Latest Rhododendron Planting Techniques and Methods Tutorial

Rhododendrons are distributed across all provinces and regions of China except Xinjiang, with the greatest variety found in Yunnan, Tibet and Sichuan, making this 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 areas and prefer cool, humid climates, and detest extreme heat and dryness.

Site Selection and Soil 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 pH between 5-6, and soil 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 should all be picked, placed indoors in a well-ventilated spot and spread out to cool, allowing them to split naturally, then the fruit shells and other impurities 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, 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; fill the pot ± with acidic soil that has good permeability, is moist and fertile, and rich in organic matter. To achieve uniform emergence, mix the seeds with some fine soil, scatter them into the pot and cover with a thin layer of fine soil, water by the seepage method to let water penetrate into the pot, place the pot on the front windowsill, and cover with a layer of glass or plastic film, the purpose being 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 variations, 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 plant and row spacing of 2-3cm, when seedlings are 2-3cm tall (around November), transplant large seedlings one plant per 10cm pot, and small seedlings three plants per pot. Water with a fine spray pot 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, transfer to 13.3cm pots; in the third year the plants are 20cm tall, already have several branches and some flower buds appearing, transfer to 16.7cm pots, and afterwards, according to the size of the plant, repot year by year.

2. Cutting propagation: The cutting method is the most commonly used propagation method for rhododendrons; apart from a very few that cannot root, all can survive through cuttings, they are easy to collect, 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, times vary: Western azaleas late May to early June, Mao azaleas early June, Eastern azaleas and Summer azaleas mid to late June.

① Selection of cuttings: Choose cuttings that are robust in growth, free of pests and diseases, new shoots of the current year, semi-mature branches, already lignified; terminal shoots from the outer canopy are best. In principle, if the mother plant has 3 new shoots, take one as a cutting; if 5, take 2 as cuttings. The lengths are respectively: Western azaleas 5-7cm, Eastern azaleas and Summer azaleas 6-8cm, Mao azaleas 8-10cm. 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, put coarse-grained soil in the bottom layer 1/3 as a base; fill 2/3 with fine soil, make holes, insert the cuttings 1/2-1/3 deep, with spacing of 3cm, and water by the seepage method with a fine-hole spray pot.

③ Ground cutting method: Use bricks to enclose the surroundings at a height of 15-20cm and a width of 80-100cm, fill the inside with coarse-grained soil and fine soil, the method being the same as pot cutting.

Both of the above methods should be under semi-shaded conditions. Build a shade shed to prevent side sunlight exposure, hang curtains, keep air humidity at 85%, pay attention to water management, and always keep the cuttings fresh without dropping leaves; in this way they can root in one month.

3. Grafting propagation

① Approach grafting: Approach grafting is rather troublesome, and after survival the shape is not very attractive, so it is not very suitable for large-scale production. The method is: during the growing season, select a good scion and rootstock, and at an appropriate location, smooth and without nodes, make one cut on each, with the cut surface 3-4cm long, reaching deep to the xylem. Align the cambium 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 body, cut off the rootstock above the graft union, and it becomes an independent plant. This method has reliable survival, quickly becomes a tree, and gives quick results. If it does not survive, the loss to the plant is not great.

② Green shoot cleft grafting: Cleft grafting is mostly carried out in May-June, when plants grow vigorously, scion resources are abundant, the old and tender degree is moderate, healing is fast, and the survival rate is as high as over 90%. For the rootstock, choose Mao azaleas that have been cuttings for 2 years and have no forking; for the scion, choose one with high ornamental value, free of pests and diseases, 3cm long, retain 3-4 fine leaves at the upper part, remove all lower leaves, and cut it into a wedge shape 0.5-1cm long; cut off the rootstock at 2-3cm from the base of the new shoot, make a longitudinal cleft in the center, slightly deeper than the scion cut surface. Insert the scion so that one side or both sides have the cambium of the two matching. Tie tightly with a plastic bag, cover the scion and the graft union with a plastic bag, tie a transparent bag below the graft union to prevent rain from entering and to maintain temperature. Ordinarily keep the temperature inside the bag, prevent direct sunlight, give some fertilizer water, promptly cut off the rootstock's sprouts, and after 2 months when it has survived remove the bag.

③ Green shoot side grafting: Side grafting is done from mid to late May, still using Mao azaleas as the rootstock, making a slanting cut at 6-7cm from the base down to the xylem, 0.5-1cm long; the requirements for the scion are the same as for cleft grafting, but the front is made one long and one short, with the long side inserted toward the inside and the short side facing outward, cover and cut well, align the cambium, tie and put on a bag, and place in a shaded spot.

