichever kind of soil is used, before cultivation the soil needs to be disinfected, fully decomposed and fermented, and treated alternately at high and low temperatures; only after one year can it be used. Among these, the best soil is forest humus soil, or homemade pine-needle soil, with dead branches and fallen leaves, plus a small amount of horse manure and sheep manure, and after collecting, add 1/3 fertile field soil, or soil from the foot of mountains, add cake fertilizer, human excrement and urine, pig manure, chicken and duck manure, superphosphate, etc., then add 1/3 sandy loam soil and Huangshan soil; leaf mold soil (dead branches and fallen leaves, with added excrement and urine) is second best.
2. Transplanting into pots and repotting: Before transplanting, if the pot has no drainage hole, cover it crosswise with several tile pieces, fill a layer of coarse-grained soil, then fill with fine soil that has been aged for a year, plant one seedling in the middle of the pot so that the roots spread out, add soil toward the pot up to the root crown, press firmly, shake the pot and level the soil surface. Spray finely until water drips from the potted flower; after a 7-10 day recovery period, place it in front of or behind the house, and it should be placed in a semi-shaded place in the greenhouse. Repotting is because after the plant grows larger it can no longer meet ecological conditions, so new culture soil must be used; take the plant out of the old pot, remove some of the compacted soil, spread the surrounding roots out, and cut off diseased roots, with the operation the same as above. Generally repot once every 2-3 years. For large plants, once every 3-5 years; extra-large ones may not be repotted, and technical measures such as topdressing are carried out, applied in the space of the rhododendron, maintaining relative humidity above 70 percent.
3. Watering: For watering rhododendrons, rainwater, river water, lake water, pond water, and water from fish tanks are best. Do not use alkaline water, well water, or tap water. If they must be used, treat well water so its temperature is similar to the air temperature, and let tap water dechlorinate by standing for several days to let the chlorine volatilize before use. For alkaline water, add sulfuric acid to adjust the pH. The amount of watering should differ according to different growth stages; less during the dormant period. In Jiangnan from November to February, and from March to June when rhododendrons bloom and grow vigorously, water more; on sunny days water once a day, and in some cases replenish water in the evening. During the plum rain season, pay attention to drainage; in July-August, check at all times, water whenever the pot is dry, and also spray the leaves. In winter, water in the morning; in summer, water in the morning; in spring and autumn, water at any time except noon. In heated greenhouses indoors, water once every 2-3 days; in unheated ones, water once every 4-5 days. In late February, as the temperature rises, the flower buds begin to swell and the leaf buds also begin to sprout, so the water amount should be appropriately increased.
4. Fertilization: Fertilization includes base fertilizer, and long-lasting hooves, hoof pieces, bone meal, cake fertilizer, dried manure, etc., mixed with the soil when potting or repotting. For topdressing, use human excrement and urine, chemical fertilizers, and quick-acting fertilizers. Rhododendrons need different fertilizers at different growth stages. Before flowering, apply more phosphorus fertilizer to promote flowering, making the flowers larger, the petals thicker, the color better, and the flowering period longer; apply once every 10 days, 2-3 times. Stop fertilizing during the flowering period. After flowering, in order to restore tree vigor and promote new shoots and leaf growth, apply nitrogen fertilizer. In the high-temperature season (July-August) growth stops, and it is not easy to fertilize. In autumn, before moving indoors, it is the flower bud differentiation period, so apply more phosphorus fertilizer, once every 7-10 days; stop fertilizing during the winter dormant period. The principles to grasp in fertilization: the fertilizer must be fully decomposed, apply thin fertilizer frequently, never apply fertilizer on the leaves, apply on sunny days, apply when the potting soil is dry, apply in the evening, and water the next morning.
5. Pruning: In order to maintain the beauty of the rhododendron's tree shape, prune excessive branches and leggy branches, buds, shoots, etc. every year. After the rhododendron tree grows to a certain height, remove the terminal buds to control height and stimulate side branches. After pinching, many side branches grow again in a whorled pattern. When the side branches grow to a certain height, pinch them again. This stimulates the next level of side branches. Bud removal: when the flower buds grow to a certain extent, remove the tips of the flower buds to prevent too many flowers from wasting nutrients. At the same time, wipe off adventitious buds. In order to maintain the beauty of the tree shape, the tree vigor should be strengthened, branches renewed, ventilation conditions adjusted, and those weak branches, diseased branches, dead branches, crossing branches, overlapping branches, overly dense branches, and leggy branches should be cut off from the base. For old plants, pruning should be carried out for rejuvenation; in early spring before the new buds sprout, cut off the upper part of the branches leaving about 30 cm. Do not cut all at once; do it in stages, each time cutting 1/3-1/5, shortening them, completing it over 3-5 years. In this way it does not affect flower viewing, and after pruning, with careful management and fertilization, the plant can be kept growing vigorously without decline for a relatively long time.