4. Layering propagation: Layering propagation is mostly carried out between May and June, divided into two methods: one is air layering, the other is ground layering.

① Air layering: Choose 2-3 year old branches, at an appropriate location cut a ring, deep to the xylem, with a width of 0.5cm, remove the bark, then take a prepared bamboo tube, 8cm long, 2-3cm in diameter, with a node on one side, split it from the middle, at the node carve a semicircular hole according to the thickness of the plant stem, and fit the two halves of the bamboo tube around the cut. Fill the tube with acidic soil, water it, and keep the soil in the tube moist; after 3 months many fibrous roots 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 makes use of shoots sprouting from field-grown rhododendrons; at an appropriate location, cut a ring, remove the bark, bury it in the soil, water after 3 days, keep the soil moist, and after 3 months, once 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, ornamental garden rhododendrons are still mainly grown in pots, as 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; no matter which kind of soil is used, before cultivation the soil needs to be disinfected, fully decomposed and fermented, and treated with alternating high and low temperatures, and only after one year can it be used. Among these, the best soil is forest humus soil, or self-prepared 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 a mountain, add dried cake, human waste, 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 human waste) is second best.

2. Transplanting into pots and repotting: Before transplanting, if the pot has no drainage hole, cover it crosswise with several pieces of tile, 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 the roots spread out, add soil toward the pot up to the root crown, press firm, shake the pot and level the soil surface. Spray finely until water drips from the potted flower; after a 7-10 day pot-covering stage, place it at the front and back of the house, and it should be placed in a semi-shaded spot in the greenhouse. Repotting is because after the plant grows larger it can no longer be satisfied by the ecological conditions, so new culture soil must be used; take the plant out of the old pot, remove some of the shrunken 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, every 3-5 years; extra-large ones need not be repotted, and technical measures such as topdressing are carried out, applied in the rhododendron's space, keeping relative humidity above 70%.

3. Watering: For watering rhododendrons, rainwater, river water, lake water, pond water, and fish-tank water are best. Alkaline water, well water and tap water are forbidden. If they really must be used, bring well water temperature to be similar to the air temperature; tap water must be dechlorinated by leaving it for several days to let the chlorine volatilize before use. For alkaline water, add sulfuric acid to adjust the pH. The amount of watering varies according to different growth stages: less during the dormant period; in Jiangnan from November to February; from March to June the rhododendrons bloom and grow vigorously, so water more, once a day on sunny days, and in some cases supplementary watering in the evening. During the plum rain season, pay attention to drainage; in July-August it must be checked at all times, and when the pot is dry it must be watered, while also spraying the leaves with water; in winter water in the morning, in summer water in the early morning, and in spring and autumn water at any time except noon. In heated greenhouses indoors, water once every 2-3 days; in unheated ones, 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. Fertilizing: Fertilizers include base fertilizer, long-acting hoof, nail pieces, bone meal, cake fertilizer, dried manure, etc., mixed with the soil when potting or repotting. For topdressing, use human waste and chemical fertilizers, and quick-acting fertilizers. Rhododendrons need different fertilizers at different growth stages. Before flowering, apply more phosphorus fertilizer to promote flowering, so the flowers are larger, the petals thicker, the color better, and the flowering period longer, applying once every 10 days, 2-3 times. Stop fertilizing during the flowering period. After flowering, in order to restore the tree's vigor and promote new shoots and leaf growth, apply nitrogen fertilizer; in the high-temperature season (July-August) growth stops, so it is not easy to fertilize. Before moving indoors in autumn is the bud pregnancy period, so apply more phosphorus fertilizer, once every 7-10 days; stop fertilizing during the winter dormant period. The principles to grasp in fertilizing: the fertilizer must be fully decomposed, apply diluted 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, excess branches and leggy branches, buds, shoots, etc. must be pruned every year. When the rhododendron tree grows to a certain height, remove the terminal bud to control height and stimulate side branches. After pinching, many side branches grow again, in a whorled arrangement. When the side branches grow to a certain height, pinch again. This stimulates the next level of side branches. Bud removal: when the flower buds grow to a certain degree, remove the tips of the flower buds to prevent too many flowers from blooming and wasting nutrients. At the same time, wipe off adventitious buds; in order to maintain the tree shape's elegance, the tree's vigor should be enhanced, 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 to rejuvenate them; in early spring before new buds sprout, cut the branches leaving about 30cm and remove the upper part. Do not cut everything at once; do it in stages, cutting 1/3-1/5 each time, cutting short, completing in 3-5 years. In this way it does not affect flower viewing, and after pruning, with careful management and fertilizing, the plant can remain vigorous without decline for a relatively long time.